2022
Riva, F.; Steiner, O.
Methodology for estimating the magnetic Prandtl number and application to solar surface small-scale dynamo simulations Journal Article
In: Astronomy and Astrophysics, vol. 660, pp. A115, 2022.
@article{refId0k,
title = {Methodology for estimating the magnetic Prandtl number and application to solar surface small-scale dynamo simulations},
author = {F. Riva and O. Steiner},
url = {https://doi.org/10.1051/0004-6361/202142644},
doi = {10.1051/0004-6361/202142644},
year = {2022},
date = {2022-04-23},
urldate = {2022-04-23},
journal = {Astronomy and Astrophysics},
volume = {660},
pages = {A115},
abstract = {Context. A crucial step in the numerical investigation of small-scale dynamos in the solar atmosphere consists of an accurate determination of the magnetic Prandtl number, Prm, stemming from radiative magneto-hydrodynamic (MHD) simulations.
Aims. The aims are to provide a reliable methodology for estimating the effective Reynolds and magnetic Reynolds numbers, Re and Rem, and their ratio Prm = Rem/Re (the magnetic Prandlt number), that characterise MHD simulations and to categorise small-scale dynamo simulations in terms of these dimensionless parameters.
Methods. The methodology proposed for computing Re and Rem is based on the method of projection on proper elements and it relies on a post-processing step carried out using higher order accurate numerical operators than the ones in the simulation code. A number of radiative MHD simulations with different effective viscosities and plasma resistivities were carried out with the CO5BOLD code, and the resulting growth rate of the magnetic energy and saturated magnetic field strengths were characterised in terms of Re and Rem.
Results. Overall, the proposed methodology provides a solid estimate of the dissipation coefficients affecting the momentum and induction equations of MHD simulation codes, and consequently also a reliable evaluation of the magnetic Prandtl number characterising the numerical results. Additionally, it is found that small-scale dynamos are active and can amplify a small seed magnetic field up to significant values in CO5BOLD simulations with a grid spacing smaller than h = 12 km, even at Prm ≃ 0.65. However, it is also evident that it is difficult to categorise dynamo simulations in terms of Prm alone, because it is not only important to estimate the amplitude of the dissipation coefficients, but also at which scales energy dissipation takes place.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Context. A crucial step in the numerical investigation of small-scale dynamos in the solar atmosphere consists of an accurate determination of the magnetic Prandtl number, Prm, stemming from radiative magneto-hydrodynamic (MHD) simulations.
Aims. The aims are to provide a reliable methodology for estimating the effective Reynolds and magnetic Reynolds numbers, Re and Rem, and their ratio Prm = Rem/Re (the magnetic Prandlt number), that characterise MHD simulations and to categorise small-scale dynamo simulations in terms of these dimensionless parameters.
Methods. The methodology proposed for computing Re and Rem is based on the method of projection on proper elements and it relies on a post-processing step carried out using higher order accurate numerical operators than the ones in the simulation code. A number of radiative MHD simulations with different effective viscosities and plasma resistivities were carried out with the CO5BOLD code, and the resulting growth rate of the magnetic energy and saturated magnetic field strengths were characterised in terms of Re and Rem.
Results. Overall, the proposed methodology provides a solid estimate of the dissipation coefficients affecting the momentum and induction equations of MHD simulation codes, and consequently also a reliable evaluation of the magnetic Prandtl number characterising the numerical results. Additionally, it is found that small-scale dynamos are active and can amplify a small seed magnetic field up to significant values in CO5BOLD simulations with a grid spacing smaller than h = 12 km, even at Prm ≃ 0.65. However, it is also evident that it is difficult to categorise dynamo simulations in terms of Prm alone, because it is not only important to estimate the amplitude of the dissipation coefficients, but also at which scales energy dissipation takes place.
Aims. The aims are to provide a reliable methodology for estimating the effective Reynolds and magnetic Reynolds numbers, Re and Rem, and their ratio Prm = Rem/Re (the magnetic Prandlt number), that characterise MHD simulations and to categorise small-scale dynamo simulations in terms of these dimensionless parameters.
Methods. The methodology proposed for computing Re and Rem is based on the method of projection on proper elements and it relies on a post-processing step carried out using higher order accurate numerical operators than the ones in the simulation code. A number of radiative MHD simulations with different effective viscosities and plasma resistivities were carried out with the CO5BOLD code, and the resulting growth rate of the magnetic energy and saturated magnetic field strengths were characterised in terms of Re and Rem.
Results. Overall, the proposed methodology provides a solid estimate of the dissipation coefficients affecting the momentum and induction equations of MHD simulation codes, and consequently also a reliable evaluation of the magnetic Prandtl number characterising the numerical results. Additionally, it is found that small-scale dynamos are active and can amplify a small seed magnetic field up to significant values in CO5BOLD simulations with a grid spacing smaller than h = 12 km, even at Prm ≃ 0.65. However, it is also evident that it is difficult to categorise dynamo simulations in terms of Prm alone, because it is not only important to estimate the amplitude of the dissipation coefficients, but also at which scales energy dissipation takes place.
2021
Marassi, Alessandro; Monstein, Christian
Trieste CALLISTO Station Setup and Observations of Solar Radio Bursts Journal Article
In: Advances in Space Research, 2021, ISSN: 0273-1177.
@article{MARASSI2021,
title = {Trieste CALLISTO Station Setup and Observations of Solar Radio Bursts},
author = {Alessandro Marassi and Christian Monstein},
url = {https://www.sciencedirect.com/science/article/pii/S0273117721009704},
doi = {10.1016/j.asr.2021.12.043},
issn = {0273-1177},
year = {2021},
date = {2021-12-31},
urldate = {2021-01-01},
journal = {Advances in Space Research},
abstract = {The Trieste CALLISTO station (http://radiosun.oats.inaf.it) was established in 2012 at the Basovizza Observing Station (45°38'37” N, 13°52'34 E”, 398m above MSL) operated by the Italian National Institute for Astrophysics (INAF) - Astronomical Observatory of Trieste (Italy) to study solar radio bursts and the response of the Earth’s ionosphere and geomagnetic field. To date, three ‘Compound Astronomical Low-cost Low frequency Instrument for Spectroscopy and Transportable Observatory’ (CALLISTO) spectrometers have been installed, with the capability of observing in the frequency ranges 45-80 MHz (from 30 December 2014), 220-420 MHz (from 1 June 2012 to 23 October 2012 and from 05 October 2013), 905-1730 MHz (from 30 December 2019). The three receivers are fed respectively by a dipole, log-periodic and cross-dipole antenna. Nominally, frequency spectra are obtained with 4 sweeps per second over in total 600 channels. Here, we describe the Trieste CALLISTO station set-up, the local e-Callisto network digital archive, Trieste CALLISTO Radio Bursts Detection and Visualization System available via web and present dynamic spectra of a sample of Type I, II, III, IV and V radio bursts. As an additional feature, we show also its capability to record lightning strikes.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The Trieste CALLISTO station (http://radiosun.oats.inaf.it) was established in 2012 at the Basovizza Observing Station (45°38'37” N, 13°52'34 E”, 398m above MSL) operated by the Italian National Institute for Astrophysics (INAF) - Astronomical Observatory of Trieste (Italy) to study solar radio bursts and the response of the Earth’s ionosphere and geomagnetic field. To date, three ‘Compound Astronomical Low-cost Low frequency Instrument for Spectroscopy and Transportable Observatory’ (CALLISTO) spectrometers have been installed, with the capability of observing in the frequency ranges 45-80 MHz (from 30 December 2014), 220-420 MHz (from 1 June 2012 to 23 October 2012 and from 05 October 2013), 905-1730 MHz (from 30 December 2019). The three receivers are fed respectively by a dipole, log-periodic and cross-dipole antenna. Nominally, frequency spectra are obtained with 4 sweeps per second over in total 600 channels. Here, we describe the Trieste CALLISTO station set-up, the local e-Callisto network digital archive, Trieste CALLISTO Radio Bursts Detection and Visualization System available via web and present dynamic spectra of a sample of Type I, II, III, IV and V radio bursts. As an additional feature, we show also its capability to record lightning strikes.
Ndacyayisenga, T.; Umuhire, A. C.; Uwamahoro, J.; Monstein, C.
Space weather study through analysis of solar radio bursts detected by a single-station CALLISTO spectrometer Journal Article
In: Annales Geophysicae, vol. 39, no. 5, pp. 945–959, 2021.
@article{angeo-39-945-2021,
title = {Space weather study through analysis of solar radio bursts detected by a single-station CALLISTO spectrometer},
author = {T. Ndacyayisenga and A. C. Umuhire and J. Uwamahoro and C. Monstein},
url = {https://angeo.copernicus.org/articles/39/945/2021/},
doi = {10.5194/angeo-39-945-2021},
year = {2021},
date = {2021-10-29},
urldate = {2021-01-01},
journal = {Annales Geophysicae},
volume = {39},
number = {5},
pages = {945–959},
abstract = {This article summarises the results of an analysis of solar radio bursts (SRBs) detected by the Compound Astronomical Low-cost Low-frequency Instrument for Spectroscopy and Transportable Observatory (CALLISTO) spectrometer hosted by the University of Rwanda. The data analysed were detected during the first year (2014–2015) of the instrument operation. Using quick plots provided by the e-CALLISTO website, a total of 201 intense and well-separated solar radio bursts detected by the CALLISTO station located in Rwanda, are found consisting of 4 type II, 175 type III and 22 type IV radio bursts. It is found that all analysed type II and ∼ 37 % of type III bursts are associated with impulsive solar flares, while the minority (∼ 13 %) of type IV radio bursts are associated with solar flares. Furthermore, all type II radio bursts are associated with coronal mass ejections (CMEs), ∼ 44 % of type III bursts are associated with CMEs, and the majority (∼ 82 %) of type IV bursts were accompanied by CMEs. With aid of the atmospheric imaging assembly (AIA) images on board the Solar Dynamics Observatory (SDO), the location of open magnetic field lines of non-flare-associated type III radio bursts are shown. The same images are used to show the magnetic loops in the solar corona for type IV radio bursts observed in the absence of solar flares and/or CMEs. Findings from this study indicate that analysis of SRBs that are observed from the ground can provide a significant contribution to the early diagnosis of solar transients phenomena, such as solar flares and CMEs, which are major drivers of potential space weather hazards.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
This article summarises the results of an analysis of solar radio bursts (SRBs) detected by the Compound Astronomical Low-cost Low-frequency Instrument for Spectroscopy and Transportable Observatory (CALLISTO) spectrometer hosted by the University of Rwanda. The data analysed were detected during the first year (2014–2015) of the instrument operation. Using quick plots provided by the e-CALLISTO website, a total of 201 intense and well-separated solar radio bursts detected by the CALLISTO station located in Rwanda, are found consisting of 4 type II, 175 type III and 22 type IV radio bursts. It is found that all analysed type II and ∼ 37 % of type III bursts are associated with impulsive solar flares, while the minority (∼ 13 %) of type IV radio bursts are associated with solar flares. Furthermore, all type II radio bursts are associated with coronal mass ejections (CMEs), ∼ 44 % of type III bursts are associated with CMEs, and the majority (∼ 82 %) of type IV bursts were accompanied by CMEs. With aid of the atmospheric imaging assembly (AIA) images on board the Solar Dynamics Observatory (SDO), the location of open magnetic field lines of non-flare-associated type III radio bursts are shown. The same images are used to show the magnetic loops in the solar corona for type IV radio bursts observed in the absence of solar flares and/or CMEs. Findings from this study indicate that analysis of SRBs that are observed from the ground can provide a significant contribution to the early diagnosis of solar transients phenomena, such as solar flares and CMEs, which are major drivers of potential space weather hazards.
Clette, Frédéric; Lefèvre, Laure; Bechet, Sabrina; Ramelli, Renzo; Cagnotti, Marco
Reconstruction of the Sunspot Number Source Database and the 1947 Zurich Discontinuity Journal Article
In: Solar Physics, vol. 296, pp. 137, 2021.
@article{article,
title = {Reconstruction of the Sunspot Number Source Database and the 1947 Zurich Discontinuity},
author = {Frédéric Clette and Laure Lefèvre and Sabrina Bechet and Renzo Ramelli and Marco Cagnotti},
doi = {10.1007/s11207-021-01882-6},
year = {2021},
date = {2021-09-15},
urldate = {2021-01-01},
journal = {Solar Physics},
volume = {296},
pages = {137},
abstract = {The recalibration of the sunspot number series, the primary long-term record of the solar cycle, requires the recovery of the entire collection of raw sunspot counts collected by the Zurich Observatory for the production of this index between 1849 and 1980.
Here, we report about the major progresses accomplished recently in the construction of this global digital sunspot number database, and we derive global statistics of all the individual observers and professional observatories who provided sunspot data over more than 130 years.
First, we can announce the full recovery of long-lost source-data tables covering the last 34 years between 1945 and 1979, and we describe the unique information available in those tables. We then also retrace the evolution of the core observing team in Zurich and of the auxiliary stations. In 1947, we find a major disruption in the composition of both the Zurich team and the international network of auxiliary stations.
This sharp transition is unique in the history of the Zurich Observatory and coincides with the main scale-jump found in the original Zurich sunspot number series, the so-called “Waldmeier” jump. This adds key historical evidence explaining why methodological changes introduced progressively in the early 20th century could play a role precisely at that time. We conclude on the remaining steps needed to fully complete this new sunspot data resource.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The recalibration of the sunspot number series, the primary long-term record of the solar cycle, requires the recovery of the entire collection of raw sunspot counts collected by the Zurich Observatory for the production of this index between 1849 and 1980.
Here, we report about the major progresses accomplished recently in the construction of this global digital sunspot number database, and we derive global statistics of all the individual observers and professional observatories who provided sunspot data over more than 130 years.
First, we can announce the full recovery of long-lost source-data tables covering the last 34 years between 1945 and 1979, and we describe the unique information available in those tables. We then also retrace the evolution of the core observing team in Zurich and of the auxiliary stations. In 1947, we find a major disruption in the composition of both the Zurich team and the international network of auxiliary stations.
This sharp transition is unique in the history of the Zurich Observatory and coincides with the main scale-jump found in the original Zurich sunspot number series, the so-called “Waldmeier” jump. This adds key historical evidence explaining why methodological changes introduced progressively in the early 20th century could play a role precisely at that time. We conclude on the remaining steps needed to fully complete this new sunspot data resource.
Here, we report about the major progresses accomplished recently in the construction of this global digital sunspot number database, and we derive global statistics of all the individual observers and professional observatories who provided sunspot data over more than 130 years.
First, we can announce the full recovery of long-lost source-data tables covering the last 34 years between 1945 and 1979, and we describe the unique information available in those tables. We then also retrace the evolution of the core observing team in Zurich and of the auxiliary stations. In 1947, we find a major disruption in the composition of both the Zurich team and the international network of auxiliary stations.
This sharp transition is unique in the history of the Zurich Observatory and coincides with the main scale-jump found in the original Zurich sunspot number series, the so-called “Waldmeier” jump. This adds key historical evidence explaining why methodological changes introduced progressively in the early 20th century could play a role precisely at that time. We conclude on the remaining steps needed to fully complete this new sunspot data resource.
Lightner, Carin R.; Gisler, Daniel; Meyer, Stefan A.; Niese, Hannah; Keitel, Robert C.; Norris, David J.
Measurement of Raman Optical Activity with High-Frequency Polarization Modulation Journal Article
In: Journal of Physical Chemistry A, vol. 125, no. 36, pp. 8132-8139, 2021.
@article{2021JPCA..125.8132L,
title = {Measurement of Raman Optical Activity with High-Frequency Polarization Modulation},
author = {Carin R. Lightner and Daniel Gisler and Stefan A. Meyer and Hannah Niese and Robert C. Keitel and David J. Norris},
doi = {10.1021/acs.jpca.1c06132},
year = {2021},
date = {2021-09-01},
urldate = {2021-09-01},
journal = {Journal of Physical Chemistry A},
volume = {125},
number = {36},
pages = {8132-8139},
abstract = {Many chiroptical spectroscopic techniques have been developed to detect chirality in molecular species and probe its role in biological processes. Raman optical activity (ROA) should be one of the most powerful methods, as ROA yields vibrational and chirality information simultaneously and can measure analytes in aqueous and biologically relevant solvents. However, despite its promise, the use of ROA has been limited, largely due to challenges in instrumentation. Here, we report a new approach to ROA that exploits high-frequency polarization modulation. High-frequency polarization modulation, usually implemented with a photoelastic modulator (PEM), has long been the standard technique in other chiroptical spectroscopies. Unfortunately, the need for simultaneous spectral and polarization resolution has precluded the use of PEMs in ROA instruments. We combine a specialized camera system (the Zurich imaging polarimeter, or ZIMPOL) with PEM modulation to perform ROA measurements. We demonstrate performance similar to the current standard in ROA instrumentation while reducing complexity and polarization artifacts. This development should aid researchers in exploiting the full potential of ROA for chemical and biological analysis.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Many chiroptical spectroscopic techniques have been developed to detect chirality in molecular species and probe its role in biological processes. Raman optical activity (ROA) should be one of the most powerful methods, as ROA yields vibrational and chirality information simultaneously and can measure analytes in aqueous and biologically relevant solvents. However, despite its promise, the use of ROA has been limited, largely due to challenges in instrumentation. Here, we report a new approach to ROA that exploits high-frequency polarization modulation. High-frequency polarization modulation, usually implemented with a photoelastic modulator (PEM), has long been the standard technique in other chiroptical spectroscopies. Unfortunately, the need for simultaneous spectral and polarization resolution has precluded the use of PEMs in ROA instruments. We combine a specialized camera system (the Zurich imaging polarimeter, or ZIMPOL) with PEM modulation to perform ROA measurements. We demonstrate performance similar to the current standard in ROA instrumentation while reducing complexity and polarization artifacts. This development should aid researchers in exploiting the full potential of ROA for chemical and biological analysis.
Ernest, Alsina Ballester; Belluzzi, Luca; Javier, Trujillo Bueno
Solving the Paradox of the Solar Sodium D$_1$ Line Polarization Journal Article
In: Physical Review Letters, vol. 127, no. 8, pp. 081101, 2021.
@article{2021PhRvL.127h1101A,
title = {Solving the Paradox of the Solar Sodium D$_1$ Line Polarization},
author = {Ernest, Alsina Ballester and Luca Belluzzi and Javier, Trujillo Bueno},
doi = {10.1103/PhysRevLett.127.081101},
year = {2021},
date = {2021-08-01},
urldate = {2021-08-01},
journal = {Physical Review Letters},
volume = {127},
number = {8},
pages = {081101},
abstract = {Twenty-five years ago, enigmatic linear polarization signals were
discovered in the core of the sodium D$_1$ line. The only
explanation that could be found implied that the solar
chromosphere is practically unmagnetized, in contradiction with
other evidences. This opened a paradox that has challenged
physicists for many years. Here we present its solution,
demonstrating that these polarization signals can be properly
explained in the presence of magnetic fields in the gauss range.
This result opens a novel diagnostic window for exploring the
elusive magnetism of the solar chromosphere.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Twenty-five years ago, enigmatic linear polarization signals were
discovered in the core of the sodium D$_1$ line. The only
explanation that could be found implied that the solar
chromosphere is practically unmagnetized, in contradiction with
other evidences. This opened a paradox that has challenged
physicists for many years. Here we present its solution,
demonstrating that these polarization signals can be properly
explained in the presence of magnetic fields in the gauss range.
This result opens a novel diagnostic window for exploring the
elusive magnetism of the solar chromosphere.
discovered in the core of the sodium D$_1$ line. The only
explanation that could be found implied that the solar
chromosphere is practically unmagnetized, in contradiction with
other evidences. This opened a paradox that has challenged
physicists for many years. Here we present its solution,
demonstrating that these polarization signals can be properly
explained in the presence of magnetic fields in the gauss range.
This result opens a novel diagnostic window for exploring the
elusive magnetism of the solar chromosphere.
Peter, H.; E., Alsina Ballester; Andretta, V.; Auch`ere, F.; Belluzzi, L.; Bemporad, A.; Berghmans, D.; Buchlin, E.; Calcines, A.; Chitta, L. P.; Dalmasse, K.; del Pino Alemán, T.; Feller, A.; Froment, C.; Harrison, R.; Janvier, M.; Matthews, S.; Parenti, S.; Przybylski, D.; Solanki, S. K.; Štěpán, J.; Teriaca, L.; Bueno, J. Trujillo
Magnetic imaging of the outer solar atmosphere (MImOSA) Journal Article
In: Experimental Astronomy, 2021.
@article{2021ExA...tmp...95P,
title = {Magnetic imaging of the outer solar atmosphere (MImOSA)},
author = {H. Peter and E., Alsina Ballester and V. Andretta and F. Auch`ere and L. Belluzzi and A. Bemporad and D. Berghmans and E. Buchlin and A. Calcines and L. P. Chitta and K. Dalmasse and T. del Pino Alemán and A. Feller and C. Froment and R. Harrison and M. Janvier and S. Matthews and S. Parenti and D. Przybylski and S. K. Solanki and J. Štěpán and L. Teriaca and J. Trujillo Bueno},
doi = {10.1007/s10686-021-09774-0},
year = {2021},
date = {2021-08-01},
urldate = {2021-08-01},
journal = {Experimental Astronomy},
abstract = {The magnetic activity of the Sun directly impacts the Earth and human
life. Likewise, other stars will have an impact on the
habitability of planets orbiting these host stars. Although the
magnetic field at the surface of the Sun is reasonably well
characterised by observations, the information on the magnetic
field in the higher atmospheric layers is mainly indirect. This
lack of information hampers our progress in understanding solar
magnetic activity. Overcoming this limitation would allow us to
address four paramount long-standing questions: (1) How does the
magnetic field couple the different layers of the atmosphere,
and how does it transport energy? (2) How does the magnetic
field structure, drive and interact with the plasma in the
chromosphere and upper atmosphere? (3) How does the magnetic
field destabilise the outer solar atmosphere and thus affect the
interplanetary environment? (4) How do magnetic processes
accelerate particles to high energies? New ground-breaking
observations are needed to address these science questions. We
suggest a suite of three instruments that far exceed current
capabilities in terms of spatial resolution, light-gathering
power, and polarimetric performance: (a) A large-aperture UV-to-
IR telescope of the 1-3 m class aimed mainly to measure the
magnetic field in the chromosphere by combining high spatial
resolution and high sensitivity. (b) An extreme-UV-to-IR
coronagraph that is designed to measure the large-scale magnetic
field in the corona with an aperture of about 40 cm. (c) An
extreme-UV imaging polarimeter based on a 30 cm telescope that
combines high throughput in the extreme UV with polarimetry to
connect the magnetic measurements of the other two instruments.
Placed in a near-Earth orbit, the data downlink would be
maximised, while a location at L4 or L5 would provide
stereoscopic observations of the Sun in combination with Earth-
based observatories. This mission to measure the magnetic field
will finally unlock the driver of the dynamics in the outer
solar atmosphere and thereby will greatly advance our
understanding of the Sun and the heliosphere.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The magnetic activity of the Sun directly impacts the Earth and human
life. Likewise, other stars will have an impact on the
habitability of planets orbiting these host stars. Although the
magnetic field at the surface of the Sun is reasonably well
characterised by observations, the information on the magnetic
field in the higher atmospheric layers is mainly indirect. This
lack of information hampers our progress in understanding solar
magnetic activity. Overcoming this limitation would allow us to
address four paramount long-standing questions: (1) How does the
magnetic field couple the different layers of the atmosphere,
and how does it transport energy? (2) How does the magnetic
field structure, drive and interact with the plasma in the
chromosphere and upper atmosphere? (3) How does the magnetic
field destabilise the outer solar atmosphere and thus affect the
interplanetary environment? (4) How do magnetic processes
accelerate particles to high energies? New ground-breaking
observations are needed to address these science questions. We
suggest a suite of three instruments that far exceed current
capabilities in terms of spatial resolution, light-gathering
power, and polarimetric performance: (a) A large-aperture UV-to-
IR telescope of the 1-3 m class aimed mainly to measure the
magnetic field in the chromosphere by combining high spatial
resolution and high sensitivity. (b) An extreme-UV-to-IR
coronagraph that is designed to measure the large-scale magnetic
field in the corona with an aperture of about 40 cm. (c) An
extreme-UV imaging polarimeter based on a 30 cm telescope that
combines high throughput in the extreme UV with polarimetry to
connect the magnetic measurements of the other two instruments.
Placed in a near-Earth orbit, the data downlink would be
maximised, while a location at L4 or L5 would provide
stereoscopic observations of the Sun in combination with Earth-
based observatories. This mission to measure the magnetic field
will finally unlock the driver of the dynamics in the outer
solar atmosphere and thereby will greatly advance our
understanding of the Sun and the heliosphere.
life. Likewise, other stars will have an impact on the
habitability of planets orbiting these host stars. Although the
magnetic field at the surface of the Sun is reasonably well
characterised by observations, the information on the magnetic
field in the higher atmospheric layers is mainly indirect. This
lack of information hampers our progress in understanding solar
magnetic activity. Overcoming this limitation would allow us to
address four paramount long-standing questions: (1) How does the
magnetic field couple the different layers of the atmosphere,
and how does it transport energy? (2) How does the magnetic
field structure, drive and interact with the plasma in the
chromosphere and upper atmosphere? (3) How does the magnetic
field destabilise the outer solar atmosphere and thus affect the
interplanetary environment? (4) How do magnetic processes
accelerate particles to high energies? New ground-breaking
observations are needed to address these science questions. We
suggest a suite of three instruments that far exceed current
capabilities in terms of spatial resolution, light-gathering
power, and polarimetric performance: (a) A large-aperture UV-to-
IR telescope of the 1-3 m class aimed mainly to measure the
magnetic field in the chromosphere by combining high spatial
resolution and high sensitivity. (b) An extreme-UV-to-IR
coronagraph that is designed to measure the large-scale magnetic
field in the corona with an aperture of about 40 cm. (c) An
extreme-UV imaging polarimeter based on a 30 cm telescope that
combines high throughput in the extreme UV with polarimetry to
connect the magnetic measurements of the other two instruments.
Placed in a near-Earth orbit, the data downlink would be
maximised, while a location at L4 or L5 would provide
stereoscopic observations of the Sun in combination with Earth-
based observatories. This mission to measure the magnetic field
will finally unlock the driver of the dynamics in the outer
solar atmosphere and thereby will greatly advance our
understanding of the Sun and the heliosphere.
Kharayat, Hema; Joshi, Bhuwan; Mitra, Prabir K; Manoharan, P K; Monstein, Christian
A Transient Coronal Sigmoid in Active Region NOAA 11909: Build-up Phase, M-class Eruptive Flare, and Associated Fast Coronal Mass Ejection Journal Article
In: Solar Physics, vol. 296, pp. 99, 2021.
@article{2021arXiv210500411K,
title = {A Transient Coronal Sigmoid in Active Region NOAA 11909: Build-up Phase, M-class Eruptive Flare, and Associated Fast Coronal Mass Ejection},
author = {Hema {Kharayat} and Bhuwan {Joshi} and Prabir K {Mitra} and P ~K {Manoharan} and Christian {Monstein}},
url = {https://ui.adsabs.harvard.edu/link_gateway/2021arXiv210500411K/EPRINT_PDF},
doi = {10.1007/s11207-021-01830-4},
year = {2021},
date = {2021-06-22},
journal = {Solar Physics},
volume = {296},
pages = {99},
abstract = {In this article, we investigate the formation and disruption of a
coronal sigmoid from the active region (AR) NOAA 11909 on 07
December 2013, by analyzing multi-wavelength and multi-
instrument observations. Our analysis suggests that the
formation of `transient' sigmoid initiated $approx$1 hour
before its eruption through a coupling between two twisted
coronal loop systems. A comparison between coronal and
photospheric images suggests that the coronal sigmoid was formed
over a simple $beta$-type AR which also possessed dispersed
magnetic field structure in the photosphere. The line-of-sight
photospheric magnetograms also reveal moving magnetic features,
small-scale flux cancellation events near the PIL, and overall
flux cancellation during the extended pre-eruption phase which
suggest the role of tether-cutting reconnection toward the
build-up of the flux rope. The disruption of the sigmoid
proceeded with a two-ribbon eruptive M1.2 flare
(SOL2013-12-07T07:29). In radio frequencies, we observe type III
and type II bursts in meter wavelengths during the impulsive
phase of the flare. The successful eruption of the flux rope
leads to a fast coronal mass ejection (with a linear speed of
$approx$1085 km s -1 ) in SOHO/LASCO field-of-view. During the
evolution of the flare, we clearly observe typical ``sigmoid-to-
arcade'' transformation. Prior to the onset of the impulsive
phase of the flare, flux rope undergoes a slow rise ($approx$15
km s -1 ) which subsequently transitions into a fast eruption
($approx$110 km s -1 ). The two-phase evolution of the flux
rope shows temporal associations with the soft X-ray precursor
and impulsive phase emissions of the M-class flare,
respectively, thus pointing toward a feedback relationship
between magnetic reconnection and early CME dynamics.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
In this article, we investigate the formation and disruption of a
coronal sigmoid from the active region (AR) NOAA 11909 on 07
December 2013, by analyzing multi-wavelength and multi-
instrument observations. Our analysis suggests that the
formation of `transient' sigmoid initiated $approx$1 hour
before its eruption through a coupling between two twisted
coronal loop systems. A comparison between coronal and
photospheric images suggests that the coronal sigmoid was formed
over a simple $beta$-type AR which also possessed dispersed
magnetic field structure in the photosphere. The line-of-sight
photospheric magnetograms also reveal moving magnetic features,
small-scale flux cancellation events near the PIL, and overall
flux cancellation during the extended pre-eruption phase which
suggest the role of tether-cutting reconnection toward the
build-up of the flux rope. The disruption of the sigmoid
proceeded with a two-ribbon eruptive M1.2 flare
(SOL2013-12-07T07:29). In radio frequencies, we observe type III
and type II bursts in meter wavelengths during the impulsive
phase of the flare. The successful eruption of the flux rope
leads to a fast coronal mass ejection (with a linear speed of
$approx$1085 km s -1 ) in SOHO/LASCO field-of-view. During the
evolution of the flare, we clearly observe typical ``sigmoid-to-
arcade'' transformation. Prior to the onset of the impulsive
phase of the flare, flux rope undergoes a slow rise ($approx$15
km s -1 ) which subsequently transitions into a fast eruption
($approx$110 km s -1 ). The two-phase evolution of the flux
rope shows temporal associations with the soft X-ray precursor
and impulsive phase emissions of the M-class flare,
respectively, thus pointing toward a feedback relationship
between magnetic reconnection and early CME dynamics.
coronal sigmoid from the active region (AR) NOAA 11909 on 07
December 2013, by analyzing multi-wavelength and multi-
instrument observations. Our analysis suggests that the
formation of `transient' sigmoid initiated $approx$1 hour
before its eruption through a coupling between two twisted
coronal loop systems. A comparison between coronal and
photospheric images suggests that the coronal sigmoid was formed
over a simple $beta$-type AR which also possessed dispersed
magnetic field structure in the photosphere. The line-of-sight
photospheric magnetograms also reveal moving magnetic features,
small-scale flux cancellation events near the PIL, and overall
flux cancellation during the extended pre-eruption phase which
suggest the role of tether-cutting reconnection toward the
build-up of the flux rope. The disruption of the sigmoid
proceeded with a two-ribbon eruptive M1.2 flare
(SOL2013-12-07T07:29). In radio frequencies, we observe type III
and type II bursts in meter wavelengths during the impulsive
phase of the flare. The successful eruption of the flux rope
leads to a fast coronal mass ejection (with a linear speed of
$approx$1085 km s -1 ) in SOHO/LASCO field-of-view. During the
evolution of the flare, we clearly observe typical ``sigmoid-to-
arcade'' transformation. Prior to the onset of the impulsive
phase of the flare, flux rope undergoes a slow rise ($approx$15
km s -1 ) which subsequently transitions into a fast eruption
($approx$110 km s -1 ). The two-phase evolution of the flux
rope shows temporal associations with the soft X-ray precursor
and impulsive phase emissions of the M-class flare,
respectively, thus pointing toward a feedback relationship
between magnetic reconnection and early CME dynamics.
Joshi, Bhuwan; Mitra, Prabir K; Bhattacharyya, R; Upadhyay, Kushagra; Oberoi, Divya; Raja, K Sasikumar; Monstein, Christian
In: Solar Physics, vol. 296, no. 6, pp. 85, 2021.
@article{2021SoPh..296...85J,
title = {Two-Stage Evolution of an Extended C-Class Eruptive Flaring Activity from Sigmoid Active Region NOAA 12734: SDO and Udaipur-CALLISTO Observations},
author = {Bhuwan {Joshi} and Prabir K {Mitra} and R {Bhattacharyya} and Kushagra {Upadhyay} and Divya {Oberoi} and K {Sasikumar Raja} and Christian {Monstein}},
doi = {10.1007/s11207-021-01820-6},
year = {2021},
date = {2021-06-03},
journal = {Solar Physics},
volume = {296},
number = {6},
pages = {85},
abstract = {In this article, we present a multi-wavelength investigation of a
C-class flaring activity that occurred in the active region NOAA
12734 on 8 March 2019. The investigation utilizes data from the
Atmospheric Imaging Assembly (AIA) and the Helioseismic Magnetic
Imager (HMI) on board the Solar Dynamics Observatory (SDO) and
the Udaipur-CALLISTO solar radio spectrograph of the Physical
Research Laboratory. This low intensity C1.3 event is
characterized by typical features of a long-duration event
(LDE), viz. extended flare arcade, large-scale two-ribbon
structures and twin coronal dimmings. The eruptive event
occurred in a coronal sigmoid and displayed two distinct stages
of energy release, manifested in terms of temporal and spatial
evolution. The formation of twin-dimming regions are consistent
with the eruption of a large flux rope with footpoints lying in
the western and eastern edges of the coronal sigmoid. The metric
radio observations obtained from Udaipur-CALLISTO reveals a
broad-band (ensuremathapprox50 -180 MHz), stationary
plasma emission for ensuremathapprox7 min during the
second stage of the flaring activity that resemble a type IV
radio burst. A type III decametre-hectometre radio bursts with
starting frequency of ensuremathapprox2.5 MHz precedes the
stationary type IV burst observed by Udaipur-CALLISTO by
ensuremathapprox5 min. The synthesis of multi-wavelength
observations and non-linear force-free field (NLFFF) coronal
modeling together with magnetic decay index analysis suggest
that the sigmoid flux rope underwent a zipping-like uprooting
from its western to eastern footpoints in response to the
overlying asymmetric magnetic field confinement. The
asymmetrical eruption of the flux rope also accounts for the
observed large-scale structures viz. apparent eastward shift of
flare ribbons and post-flare loops along the polarity inversion
line (PIL), and provides evidence for lateral progression of
magnetic reconnection site as the eruption proceeds.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
In this article, we present a multi-wavelength investigation of a
C-class flaring activity that occurred in the active region NOAA
12734 on 8 March 2019. The investigation utilizes data from the
Atmospheric Imaging Assembly (AIA) and the Helioseismic Magnetic
Imager (HMI) on board the Solar Dynamics Observatory (SDO) and
the Udaipur-CALLISTO solar radio spectrograph of the Physical
Research Laboratory. This low intensity C1.3 event is
characterized by typical features of a long-duration event
(LDE), viz. extended flare arcade, large-scale two-ribbon
structures and twin coronal dimmings. The eruptive event
occurred in a coronal sigmoid and displayed two distinct stages
of energy release, manifested in terms of temporal and spatial
evolution. The formation of twin-dimming regions are consistent
with the eruption of a large flux rope with footpoints lying in
the western and eastern edges of the coronal sigmoid. The metric
radio observations obtained from Udaipur-CALLISTO reveals a
broad-band (ensuremathapprox50 -180 MHz), stationary
plasma emission for ensuremathapprox7 min during the
second stage of the flaring activity that resemble a type IV
radio burst. A type III decametre-hectometre radio bursts with
starting frequency of ensuremathapprox2.5 MHz precedes the
stationary type IV burst observed by Udaipur-CALLISTO by
ensuremathapprox5 min. The synthesis of multi-wavelength
observations and non-linear force-free field (NLFFF) coronal
modeling together with magnetic decay index analysis suggest
that the sigmoid flux rope underwent a zipping-like uprooting
from its western to eastern footpoints in response to the
overlying asymmetric magnetic field confinement. The
asymmetrical eruption of the flux rope also accounts for the
observed large-scale structures viz. apparent eastward shift of
flare ribbons and post-flare loops along the polarity inversion
line (PIL), and provides evidence for lateral progression of
magnetic reconnection site as the eruption proceeds.
C-class flaring activity that occurred in the active region NOAA
12734 on 8 March 2019. The investigation utilizes data from the
Atmospheric Imaging Assembly (AIA) and the Helioseismic Magnetic
Imager (HMI) on board the Solar Dynamics Observatory (SDO) and
the Udaipur-CALLISTO solar radio spectrograph of the Physical
Research Laboratory. This low intensity C1.3 event is
characterized by typical features of a long-duration event
(LDE), viz. extended flare arcade, large-scale two-ribbon
structures and twin coronal dimmings. The eruptive event
occurred in a coronal sigmoid and displayed two distinct stages
of energy release, manifested in terms of temporal and spatial
evolution. The formation of twin-dimming regions are consistent
with the eruption of a large flux rope with footpoints lying in
the western and eastern edges of the coronal sigmoid. The metric
radio observations obtained from Udaipur-CALLISTO reveals a
broad-band (ensuremathapprox50 -180 MHz), stationary
plasma emission for ensuremathapprox7 min during the
second stage of the flaring activity that resemble a type IV
radio burst. A type III decametre-hectometre radio bursts with
starting frequency of ensuremathapprox2.5 MHz precedes the
stationary type IV burst observed by Udaipur-CALLISTO by
ensuremathapprox5 min. The synthesis of multi-wavelength
observations and non-linear force-free field (NLFFF) coronal
modeling together with magnetic decay index analysis suggest
that the sigmoid flux rope underwent a zipping-like uprooting
from its western to eastern footpoints in response to the
overlying asymmetric magnetic field confinement. The
asymmetrical eruption of the flux rope also accounts for the
observed large-scale structures viz. apparent eastward shift of
flare ribbons and post-flare loops along the polarity inversion
line (PIL), and provides evidence for lateral progression of
magnetic reconnection site as the eruption proceeds.
Kouloumvakos, Athanasios; Rouillard, Alexis; Warmuth, Alexander; Magdalenic, Jasmina; Jebaraj, Immanuel. C; Mann, Gottfried; Vainio, Rami; Monstein, Christian
Coronal Conditions for the Occurrence of Type II Radio Bursts Journal Article
In: Astrophysical Journal, vol. 913, no. 2, pp. 99, 2021.
@article{2021ApJ...913...99K,
title = {Coronal Conditions for the Occurrence of Type II Radio Bursts},
author = {Athanasios {Kouloumvakos} and Alexis {Rouillard} and Alexander {Warmuth} and Jasmina {Magdalenic} and Immanuel. C {Jebaraj} and Gottfried {Mann} and Rami {Vainio} and Christian {Monstein}},
doi = {10.3847/1538-4357/abf435},
year = {2021},
date = {2021-05-28},
journal = {Astrophysical Journal},
volume = {913},
number = {2},
pages = {99},
abstract = {Type II radio bursts are generally observed in association with flare-
generated or coronal-mass-ejection-driven shock waves. The exact
shock and coronal conditions necessary for the production of
type II radio emission are still under debate. Shock waves are
important for the acceleration of electrons necessary for the
generation of the radio emission. Additionally, the shock
geometry and closed field line topology, e.g., quasi-
perpendicular shock regions or shocks interacting with
streamers, play an important role for the production of the
emission. In this study we perform a 3D reconstruction and
modeling of a shock wave observed during the 2014 November 5
solar event. We determine the spatial and temporal evolution of
the shock properties and examine the conditions responsible for
the generation and evolution of type II radio emission. Our
results suggest that the formation and evolution of a strong,
supercritical, quasi-perpendicular shock wave interacting with a
coronal streamer were responsible for producing type II radio
emission. We find that the shock wave is subcritical before and
supercritical after the start of the type II emission. The shock
geometry is mostly quasi-perpendicular throughout the event. Our
analysis shows that the radio emission is produced in regions
where the supercritical shock develops with an oblique to quasi-
perpendicular geometry.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Type II radio bursts are generally observed in association with flare-
generated or coronal-mass-ejection-driven shock waves. The exact
shock and coronal conditions necessary for the production of
type II radio emission are still under debate. Shock waves are
important for the acceleration of electrons necessary for the
generation of the radio emission. Additionally, the shock
geometry and closed field line topology, e.g., quasi-
perpendicular shock regions or shocks interacting with
streamers, play an important role for the production of the
emission. In this study we perform a 3D reconstruction and
modeling of a shock wave observed during the 2014 November 5
solar event. We determine the spatial and temporal evolution of
the shock properties and examine the conditions responsible for
the generation and evolution of type II radio emission. Our
results suggest that the formation and evolution of a strong,
supercritical, quasi-perpendicular shock wave interacting with a
coronal streamer were responsible for producing type II radio
emission. We find that the shock wave is subcritical before and
supercritical after the start of the type II emission. The shock
geometry is mostly quasi-perpendicular throughout the event. Our
analysis shows that the radio emission is produced in regions
where the supercritical shock develops with an oblique to quasi-
perpendicular geometry.
generated or coronal-mass-ejection-driven shock waves. The exact
shock and coronal conditions necessary for the production of
type II radio emission are still under debate. Shock waves are
important for the acceleration of electrons necessary for the
generation of the radio emission. Additionally, the shock
geometry and closed field line topology, e.g., quasi-
perpendicular shock regions or shocks interacting with
streamers, play an important role for the production of the
emission. In this study we perform a 3D reconstruction and
modeling of a shock wave observed during the 2014 November 5
solar event. We determine the spatial and temporal evolution of
the shock properties and examine the conditions responsible for
the generation and evolution of type II radio emission. Our
results suggest that the formation and evolution of a strong,
supercritical, quasi-perpendicular shock wave interacting with a
coronal streamer were responsible for producing type II radio
emission. We find that the shock wave is subcritical before and
supercritical after the start of the type II emission. The shock
geometry is mostly quasi-perpendicular throughout the event. Our
analysis shows that the radio emission is produced in regions
where the supercritical shock develops with an oblique to quasi-
perpendicular geometry.
Battaglia, Andrea Francesco; Cuissa, José Roberto Canivete; Calvo, Flavio; Bossart, Aleksi Antoine; Steiner, Oskar
The Alfvénic nature of chromospheric swirls Journal Article
In: A&A, vol. 649, pp. A121, 2021.
@article{refId0j,
title = {The Alfvénic nature of chromospheric swirls},
author = {Andrea Francesco {Battaglia} and José Roberto {Canivete Cuissa} and Flavio {Calvo} and Aleksi Antoine {Bossart} and Oskar {Steiner}},
url = {https://doi.org/10.1051/0004-6361/202040110
https://arxiv.org/abs/2103.07366},
doi = {10.1051/0004-6361/202040110},
year = {2021},
date = {2021-05-26},
journal = {A&A},
volume = {649},
pages = {A121},
abstract = {Context. Observations show that small-scale vortical plasma motions are ubiquitous in the quiet solar atmosphere. They have received increasing attention in recent years because they are a viable candidate mechanism for the heating of the outer solar atmospheric layers. However, the true nature and the origin of these swirls, and their effective role in the energy transport, are still unclear.
Aims. We investigate the evolution and origin of chromospheric swirls by analyzing numerical simulations of the quiet solar atmosphere. In particular, we are interested in finding their relation with magnetic field perturbations and in the processes driving their evolution.
Methods. The radiative magnetohydrodynamic code CO5BOLD is used to perform realistic numerical simulations of a small portion of the solar atmosphere, ranging from the top layers of the convection zone to the middle chromosphere. For the analysis, the swirling strength criterion and its evolution equation are applied in order to identify vortical motions and to study their dynamics. As a new criterion, we introduce the magnetic swirling strength, which allows us to recognize torsional perturbations in the magnetic field.
Results. We find a strong correlation between swirling strength and magnetic swirling strength, in particular in intense magnetic flux concentrations, which suggests a tight relation between vortical motions and torsional magnetic field perturbations. Furthermore, we find that swirls propagate upward with the local Alfvén speed as unidirectional swirls driven by magnetic tension forces alone. In the photosphere and low chromosphere, the rotation of the plasma co-occurs with a twist in the upwardly directed magnetic field that is in the opposite direction of the plasma flow. All together, these are clear characteristics of torsional Alfvén waves. Yet, the Alfvén wave is not oscillatory but takes the form of a unidirectional pulse. The novelty of the present work is that these Alfvén pulses naturally emerge from realistic numerical simulations of the solar atmosphere. We also find indications of an imbalance between the hydrodynamic and magnetohydrodynamic baroclinic effects being at the origin of the swirls. At the base of the chromosphere, we find a mean net upwardly directed Poynting flux of 12.8 ± 6.5 kW m−2, which is mainly due to swirling motions. This energy flux is mostly associated with large and complex swirling structures, which we interpret as the superposition of various small-scale vortices.
Conclusions. We conclude that the ubiquitous swirling events observed in numerical simulations are tightly correlated with perturbations of the magnetic field. At photospheric and chromospheric levels, they form Alfvén pulses that propagate upward and may contribute to chromospheric heating.
},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Context. Observations show that small-scale vortical plasma motions are ubiquitous in the quiet solar atmosphere. They have received increasing attention in recent years because they are a viable candidate mechanism for the heating of the outer solar atmospheric layers. However, the true nature and the origin of these swirls, and their effective role in the energy transport, are still unclear.
Aims. We investigate the evolution and origin of chromospheric swirls by analyzing numerical simulations of the quiet solar atmosphere. In particular, we are interested in finding their relation with magnetic field perturbations and in the processes driving their evolution.
Methods. The radiative magnetohydrodynamic code CO5BOLD is used to perform realistic numerical simulations of a small portion of the solar atmosphere, ranging from the top layers of the convection zone to the middle chromosphere. For the analysis, the swirling strength criterion and its evolution equation are applied in order to identify vortical motions and to study their dynamics. As a new criterion, we introduce the magnetic swirling strength, which allows us to recognize torsional perturbations in the magnetic field.
Results. We find a strong correlation between swirling strength and magnetic swirling strength, in particular in intense magnetic flux concentrations, which suggests a tight relation between vortical motions and torsional magnetic field perturbations. Furthermore, we find that swirls propagate upward with the local Alfvén speed as unidirectional swirls driven by magnetic tension forces alone. In the photosphere and low chromosphere, the rotation of the plasma co-occurs with a twist in the upwardly directed magnetic field that is in the opposite direction of the plasma flow. All together, these are clear characteristics of torsional Alfvén waves. Yet, the Alfvén wave is not oscillatory but takes the form of a unidirectional pulse. The novelty of the present work is that these Alfvén pulses naturally emerge from realistic numerical simulations of the solar atmosphere. We also find indications of an imbalance between the hydrodynamic and magnetohydrodynamic baroclinic effects being at the origin of the swirls. At the base of the chromosphere, we find a mean net upwardly directed Poynting flux of 12.8 ± 6.5 kW m−2, which is mainly due to swirling motions. This energy flux is mostly associated with large and complex swirling structures, which we interpret as the superposition of various small-scale vortices.
Conclusions. We conclude that the ubiquitous swirling events observed in numerical simulations are tightly correlated with perturbations of the magnetic field. At photospheric and chromospheric levels, they form Alfvén pulses that propagate upward and may contribute to chromospheric heating.
Aims. We investigate the evolution and origin of chromospheric swirls by analyzing numerical simulations of the quiet solar atmosphere. In particular, we are interested in finding their relation with magnetic field perturbations and in the processes driving their evolution.
Methods. The radiative magnetohydrodynamic code CO5BOLD is used to perform realistic numerical simulations of a small portion of the solar atmosphere, ranging from the top layers of the convection zone to the middle chromosphere. For the analysis, the swirling strength criterion and its evolution equation are applied in order to identify vortical motions and to study their dynamics. As a new criterion, we introduce the magnetic swirling strength, which allows us to recognize torsional perturbations in the magnetic field.
Results. We find a strong correlation between swirling strength and magnetic swirling strength, in particular in intense magnetic flux concentrations, which suggests a tight relation between vortical motions and torsional magnetic field perturbations. Furthermore, we find that swirls propagate upward with the local Alfvén speed as unidirectional swirls driven by magnetic tension forces alone. In the photosphere and low chromosphere, the rotation of the plasma co-occurs with a twist in the upwardly directed magnetic field that is in the opposite direction of the plasma flow. All together, these are clear characteristics of torsional Alfvén waves. Yet, the Alfvén wave is not oscillatory but takes the form of a unidirectional pulse. The novelty of the present work is that these Alfvén pulses naturally emerge from realistic numerical simulations of the solar atmosphere. We also find indications of an imbalance between the hydrodynamic and magnetohydrodynamic baroclinic effects being at the origin of the swirls. At the base of the chromosphere, we find a mean net upwardly directed Poynting flux of 12.8 ± 6.5 kW m−2, which is mainly due to swirling motions. This energy flux is mostly associated with large and complex swirling structures, which we interpret as the superposition of various small-scale vortices.
Conclusions. We conclude that the ubiquitous swirling events observed in numerical simulations are tightly correlated with perturbations of the magnetic field. At photospheric and chromospheric levels, they form Alfvén pulses that propagate upward and may contribute to chromospheric heating.
Ndacyayisenga, Theogene; Uwamahoro, Jean; Raja, K Sasikumar; Monstein, Christian
A statistical study of solar radio Type III bursts and space weather implication Journal Article
In: Advances in Space Research, vol. 67, no. 4, pp. 1425-1435, 2021.
@article{2021AdSpR..67.1425N,
title = {A statistical study of solar radio Type III bursts and space weather implication},
author = {Theogene {Ndacyayisenga} and Jean {Uwamahoro} and K {Sasikumar Raja} and Christian {Monstein}},
url = {https://arxiv.org/abs/2012.01210},
doi = {10.1016/j.asr.2020.11.022},
year = {2021},
date = {2021-02-01},
journal = {Advances in Space Research},
volume = {67},
number = {4},
pages = {1425-1435},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Paganini, A; Hashemi, B; Ballester, E Alsina; Belluzzi, L
Fast and accurate approximation of the angle-averaged redistribution function for polarized radiation Journal Article
In: Astronomy and Astrophysics, vol. 645, pp. A4, 2021.
@article{2021A&A...645A...4P,
title = {Fast and accurate approximation of the angle-averaged redistribution function for polarized radiation},
author = {A {Paganini} and B {Hashemi} and E {Alsina Ballester} and L {Belluzzi}},
url = {https://arxiv.org/abs/2010.03508},
doi = {10.1051/0004-6361/201937149},
year = {2021},
date = {2021-01-01},
journal = {Astronomy and Astrophysics},
volume = {645},
pages = {A4},
abstract = {Context. Modeling spectral line profiles taking frequency redistribution
effects into account is a notoriously challenging problem from
the computational point of view, especially when polarization
phenomena (atomic polarization and polarized radiation) are
taken into account. Frequency redistribution effects are
conveniently described through the redistribution function
formalism, and the angle-averaged approximation is often
introduced to simplify the problem. Even in this case, the
evaluation of the emission coefficient for polarized radiation
remains computationally costly, especially when magnetic fields
are present or complex atomic models are considered.
textbackslash Aims: We aim to develop an efficient algorithm to
numerically evaluate the angle-averaged redistribution function
for polarized radiation. textbackslash Methods: The proposed
approach is based on a low-rank approximation via trivariate
polynomials whose univariate components are represented in the
Chebyshev basis. textbackslash Results: The resulting algorithm
is significantly faster than standard quadrature-based schemes
for any target accuracy in the range [10$^-6$, 10$^-2$].},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Context. Modeling spectral line profiles taking frequency redistribution
effects into account is a notoriously challenging problem from
the computational point of view, especially when polarization
phenomena (atomic polarization and polarized radiation) are
taken into account. Frequency redistribution effects are
conveniently described through the redistribution function
formalism, and the angle-averaged approximation is often
introduced to simplify the problem. Even in this case, the
evaluation of the emission coefficient for polarized radiation
remains computationally costly, especially when magnetic fields
are present or complex atomic models are considered.
textbackslash Aims: We aim to develop an efficient algorithm to
numerically evaluate the angle-averaged redistribution function
for polarized radiation. textbackslash Methods: The proposed
approach is based on a low-rank approximation via trivariate
polynomials whose univariate components are represented in the
Chebyshev basis. textbackslash Results: The resulting algorithm
is significantly faster than standard quadrature-based schemes
for any target accuracy in the range [10$^-6$, 10$^-2$].
effects into account is a notoriously challenging problem from
the computational point of view, especially when polarization
phenomena (atomic polarization and polarized radiation) are
taken into account. Frequency redistribution effects are
conveniently described through the redistribution function
formalism, and the angle-averaged approximation is often
introduced to simplify the problem. Even in this case, the
evaluation of the emission coefficient for polarized radiation
remains computationally costly, especially when magnetic fields
are present or complex atomic models are considered.
textbackslash Aims: We aim to develop an efficient algorithm to
numerically evaluate the angle-averaged redistribution function
for polarized radiation. textbackslash Methods: The proposed
approach is based on a low-rank approximation via trivariate
polynomials whose univariate components are represented in the
Chebyshev basis. textbackslash Results: The resulting algorithm
is significantly faster than standard quadrature-based schemes
for any target accuracy in the range [10$^-6$, 10$^-2$].
2020
R, Di Campli; Ramelli, R; Bianda, M; Furno, I; S, Kumar Dhara; Belluzzi, L
Imaging spectropolarimetry for magnetic field diagnostics in solar prominences Journal Article
In: A&A, vol. 644, pp. A89, 2020.
@article{refId0c,
title = {Imaging spectropolarimetry for magnetic field diagnostics in solar prominences},
author = {R, {Di Campli} and R Ramelli and M Bianda and I Furno and S, Kumar Dhara and L Belluzzi},
url = {https://doi.org/10.1051/0004-6361/202037931},
doi = {10.1051/0004-6361/202037931},
year = {2020},
date = {2020-12-01},
urldate = {2020-12-01},
journal = {A&A},
volume = {644},
pages = {A89},
abstract = {Context. Narrowband imaging spectropolarimetry is one of the most powerful tools available to infer information about the intensity and topology of the magnetic fields present in extended plasma structures in the solar atmosphere.
Aims. We describe the instrumental set-up and the observing procedure that we have developed and optimized at the Istituto Ricerche Solari Locarno in order to perform imaging spectropolarimetry. A measurement that highlights the potential of the ensuing observations for magnetic field diagnostics in solar prominences is presented.
Methods. Monochromatic images of solar prominences were obtained by combining a tunable narrowband filter, based on two Fabry-Perot etalons, with a Czerny-Turner spectrograph. Linear and circular polarization were measured at every pixel of the monochromatic image with the Zurich Imaging Polarimeter, ZIMPOL. A wavelength scan was performed across the profile of the considered spectral line. The HAZEL inversion code was applied to the observed Stokes profiles to infer a series of physical properties of the observed structure.
Results. We carried out a spectropolarimetric observation of a prominence, consisting of a set of quasi-monochromatic images across the He I D3 line at 5876 Å in the four Stokes parameters. The map of observed Stokes profiles was inverted with HAZEL, finding magnetic fields with intensities between 15 and 30 G and directed along the spine of the prominence, which is in agreement with the results of previous works.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Context. Narrowband imaging spectropolarimetry is one of the most powerful tools available to infer information about the intensity and topology of the magnetic fields present in extended plasma structures in the solar atmosphere.
Aims. We describe the instrumental set-up and the observing procedure that we have developed and optimized at the Istituto Ricerche Solari Locarno in order to perform imaging spectropolarimetry. A measurement that highlights the potential of the ensuing observations for magnetic field diagnostics in solar prominences is presented.
Methods. Monochromatic images of solar prominences were obtained by combining a tunable narrowband filter, based on two Fabry-Perot etalons, with a Czerny-Turner spectrograph. Linear and circular polarization were measured at every pixel of the monochromatic image with the Zurich Imaging Polarimeter, ZIMPOL. A wavelength scan was performed across the profile of the considered spectral line. The HAZEL inversion code was applied to the observed Stokes profiles to infer a series of physical properties of the observed structure.
Results. We carried out a spectropolarimetric observation of a prominence, consisting of a set of quasi-monochromatic images across the He I D3 line at 5876 Å in the four Stokes parameters. The map of observed Stokes profiles was inverted with HAZEL, finding magnetic fields with intensities between 15 and 30 G and directed along the spine of the prominence, which is in agreement with the results of previous works.
Aims. We describe the instrumental set-up and the observing procedure that we have developed and optimized at the Istituto Ricerche Solari Locarno in order to perform imaging spectropolarimetry. A measurement that highlights the potential of the ensuing observations for magnetic field diagnostics in solar prominences is presented.
Methods. Monochromatic images of solar prominences were obtained by combining a tunable narrowband filter, based on two Fabry-Perot etalons, with a Czerny-Turner spectrograph. Linear and circular polarization were measured at every pixel of the monochromatic image with the Zurich Imaging Polarimeter, ZIMPOL. A wavelength scan was performed across the profile of the considered spectral line. The HAZEL inversion code was applied to the observed Stokes profiles to infer a series of physical properties of the observed structure.
Results. We carried out a spectropolarimetric observation of a prominence, consisting of a set of quasi-monochromatic images across the He I D3 line at 5876 Å in the four Stokes parameters. The map of observed Stokes profiles was inverted with HAZEL, finding magnetic fields with intensities between 15 and 30 G and directed along the spine of the prominence, which is in agreement with the results of previous works.
Mahender, Aroori; Raja, K Sasikumar; Ramesh, R; Panditi, Vemareddy; Monstein, Christian; Ganji, Yellaiah
A Statistical Study of Low-Frequency Solar Radio Type III Bursts Journal Article
In: Solar Physics, vol. 295, no. 11, pp. 153, 2020, ISSN: 1573-093X.
@article{Mahender2020,
title = {A Statistical Study of Low-Frequency Solar Radio Type III Bursts},
author = {Aroori Mahender and K {Sasikumar Raja} and R Ramesh and Vemareddy Panditi and Christian Monstein and Yellaiah Ganji},
doi = {10.1007/s11207-020-01722-z},
issn = {1573-093X},
year = {2020},
date = {2020-11-09},
journal = {Solar Physics},
volume = {295},
number = {11},
pages = {153},
abstract = {We have studied low-frequency (45 – 410 MHz) type III solar radio bursts observed using the e-Compound Astronomical Low-cost Low-frequency Instrument for Spectroscopy and Transportable Observatory (e-CALLISTO) spectrometer located at Gauribidanur Radio Observatory, India, during 2013 – 2017. After inspecting 1531 type III bursts we found that 426 bursts were associated with flares, while the others might have been triggered by small scale features/weak energy events present in the solar corona. In this study, we have carried out a statistical analysis of various observational parameters like start time, lower- and upper-frequency cut-offs of type III bursts and their association with flares, variation of such parameters with flare parameters such as location, class, onset, and peak times. From this study, we found that most of the high frequency bursts (whose upper-frequency cut-off is >350MHz$>350~mboxMHz$) originate from western longitudes. We interpret that this could be due to the fact that Parker spirals from these longitudes are directed towards the Earth and high frequency bursts are more directive. Further we report that the number of bursts that reach Earth from western longitudes is higher than from eastern longitudes.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We have studied low-frequency (45 – 410 MHz) type III solar radio bursts observed using the e-Compound Astronomical Low-cost Low-frequency Instrument for Spectroscopy and Transportable Observatory (e-CALLISTO) spectrometer located at Gauribidanur Radio Observatory, India, during 2013 – 2017. After inspecting 1531 type III bursts we found that 426 bursts were associated with flares, while the others might have been triggered by small scale features/weak energy events present in the solar corona. In this study, we have carried out a statistical analysis of various observational parameters like start time, lower- and upper-frequency cut-offs of type III bursts and their association with flares, variation of such parameters with flare parameters such as location, class, onset, and peak times. From this study, we found that most of the high frequency bursts (whose upper-frequency cut-off is >350MHz$>350~mboxMHz$) originate from western longitudes. We interpret that this could be due to the fact that Parker spirals from these longitudes are directed towards the Earth and high frequency bursts are more directive. Further we report that the number of bursts that reach Earth from western longitudes is higher than from eastern longitudes.
Fischer, C E; Vigeesh, G; Lindner, P; Borrero, J M; Calvo, F; Steiner, O
Interaction of Magnetic Fields with a Vortex Tube at Solar Subgranular Scale Journal Article
In: Astrophysical Journal, Letters, vol. 903, no. 1, pp. L10, 2020.
@article{2020ApJ...903L..10F,
title = {Interaction of Magnetic Fields with a Vortex Tube at Solar Subgranular Scale},
author = {C ~E {Fischer} and G {Vigeesh} and P {Lindner} and J ~M {Borrero} and F {Calvo} and O {Steiner}},
url = {https://arxiv.org/abs/2010.05577},
doi = {10.3847/2041-8213/abbada},
year = {2020},
date = {2020-11-01},
journal = {Astrophysical Journal, Letters},
volume = {903},
number = {1},
pages = {L10},
abstract = {Using high-resolution spectropolarimetric data recorded with the Swedish
1 m Solar Telescope, we have identified several instances of
granular lanes traveling into granules. These are believed to be
the observational signature of underlying tubes of vortical flow
with their axis oriented parallel to the solar surface.
Associated with these horizontal vortex tubes, we detect in some
cases a significant signal in linear polarization, located at
the trailing dark edge of the granular lane. The linear
polarization appears at a later stage of the granular lane
development, and is flanked by patches of circular polarization.
Stokes inversions show that the elongated patch of linear
polarization signal arises from the horizontal magnetic field
aligned with the granular lane. We analyze snapshots of a
magnetohydrodynamic numerical simulation and find cases in which
the horizontal vortex tube of the granular lane redistributes
and transports the magnetic field to the solar surface causing a
polarimetric signature similar to what is observed. We thus
witness a mechanism capable of transporting magnetic flux to the
solar surface within granules. This mechanism is probably an
important component of the small-scale dynamo supposedly acting
at the solar surface and generating the quiet-Sun magnetic
field.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Using high-resolution spectropolarimetric data recorded with the Swedish
1 m Solar Telescope, we have identified several instances of
granular lanes traveling into granules. These are believed to be
the observational signature of underlying tubes of vortical flow
with their axis oriented parallel to the solar surface.
Associated with these horizontal vortex tubes, we detect in some
cases a significant signal in linear polarization, located at
the trailing dark edge of the granular lane. The linear
polarization appears at a later stage of the granular lane
development, and is flanked by patches of circular polarization.
Stokes inversions show that the elongated patch of linear
polarization signal arises from the horizontal magnetic field
aligned with the granular lane. We analyze snapshots of a
magnetohydrodynamic numerical simulation and find cases in which
the horizontal vortex tube of the granular lane redistributes
and transports the magnetic field to the solar surface causing a
polarimetric signature similar to what is observed. We thus
witness a mechanism capable of transporting magnetic flux to the
solar surface within granules. This mechanism is probably an
important component of the small-scale dynamo supposedly acting
at the solar surface and generating the quiet-Sun magnetic
field.
1 m Solar Telescope, we have identified several instances of
granular lanes traveling into granules. These are believed to be
the observational signature of underlying tubes of vortical flow
with their axis oriented parallel to the solar surface.
Associated with these horizontal vortex tubes, we detect in some
cases a significant signal in linear polarization, located at
the trailing dark edge of the granular lane. The linear
polarization appears at a later stage of the granular lane
development, and is flanked by patches of circular polarization.
Stokes inversions show that the elongated patch of linear
polarization signal arises from the horizontal magnetic field
aligned with the granular lane. We analyze snapshots of a
magnetohydrodynamic numerical simulation and find cases in which
the horizontal vortex tube of the granular lane redistributes
and transports the magnetic field to the solar surface causing a
polarimetric signature similar to what is observed. We thus
witness a mechanism capable of transporting magnetic flux to the
solar surface within granules. This mechanism is probably an
important component of the small-scale dynamo supposedly acting
at the solar surface and generating the quiet-Sun magnetic
field.
Stenflo, Jan O
Cosmological constant caused by observer-induced boundary condition Journal Article
In: Journal of Physics Communications, vol. 4, no. 10, pp. 105001, 2020.
@article{2020JPhCo...4j5001S,
title = {Cosmological constant caused by observer-induced boundary condition},
author = {Jan O {Stenflo}},
url = {https://arxiv.org/abs/2010.07743},
doi = {10.1088/2399-6528/abbab8},
year = {2020},
date = {2020-10-01},
journal = {Journal of Physics Communications},
volume = {4},
number = {10},
pages = {105001},
abstract = {The evolution of the wave function in quantum mechanics is deterministic
like that of classical waves. Only when we bring in observers
the fundamentally different quantum reality emerges. Similarly
the introduction of observers changes the nature of spacetime by
causing a split between past and future, concepts that are not
well defined in the observer-free world. The induced temporal
boundary leads to a resonance condition for the oscillatory
vacuum solutions of the metric in Euclidean time. It corresponds
to an exponential de Sitter evolution in real time, which can be represented by a cosmological constant $rmŁambda =2pi
^2/r_u^2$ , where r$_u$ is the radius of the particle
horizon at the epoch when the observer exists. For the present
epoch we get a value of ensuremathŁambda that agrees with
the observed value within 2ensuremathsigma of the
observational errors. This explanation resolves the cosmic
coincidence problem. Our epoch in cosmic history does not herald
the onset of an inflationary phase driven by some dark energy.
We show that the observed accelerated expansion that is deduced
from the redshifts is an édge effect' due to the observer-
induced boundary and not representative of the intrinsic
evolution. The new theory satisfies the BBN (Big Bang
nucleosynthesis) and CMB (cosmic microwave background)
observational constraints equally well as the concordance model
of standard cosmology. There is no link between the dark energy
and dark matter problems. Previous conclusions that dark matter
is mainly non-baryonic are not affected.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The evolution of the wave function in quantum mechanics is deterministic
like that of classical waves. Only when we bring in observers
the fundamentally different quantum reality emerges. Similarly
the introduction of observers changes the nature of spacetime by
causing a split between past and future, concepts that are not
well defined in the observer-free world. The induced temporal
boundary leads to a resonance condition for the oscillatory
vacuum solutions of the metric in Euclidean time. It corresponds
to an exponential de Sitter evolution in real time, which can be represented by a cosmological constant $rmŁambda =2pi
^2/r_u^2$ , where r$_u$ is the radius of the particle
horizon at the epoch when the observer exists. For the present
epoch we get a value of ensuremathŁambda that agrees with
the observed value within 2ensuremathsigma of the
observational errors. This explanation resolves the cosmic
coincidence problem. Our epoch in cosmic history does not herald
the onset of an inflationary phase driven by some dark energy.
We show that the observed accelerated expansion that is deduced
from the redshifts is an édge effect' due to the observer-
induced boundary and not representative of the intrinsic
evolution. The new theory satisfies the BBN (Big Bang
nucleosynthesis) and CMB (cosmic microwave background)
observational constraints equally well as the concordance model
of standard cosmology. There is no link between the dark energy
and dark matter problems. Previous conclusions that dark matter
is mainly non-baryonic are not affected.
like that of classical waves. Only when we bring in observers
the fundamentally different quantum reality emerges. Similarly
the introduction of observers changes the nature of spacetime by
causing a split between past and future, concepts that are not
well defined in the observer-free world. The induced temporal
boundary leads to a resonance condition for the oscillatory
vacuum solutions of the metric in Euclidean time. It corresponds
to an exponential de Sitter evolution in real time, which can be represented by a cosmological constant $rmŁambda =2pi
^2/r_u^2$ , where r$_u$ is the radius of the particle
horizon at the epoch when the observer exists. For the present
epoch we get a value of ensuremathŁambda that agrees with
the observed value within 2ensuremathsigma of the
observational errors. This explanation resolves the cosmic
coincidence problem. Our epoch in cosmic history does not herald
the onset of an inflationary phase driven by some dark energy.
We show that the observed accelerated expansion that is deduced
from the redshifts is an édge effect' due to the observer-
induced boundary and not representative of the intrinsic
evolution. The new theory satisfies the BBN (Big Bang
nucleosynthesis) and CMB (cosmic microwave background)
observational constraints equally well as the concordance model
of standard cosmology. There is no link between the dark energy
and dark matter problems. Previous conclusions that dark matter
is mainly non-baryonic are not affected.
Baumgartner, Sandra; Bernardini, Mauro; Canivete Cuissa, José R; de Laroussilhe, Hugues; Mitchell, Alison M W; Neuenschwander, Benno A; Saha, Prasenjit; Schaeffer, Timothée; Soyuer, Deniz; Zwick, Lorenz
Towards a polarization prediction for LISA via intensity interferometry Journal Article
In: Monthly Notices of the Royal Astronomical Society, vol. 498, no. 3, pp. 4577-4589, 2020, ISSN: 0035-8711.
@article{10.1093/mnras/staa2638,
title = {Towards a polarization prediction for LISA via intensity interferometry},
author = { Sandra Baumgartner and Mauro Bernardini and José R Canivete Cuissa and Hugues de Laroussilhe and Alison M W Mitchell and Benno A Neuenschwander and Prasenjit Saha and Timothée Schaeffer and Deniz Soyuer and Lorenz Zwick},
url = {https://doi.org/10.1093/mnras/staa2638},
doi = {10.1093/mnras/staa2638},
issn = {0035-8711},
year = {2020},
date = {2020-09-02},
journal = {Monthly Notices of the Royal Astronomical Society},
volume = {498},
number = {3},
pages = {4577-4589},
abstract = {Compact Galactic Binary Systems with orbital periods of a few hours are expected to be detected in gravitational waves (GW) by Laser Interferometer Space Antenna (LISA) or a similar mission. At present, these so-called verification binaries provide predictions for GW frequency and amplitude. A full polarization prediction would provide a new method to calibrate LISA and other GW observatories, but requires resolving the orientation of the binary on the sky, which is not currently possible. We suggest a method to determine the elusive binary orientation and hence predict the GW polarization, using km-scale optical intensity interferometry. The most promising candidate is CD–30° 11223, consisting of a hot helium subdwarf with mB = 12 and a much fainter white dwarf companion, in a nearly edge-on orbit with period 70.5 min. We estimate that the brighter star is tidally stretched by 6 per cent. Resolving the tidal stretching would provide the binary orientation. The resolution needed is far beyond any current instrument, but not beyond current technology. We consider scenarios where an array of telescopes with km-scale baselines and/or the Very Large Telescope (VLT) and Extremely Large Telescope (ELT) are equipped with recently developed kilopixel sub-ns single-photon counters and used for intensity interferometry. We estimate that a team-up of the VLT and ELT could measure the orientation to ±1° at 2σ confidence in 24 h of observation.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Compact Galactic Binary Systems with orbital periods of a few hours are expected to be detected in gravitational waves (GW) by Laser Interferometer Space Antenna (LISA) or a similar mission. At present, these so-called verification binaries provide predictions for GW frequency and amplitude. A full polarization prediction would provide a new method to calibrate LISA and other GW observatories, but requires resolving the orientation of the binary on the sky, which is not currently possible. We suggest a method to determine the elusive binary orientation and hence predict the GW polarization, using km-scale optical intensity interferometry. The most promising candidate is CD–30° 11223, consisting of a hot helium subdwarf with mB = 12 and a much fainter white dwarf companion, in a nearly edge-on orbit with period 70.5 min. We estimate that the brighter star is tidally stretched by 6 per cent. Resolving the tidal stretching would provide the binary orientation. The resolution needed is far beyond any current instrument, but not beyond current technology. We consider scenarios where an array of telescopes with km-scale baselines and/or the Very Large Telescope (VLT) and Extremely Large Telescope (ELT) are equipped with recently developed kilopixel sub-ns single-photon counters and used for intensity interferometry. We estimate that a team-up of the VLT and ELT could measure the orientation to ±1° at 2σ confidence in 24 h of observation.
Capozzi, Emilia; Ernest, Alsina Ballester; Belluzzi, Luca; Bianda, Michele; Sajal, Kumar Dhara; Ramelli, Renzo
Observational indications of magneto-optical effects in the scattering polarization wings of the Ca I 4227 Å line Journal Article
In: Astronomy and Astrophysics, vol. 641, pp. A63, 2020.
@article{2020A&A...641A..63C,
title = {Observational indications of magneto-optical effects in the scattering polarization wings of the Ca I 4227 Å line},
author = { Emilia Capozzi and Ernest, Alsina Ballester and Luca Belluzzi and Michele Bianda and Sajal, Kumar Dhara and Renzo Ramelli},
url = {https://arxiv.org/abs/2006.13653},
doi = {10.1051/0004-6361/202038455},
year = {2020},
date = {2020-09-01},
urldate = {2020-09-01},
journal = {Astronomy and Astrophysics},
volume = {641},
pages = {A63},
abstract = {Context. Several strong resonance lines, such as H I
Ly-ensuremathalpha, Mg II k, Ca II K, and Ca I 4227
Å, are characterized by deep and broad absorption profiles
in the solar intensity spectrum. These resonance lines show
conspicuous linear scattering polarization signals when observed
in quiet regions close to the solar limb. Such signals show a
characteristic triplet-peak structure with a sharp peak in the
line core and extended wing lobes. The line core peak is
sensitive to the presence of magnetic fields through the Hanle
effect, which however is known not to operate in the line wings.
Recent theoretical studies indicate that, contrary to what was
previously believed, the wing linear polarization signals are
also sensitive to the magnetic field through magneto-optical
(MO) effects. textbackslash Aims: We search for observational
indications of this recently discovered physical mechanism in
the scattering polarization wings of the Ca I 4227 Å line.
textbackslash Methods: We performed a series of
spectropolarimetric observations of this line using the Zurich
IMaging POLarimeter camera at the Gregory-Coudé telescope at
Istituto Ricerche Solari Locarno in Switzerland and at the
GREGOR telescope in Tenerife (Spain). textbackslash Results:
Spatial variations of the total linear polarization degree and
linear polarization angle are clearly appreciable in the wings
of the observed line. We provide a detailed discussion of our
observational results, showing that the detected variations
always take place in regions in which longitudinal magnetic
fields are present, thus supporting the theoretical prediction
that they are produced by MO effects.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Context. Several strong resonance lines, such as H I
Ly-ensuremathalpha, Mg II k, Ca II K, and Ca I 4227
Å, are characterized by deep and broad absorption profiles
in the solar intensity spectrum. These resonance lines show
conspicuous linear scattering polarization signals when observed
in quiet regions close to the solar limb. Such signals show a
characteristic triplet-peak structure with a sharp peak in the
line core and extended wing lobes. The line core peak is
sensitive to the presence of magnetic fields through the Hanle
effect, which however is known not to operate in the line wings.
Recent theoretical studies indicate that, contrary to what was
previously believed, the wing linear polarization signals are
also sensitive to the magnetic field through magneto-optical
(MO) effects. textbackslash Aims: We search for observational
indications of this recently discovered physical mechanism in
the scattering polarization wings of the Ca I 4227 Å line.
textbackslash Methods: We performed a series of
spectropolarimetric observations of this line using the Zurich
IMaging POLarimeter camera at the Gregory-Coudé telescope at
Istituto Ricerche Solari Locarno in Switzerland and at the
GREGOR telescope in Tenerife (Spain). textbackslash Results:
Spatial variations of the total linear polarization degree and
linear polarization angle are clearly appreciable in the wings
of the observed line. We provide a detailed discussion of our
observational results, showing that the detected variations
always take place in regions in which longitudinal magnetic
fields are present, thus supporting the theoretical prediction
that they are produced by MO effects.
Ly-ensuremathalpha, Mg II k, Ca II K, and Ca I 4227
Å, are characterized by deep and broad absorption profiles
in the solar intensity spectrum. These resonance lines show
conspicuous linear scattering polarization signals when observed
in quiet regions close to the solar limb. Such signals show a
characteristic triplet-peak structure with a sharp peak in the
line core and extended wing lobes. The line core peak is
sensitive to the presence of magnetic fields through the Hanle
effect, which however is known not to operate in the line wings.
Recent theoretical studies indicate that, contrary to what was
previously believed, the wing linear polarization signals are
also sensitive to the magnetic field through magneto-optical
(MO) effects. textbackslash Aims: We search for observational
indications of this recently discovered physical mechanism in
the scattering polarization wings of the Ca I 4227 Å line.
textbackslash Methods: We performed a series of
spectropolarimetric observations of this line using the Zurich
IMaging POLarimeter camera at the Gregory-Coudé telescope at
Istituto Ricerche Solari Locarno in Switzerland and at the
GREGOR telescope in Tenerife (Spain). textbackslash Results:
Spatial variations of the total linear polarization degree and
linear polarization angle are clearly appreciable in the wings
of the observed line. We provide a detailed discussion of our
observational results, showing that the detected variations
always take place in regions in which longitudinal magnetic
fields are present, thus supporting the theoretical prediction
that they are produced by MO effects.
José R., Canivete Cuissa; Steiner, Oskar
Vortices evolution in the solar atmosphere - A dynamical equation for the swirling strength Journal Article
In: Astronomy and Astrophysics, vol. 639, pp. A118, 2020.
@article{refId0b,
title = {Vortices evolution in the solar atmosphere - A dynamical equation for the swirling strength},
author = {José R., Canivete Cuissa and Oskar Steiner},
url = {https://doi.org/10.1051/0004-6361/202038060},
doi = {10.1051/0004-6361/202038060},
year = {2020},
date = {2020-07-21},
urldate = {2020-07-21},
journal = {Astronomy and Astrophysics},
volume = {639},
pages = {A118},
abstract = {Aims. We study vortex dynamics in the solar atmosphere by employing and deriving the analytical evolution equations of two vortex identification criteria.
Methods. The two criteria used are vorticity and the swirling strength. Vorticity can be biased in the presence of shear flows, but its dynamical equation is well known; the swirling strength is a more precise criterion for the identification of vortical flows, but its evolution equation is not known yet. Therefore, we explore the possibility of deriving a dynamical equation for the swirling strength. We then apply the two equations to analyze radiative magneto-hydrodynamical simulations of the solar atmosphere produced with the CO5BOLD code.
Results. We present a detailed review of the swirling strength criterion and the mathematical derivation of its evolution equation. This equation did not exist in the literature before and it constitutes a novel tool that is suitable for the analysis of a wide range of problems in (magneto-)hydrodynamics. By applying this equation to numerical models, we find that hydrodynamical and magnetic baroclinicities are the driving physical processes responsible for vortex generation in the convection zone and the photosphere. Higher up in the chromosphere, the magnetic terms alone dominate. Moreover, we find that the swirling strength is produced at small scales in a chaotic fashion, especially inside magnetic flux concentrations.
Conclusions. The swirling strength represents an appropriate criterion for the identification of vortices in turbulent flows, such as those in the solar atmosphere. Moreover, its evolution equation, which is derived in this paper, is pivotal for obtaining precise information about the dynamics of these vortices and the physical mechanisms responsible for their production and evolution. Since this equation is available, the swirling strength is now the ideal quantity to study the dynamics of vortices in (magneto-)hydrodynamics.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Aims. We study vortex dynamics in the solar atmosphere by employing and deriving the analytical evolution equations of two vortex identification criteria.
Methods. The two criteria used are vorticity and the swirling strength. Vorticity can be biased in the presence of shear flows, but its dynamical equation is well known; the swirling strength is a more precise criterion for the identification of vortical flows, but its evolution equation is not known yet. Therefore, we explore the possibility of deriving a dynamical equation for the swirling strength. We then apply the two equations to analyze radiative magneto-hydrodynamical simulations of the solar atmosphere produced with the CO5BOLD code.
Results. We present a detailed review of the swirling strength criterion and the mathematical derivation of its evolution equation. This equation did not exist in the literature before and it constitutes a novel tool that is suitable for the analysis of a wide range of problems in (magneto-)hydrodynamics. By applying this equation to numerical models, we find that hydrodynamical and magnetic baroclinicities are the driving physical processes responsible for vortex generation in the convection zone and the photosphere. Higher up in the chromosphere, the magnetic terms alone dominate. Moreover, we find that the swirling strength is produced at small scales in a chaotic fashion, especially inside magnetic flux concentrations.
Conclusions. The swirling strength represents an appropriate criterion for the identification of vortices in turbulent flows, such as those in the solar atmosphere. Moreover, its evolution equation, which is derived in this paper, is pivotal for obtaining precise information about the dynamics of these vortices and the physical mechanisms responsible for their production and evolution. Since this equation is available, the swirling strength is now the ideal quantity to study the dynamics of vortices in (magneto-)hydrodynamics.
Methods. The two criteria used are vorticity and the swirling strength. Vorticity can be biased in the presence of shear flows, but its dynamical equation is well known; the swirling strength is a more precise criterion for the identification of vortical flows, but its evolution equation is not known yet. Therefore, we explore the possibility of deriving a dynamical equation for the swirling strength. We then apply the two equations to analyze radiative magneto-hydrodynamical simulations of the solar atmosphere produced with the CO5BOLD code.
Results. We present a detailed review of the swirling strength criterion and the mathematical derivation of its evolution equation. This equation did not exist in the literature before and it constitutes a novel tool that is suitable for the analysis of a wide range of problems in (magneto-)hydrodynamics. By applying this equation to numerical models, we find that hydrodynamical and magnetic baroclinicities are the driving physical processes responsible for vortex generation in the convection zone and the photosphere. Higher up in the chromosphere, the magnetic terms alone dominate. Moreover, we find that the swirling strength is produced at small scales in a chaotic fashion, especially inside magnetic flux concentrations.
Conclusions. The swirling strength represents an appropriate criterion for the identification of vortices in turbulent flows, such as those in the solar atmosphere. Moreover, its evolution equation, which is derived in this paper, is pivotal for obtaining precise information about the dynamics of these vortices and the physical mechanisms responsible for their production and evolution. Since this equation is available, the swirling strength is now the ideal quantity to study the dynamics of vortices in (magneto-)hydrodynamics.
Nagendra, K N; Sowmya, K; Sampoorna, M; Stenflo, J O; Anusha, L S
In: Astrophysical Journal, vol. 898, no. 1, pp. 49, 2020.
@article{2020ApJ...898...49N,
title = {Importance of Angle-dependent Partial Frequency Redistribution in Hyperfine Structure Transitions Under the Incomplete Paschen-Back Effect Regime},
author = {K N {Nagendra} and K {Sowmya} and M {Sampoorna} and J O {Stenflo} and L S {Anusha}},
url = {https://arxiv.org/abs/2007.04044},
doi = {10.3847/1538-4357/ab9747},
year = {2020},
date = {2020-07-01},
journal = {Astrophysical Journal},
volume = {898},
number = {1},
pages = {49},
abstract = {Angle-frequency coupling in scattering of polarized light on atoms is
represented by the angle-dependent (AD) partial frequency
redistribution (PRD) matrices. There are several lines in the
linearly polarized solar spectrum, for which PRD combined with
quantum interference between hyperfine structure states play a
significant role. Here we present the solution of the polarized
line transfer equation including the AD-PRD matrix for
scattering on a two-level atom with hyperfine structure
splitting and an unpolarized lower level. We account for the
effects of arbitrary magnetic fields (including the incomplete
Paschen-Back effect regime) and elastic collisions. For
exploratory purposes we consider a self-emitting isothermal
planar atmosphere and use atomic parameters that represent an
isolated Na I D$_2$ line. For this case we show that the AD-
PRD effects are significant for field strengths below about 30
G, but that the computationally much less demanding
approximation of angle-averaged PRD may be used for stronger
fields.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Angle-frequency coupling in scattering of polarized light on atoms is
represented by the angle-dependent (AD) partial frequency
redistribution (PRD) matrices. There are several lines in the
linearly polarized solar spectrum, for which PRD combined with
quantum interference between hyperfine structure states play a
significant role. Here we present the solution of the polarized
line transfer equation including the AD-PRD matrix for
scattering on a two-level atom with hyperfine structure
splitting and an unpolarized lower level. We account for the
effects of arbitrary magnetic fields (including the incomplete
Paschen-Back effect regime) and elastic collisions. For
exploratory purposes we consider a self-emitting isothermal
planar atmosphere and use atomic parameters that represent an
isolated Na I D$_2$ line. For this case we show that the AD-
PRD effects are significant for field strengths below about 30
G, but that the computationally much less demanding
approximation of angle-averaged PRD may be used for stronger
fields.
represented by the angle-dependent (AD) partial frequency
redistribution (PRD) matrices. There are several lines in the
linearly polarized solar spectrum, for which PRD combined with
quantum interference between hyperfine structure states play a
significant role. Here we present the solution of the polarized
line transfer equation including the AD-PRD matrix for
scattering on a two-level atom with hyperfine structure
splitting and an unpolarized lower level. We account for the
effects of arbitrary magnetic fields (including the incomplete
Paschen-Back effect regime) and elastic collisions. For
exploratory purposes we consider a self-emitting isothermal
planar atmosphere and use atomic parameters that represent an
isolated Na I D$_2$ line. For this case we show that the AD-
PRD effects are significant for field strengths below about 30
G, but that the computationally much less demanding
approximation of angle-averaged PRD may be used for stronger
fields.
Mokhtar, W Z A Wan; Hamidi, Z S; Abidin, Z Z; Ibrahim, Z A; Monstein, C
Data background levels of the metre and decimetre wavelength observations by E-CALLISTO network: the Gauribidanur and Greenland sites Journal Article
In: Indian Journal of Physics, 2020, ISSN: 0974-9845.
@article{WanMokhtar2020,
title = {Data background levels of the metre and decimetre wavelength observations by E-CALLISTO network: the Gauribidanur and Greenland sites},
author = {W Z A {Wan Mokhtar} and Z S Hamidi and Z Z Abidin and Z A Ibrahim and C Monstein},
doi = {10.1007/s12648-020-01765-9},
issn = {0974-9845},
year = {2020},
date = {2020-06-22},
journal = {Indian Journal of Physics},
abstract = {The instability of the Sun’s magnetic field can ignite many eruptive events on the solar surface, including flares, coronal mass ejections, and prominence eruptions. The inner heliosphere environment is affected, and consequently, these events are said to contribute to the celestial weather change. As one of the many eruptive events, a solar flare is of the most frequent due to the magnetic reconnection process in which the accelerated electrons from the reconnection sites escape into the interplanetary space and cause solar radio bursts type III (SRBT III). When it is observed near the Earth, this SRBT III is in the form of radio dynamics spectrum; thus, monitoring this spectrum is vital to the further analysis of the said SRBT III. In this paper, we investigate the background levels: short and long periods of the CALLISTO instruments from two different stations where for each site, a 10-day background-level observation is randomly selected. For the purpose of this study, the mean differences and coefficient of variation (CV) distributions for every frequency channel are determined where most of the frequency channels have displayed small mean differences between these two background levels: short and long periods and the CV distributions as well. These short-period observations, within 15 min of the background levels, are found significant enough to warrant further analysis of the solar radio bursts detected by the CALLISTO instruments.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The instability of the Sun’s magnetic field can ignite many eruptive events on the solar surface, including flares, coronal mass ejections, and prominence eruptions. The inner heliosphere environment is affected, and consequently, these events are said to contribute to the celestial weather change. As one of the many eruptive events, a solar flare is of the most frequent due to the magnetic reconnection process in which the accelerated electrons from the reconnection sites escape into the interplanetary space and cause solar radio bursts type III (SRBT III). When it is observed near the Earth, this SRBT III is in the form of radio dynamics spectrum; thus, monitoring this spectrum is vital to the further analysis of the said SRBT III. In this paper, we investigate the background levels: short and long periods of the CALLISTO instruments from two different stations where for each site, a 10-day background-level observation is randomly selected. For the purpose of this study, the mean differences and coefficient of variation (CV) distributions for every frequency channel are determined where most of the frequency channels have displayed small mean differences between these two background levels: short and long periods and the CV distributions as well. These short-period observations, within 15 min of the background levels, are found significant enough to warrant further analysis of the solar radio bursts detected by the CALLISTO instruments.
Zeuner, Franziska; Sainz, Rafael Manso; Feller, Alex; van Noort, Michiel; Solanki, Sami K; Iglesias, Francisco A; Reardon, Kevin; Pillet, Valentin Martinez
Solar Disk Center Shows Scattering Polarization in the Sr I 4607 Å Line Journal Article
In: Astrophysical Journal, Letters, vol. 893, no. 2, pp. L44, 2020.
@article{2020ApJ...893L..44Z,
title = {Solar Disk Center Shows Scattering Polarization in the Sr I 4607 Å Line},
author = {Franziska {Zeuner} and Rafael {Manso Sainz} and Alex {Feller} and Michiel {van Noort} and Sami K {Solanki} and Francisco A {Iglesias} and Kevin {Reardon} and Valentin {Martinez Pillet}},
doi = {10.3847/2041-8213/ab86b8},
year = {2020},
date = {2020-04-01},
journal = {Astrophysical Journal, Letters},
volume = {893},
number = {2},
pages = {L44},
abstract = {Magnetic fields in turbulent, convective high-ensuremathbeta
plasma naturally develop highly tangled and complex topologies -
the solar photosphere being the paradigmatic example. These
fields are mostly undetectable by standard diagnostic techniques
with finite spatio-temporal resolution due to cancellations of
Zeeman polarization signals. Observations of resonance
scattering polarization have been considered to overcome these
problems. But up to now, observations of scattering polarization
lack the necessary combination of high sensitivity and high
spatial resolution in order to directly infer the turbulent
magnetic structure at the resolution limit of solar telescopes.
Here, we report the detection of clear spatial structuring of
scattering polarization in a magnetically quiet solar region at
disk center in the Sr I 4607 Å spectral line on granular
scales, confirming theoretical expectations. We find that the
linear polarization presents a strong spatial correlation with
the local quadrupole of the radiation field. The result
indicates that polarization survives the dynamic and turbulent
magnetic environment of the middle photosphere and is thereby
usable for spatially resolved Hanle observations. This is an
important step toward the long-sought goal of directly observing
turbulent solar magnetic fields at the resolution limit and
investigating their spatial structure.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Magnetic fields in turbulent, convective high-ensuremathbeta
plasma naturally develop highly tangled and complex topologies -
the solar photosphere being the paradigmatic example. These
fields are mostly undetectable by standard diagnostic techniques
with finite spatio-temporal resolution due to cancellations of
Zeeman polarization signals. Observations of resonance
scattering polarization have been considered to overcome these
problems. But up to now, observations of scattering polarization
lack the necessary combination of high sensitivity and high
spatial resolution in order to directly infer the turbulent
magnetic structure at the resolution limit of solar telescopes.
Here, we report the detection of clear spatial structuring of
scattering polarization in a magnetically quiet solar region at
disk center in the Sr I 4607 Å spectral line on granular
scales, confirming theoretical expectations. We find that the
linear polarization presents a strong spatial correlation with
the local quadrupole of the radiation field. The result
indicates that polarization survives the dynamic and turbulent
magnetic environment of the middle photosphere and is thereby
usable for spatially resolved Hanle observations. This is an
important step toward the long-sought goal of directly observing
turbulent solar magnetic fields at the resolution limit and
investigating their spatial structure.
plasma naturally develop highly tangled and complex topologies -
the solar photosphere being the paradigmatic example. These
fields are mostly undetectable by standard diagnostic techniques
with finite spatio-temporal resolution due to cancellations of
Zeeman polarization signals. Observations of resonance
scattering polarization have been considered to overcome these
problems. But up to now, observations of scattering polarization
lack the necessary combination of high sensitivity and high
spatial resolution in order to directly infer the turbulent
magnetic structure at the resolution limit of solar telescopes.
Here, we report the detection of clear spatial structuring of
scattering polarization in a magnetically quiet solar region at
disk center in the Sr I 4607 Å spectral line on granular
scales, confirming theoretical expectations. We find that the
linear polarization presents a strong spatial correlation with
the local quadrupole of the radiation field. The result
indicates that polarization survives the dynamic and turbulent
magnetic environment of the middle photosphere and is thereby
usable for spatially resolved Hanle observations. This is an
important step toward the long-sought goal of directly observing
turbulent solar magnetic fields at the resolution limit and
investigating their spatial structure.
Pauzi, F A M; Abidin, Z Z; Guo, S J; Gao, G N; Dong, L; Monstein, C
Investigation into CME Shock Speed Resulting from Type II Solar Radio Bursts Journal Article
In: Solar Physics, vol. 295, no. 3, pp. 42, 2020, ISSN: 1573-093X.
@article{Pauzi2020,
title = {Investigation into CME Shock Speed Resulting from Type II Solar Radio Bursts},
author = {F A M Pauzi and Z Z Abidin and S J Guo and G N Gao and L Dong and C Monstein},
doi = {10.1007/s11207-019-1404-z},
issn = {1573-093X},
year = {2020},
date = {2020-03-18},
journal = {Solar Physics},
volume = {295},
number = {3},
pages = {42},
abstract = {An investigation into Type II solar radio bursts was carried out to understand the frequency gap between fundamental and harmonic emissions of the radio burst. This investigation focused on Type II solar radio bursts with flares and coronal mass ejections by relating the separation between fundamental and harmonic emissions. We used the Compound Astronomical Low-cost Low-frequency Instrument for Spectroscopy and Transportable Spectrometers (CALLISTO) and a newly designed low-frequency antenna array. This article describes the proposed new instrument in terms of its antenna design, the bandpass testing of the antenna, the new system significance in studying Type II solar radio bursts, and its comparison with other leading radio solar monitoring instruments. Upon setting up the new technology, the radio-frequency interference of the observation site at the University of Malaya was shown to emphasize the suitability of the selected site. This article also shows the preliminary results of the proposed new instrument by reporting the detection of a Type III solar radio burst that was confirmed by CALLISTO. Moreover, it also includes the optimal observation design and strategies for future detections.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
An investigation into Type II solar radio bursts was carried out to understand the frequency gap between fundamental and harmonic emissions of the radio burst. This investigation focused on Type II solar radio bursts with flares and coronal mass ejections by relating the separation between fundamental and harmonic emissions. We used the Compound Astronomical Low-cost Low-frequency Instrument for Spectroscopy and Transportable Spectrometers (CALLISTO) and a newly designed low-frequency antenna array. This article describes the proposed new instrument in terms of its antenna design, the bandpass testing of the antenna, the new system significance in studying Type II solar radio bursts, and its comparison with other leading radio solar monitoring instruments. Upon setting up the new technology, the radio-frequency interference of the observation site at the University of Malaya was shown to emphasize the suitability of the selected site. This article also shows the preliminary results of the proposed new instrument by reporting the detection of a Type III solar radio burst that was confirmed by CALLISTO. Moreover, it also includes the optimal observation design and strategies for future detections.
Prieto, Manuel; Gordo, Javier Bussons; Rodríguez-Pacheco, Javier; Martínez, Agustín; Sánchez, Sebastián; Russu, Andrés; Monstein, Christian; Fernández, Rafael
Increase in Interference Levels in the 45 – 870 MHz Band at the Spanish e-CALLISTO Sites over the Years 2012 and 2019 Journal Article
In: Solar Physics, vol. 295, no. 2, pp. 11, 2020, ISSN: 1573-093X.
@article{Prieto2020,
title = {Increase in Interference Levels in the 45 – 870 MHz Band at the Spanish e-CALLISTO Sites over the Years 2012 and 2019},
author = {Manuel Prieto and Javier Bussons Gordo and Javier Rodríguez-Pacheco and Agustín Martínez and Sebastián Sánchez and Andrés Russu and Christian Monstein and Rafael Fernández},
doi = {10.1007/s11207-019-1577-5},
issn = {1573-093X},
year = {2020},
date = {2020-01-15},
journal = {Solar Physics},
volume = {295},
number = {2},
pages = {11},
abstract = {Two sets of radio-frequency interference (RFI) measurements in the 45 – 870 MHz band are compared. The first set was taken in 2012 at various sites in the province of Guadalajara (Spain) as part of a worldwide site-testing campaign for the deployment of an international network of solar radio-spectrometers, the Compound Astronomical Low-cost Low-frequency Instrument for Spectroscopy and Transportable Observatory (e-CALLISTO) array. Peralejos de las Truchas was found to be an ideal location, even for high-sensitivity non-solar observations, with the lowest interference levels ever measured in the framework of e-CALLISTO. The same set of measurements have been repeated seven years later using the same experimental setup at the same locations. The results presented in this article show that the RFI levels after seven years have notably increased, at some places by a factor of two, thereby placing at risk broadband spectroscopic radio-astronomy studies from the ground.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Two sets of radio-frequency interference (RFI) measurements in the 45 – 870 MHz band are compared. The first set was taken in 2012 at various sites in the province of Guadalajara (Spain) as part of a worldwide site-testing campaign for the deployment of an international network of solar radio-spectrometers, the Compound Astronomical Low-cost Low-frequency Instrument for Spectroscopy and Transportable Observatory (e-CALLISTO) array. Peralejos de las Truchas was found to be an ideal location, even for high-sensitivity non-solar observations, with the lowest interference levels ever measured in the framework of e-CALLISTO. The same set of measurements have been repeated seven years later using the same experimental setup at the same locations. The results presented in this article show that the RFI levels after seven years have notably increased, at some places by a factor of two, thereby placing at risk broadband spectroscopic radio-astronomy studies from the ground.
2019
Sampoorna, M.; Nagendra, K. N.; Sowmya, K.; Stenflo, J. O.; Anusha, L. S.
Polarized Line Formation in Arbitrary Strength Magnetic Fields: The Case of a Two-level Atom with Hyperfine Structure Splitting Journal Article
In: Astrophysical Journal, vol. 883, no. 2, pp. 188, 2019.
@article{2019ApJ...883..188S,
title = {Polarized Line Formation in Arbitrary Strength Magnetic Fields: The Case of a Two-level Atom with Hyperfine Structure Splitting},
author = {M. {Sampoorna} and K.N. {Nagendra} and K. {Sowmya} and J. O. {Stenflo} and L.S. {Anusha}},
doi = {10.3847/1538-4357/ab3805},
year = {2019},
date = {2019-09-01},
journal = {Astrophysical Journal},
volume = {883},
number = {2},
pages = {188},
abstract = {Quantum interference effects, together with partial frequency
redistribution (PFR) in line scattering, produce subtle
signatures in the so-called Second Solar Spectrum (the linearly
polarized spectrum of the Sun). These signatures are modified in
the presence of arbitrary strength magnetic fields via the
Hanle, Zeeman, and Paschen─Back effects. In the present paper we
solve the problem of polarized line formation in a magnetized
atmosphere taking into account scattering in a two-level atom
with hyperfine structure splitting together with PFR. To this
end we incorporate the collisionless PFR matrix derived in
Sowmya et al. in the polarized transfer equation. We apply the
scattering expansion method to solve this transfer equation. We
study the combined effects of PFR and the Paschen─Back effect on
polarized line profiles formed in an isothermal one-dimensional
planar atmosphere. For this purpose, we consider the cases of
D$_2$ lines of Li I and Na I.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Quantum interference effects, together with partial frequency
redistribution (PFR) in line scattering, produce subtle
signatures in the so-called Second Solar Spectrum (the linearly
polarized spectrum of the Sun). These signatures are modified in
the presence of arbitrary strength magnetic fields via the
Hanle, Zeeman, and Paschen─Back effects. In the present paper we
solve the problem of polarized line formation in a magnetized
atmosphere taking into account scattering in a two-level atom
with hyperfine structure splitting together with PFR. To this
end we incorporate the collisionless PFR matrix derived in
Sowmya et al. in the polarized transfer equation. We apply the
scattering expansion method to solve this transfer equation. We
study the combined effects of PFR and the Paschen─Back effect on
polarized line profiles formed in an isothermal one-dimensional
planar atmosphere. For this purpose, we consider the cases of
D$_2$ lines of Li I and Na I.
redistribution (PFR) in line scattering, produce subtle
signatures in the so-called Second Solar Spectrum (the linearly
polarized spectrum of the Sun). These signatures are modified in
the presence of arbitrary strength magnetic fields via the
Hanle, Zeeman, and Paschen─Back effects. In the present paper we
solve the problem of polarized line formation in a magnetized
atmosphere taking into account scattering in a two-level atom
with hyperfine structure splitting together with PFR. To this
end we incorporate the collisionless PFR matrix derived in
Sowmya et al. in the polarized transfer equation. We apply the
scattering expansion method to solve this transfer equation. We
study the combined effects of PFR and the Paschen─Back effect on
polarized line profiles formed in an isothermal one-dimensional
planar atmosphere. For this purpose, we consider the cases of
D$_2$ lines of Li I and Na I.
Stenflo, J. O.
Origin of the cosmological constant Journal Article
In: Astrophysics and Space Science, vol. 364, no. 9, pp. 143, 2019.
@article{2019Ap&SS.364..143S,
title = {Origin of the cosmological constant},
author = {J. O. {Stenflo}},
doi = {10.1007/s10509-019-3636-7},
year = {2019},
date = {2019-09-01},
journal = {Astrophysics and Space Science},
volume = {364},
number = {9},
pages = {143},
abstract = {The observed value of the cosmological constant corresponds to a time
scale that is very close to the current conformal age of the
universe. Here we show that this is not a coincidence but is
caused by a periodic boundary condition, which only manifests
itself when the metric is represented in Euclidian spacetime.
The circular property of the metric in Euclidian spacetime
becomes an exponential evolution (de Sitter or
ensuremathŁambda term) in ordinary spacetime. The value of
ensuremathŁambda then gets uniquely linked to the period
in Euclidian conformal time, which corresponds to the conformal
age of the universe. Without the use of any free model
parameters we predict the value of the dimensionless parameter
ensuremathØmega_ensuremathŁambda to be 67.2%,
which is within 2ensuremathsigma of the value derived from
CMB observations.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The observed value of the cosmological constant corresponds to a time
scale that is very close to the current conformal age of the
universe. Here we show that this is not a coincidence but is
caused by a periodic boundary condition, which only manifests
itself when the metric is represented in Euclidian spacetime.
The circular property of the metric in Euclidian spacetime
becomes an exponential evolution (de Sitter or
ensuremathŁambda term) in ordinary spacetime. The value of
ensuremathŁambda then gets uniquely linked to the period
in Euclidian conformal time, which corresponds to the conformal
age of the universe. Without the use of any free model
parameters we predict the value of the dimensionless parameter
ensuremathØmega_ensuremathŁambda to be 67.2%,
which is within 2ensuremathsigma of the value derived from
CMB observations.
scale that is very close to the current conformal age of the
universe. Here we show that this is not a coincidence but is
caused by a periodic boundary condition, which only manifests
itself when the metric is represented in Euclidian spacetime.
The circular property of the metric in Euclidian spacetime
becomes an exponential evolution (de Sitter or
ensuremathŁambda term) in ordinary spacetime. The value of
ensuremathŁambda then gets uniquely linked to the period
in Euclidian conformal time, which corresponds to the conformal
age of the universe. Without the use of any free model
parameters we predict the value of the dimensionless parameter
ensuremathØmega_ensuremathŁambda to be 67.2%,
which is within 2ensuremathsigma of the value derived from
CMB observations.
Singh, Dayal; Raja, K Sasikumar; Subramanian, Prasad; Ramesh, R; Monstein, Christian
Automated Detection of Solar Radio Bursts Using a Statistical Method Journal Article
In: Solar Physics, vol. 294, no. 8, 2019, ISSN: 1573-093X.
@article{Singh_2019,
title = {Automated Detection of Solar Radio Bursts Using a Statistical Method},
author = {Dayal Singh and K Sasikumar Raja and Prasad Subramanian and R Ramesh and Christian Monstein},
url = {http://dx.doi.org/10.1007/s11207-019-1500-0},
doi = {10.1007/s11207-019-1500-0},
issn = {1573-093X},
year = {2019},
date = {2019-08-26},
journal = {Solar Physics},
volume = {294},
number = {8},
publisher = {Springer Science and Business Media LLC},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Dhara, Sajal Kumar; Capozzi, Emilia; Gisler, Daniel; a, Michele Biand; Ramelli, Renzo; Berdyugina, Svetlana; Alsina, Ernest; Belluzzi, Luca
Observations on spatial variations of the Sr I 4607 Å scattering polarization signals at different limb distances with ZIMPOL Journal Article
In: Astronomy and Astrophysics, vol. 630, pp. A67, 2019.
@article{2019A&A...630A..67Db,
title = {Observations on spatial variations of the Sr I 4607 Å scattering polarization signals at different limb distances with ZIMPOL},
author = {Sajal Kumar {Dhara} and Emilia {Capozzi} and Daniel {Gisler} and Michele {Biand a} and Renzo {Ramelli} and Svetlana {Berdyugina} and Ernest {Alsina} and Luca {Belluzzi}},
url = {https://arxiv.org/pdf/1908.03366},
doi = {10.1051/0004-6361/201935768},
year = {2019},
date = {2019-08-01},
journal = {Astronomy and Astrophysics},
volume = {630},
pages = {A67},
abstract = {Context. The Sr I 4607 Å spectral line shows one of the strongest
scattering polarization signals in the visible solar spectrum.
The amplitude of this polarization signal is expected to vary at
granular spatial scales, due to the combined action of the Hanle
effect and the local anisotropy of the radiation field.
Observing these variations would be of great interest because it
would provide precious information on the small-scale activity
of the solar photosphere. At present, few detections of such
spatial variations have been reported. This is due to the
difficulty of these measurements, which require combining high
spatial (̃0.1″), spectral (ensuremathłeq20 mÅ), and
temporal resolution (< 1 min) with increased polarimetric
sensitivity (̃10$^-4$).
Aims: We aim to detect spatial
variations at granular scales of the scattering polarization
peak of the Sr I 4607 Å line at different limb distances,
and to study the correlation with the continuum intensity.
/> Methods: Using the Zurich IMaging POLarimeter (ZIMPOL) system
mounted at the GREGOR telescope and spectrograph in Tenerife,
Spain, we carried out spectro-polarimetric measurements to
obtain the four Stokes parameters in the Sr I line at different limb distances, from ensuremathmu = 0.2 to ensuremathmu = 0.8, on the solar disk.
Results:
Spatial variations of the scattering polarization signal in the
Sr I 4607 Å line, with a spatial resolution of about
0.66″, are clearly observed at every ensuremathmu. The
spatial scale of these variations is comparable to the granular
size. A statistical analysis reveals that the linear scattering
polarization amplitude in this Sr I spectral line is positively
correlated with the intensity in the continuum, corresponding to
the granules, at every ensuremathmu.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Context. The Sr I 4607 Å spectral line shows one of the strongest
scattering polarization signals in the visible solar spectrum.
The amplitude of this polarization signal is expected to vary at
granular spatial scales, due to the combined action of the Hanle
effect and the local anisotropy of the radiation field.
Observing these variations would be of great interest because it
would provide precious information on the small-scale activity
of the solar photosphere. At present, few detections of such
spatial variations have been reported. This is due to the
difficulty of these measurements, which require combining high
spatial (̃0.1″), spectral (ensuremathłeq20 mÅ), and
temporal resolution (< 1 min) with increased polarimetric
sensitivity (̃10$^-4$). <BR /> Aims: We aim to detect spatial
variations at granular scales of the scattering polarization
peak of the Sr I 4607 Å line at different limb distances,
and to study the correlation with the continuum intensity. <BR
/> Methods: Using the Zurich IMaging POLarimeter (ZIMPOL) system
mounted at the GREGOR telescope and spectrograph in Tenerife,
Spain, we carried out spectro-polarimetric measurements to
obtain the four Stokes parameters in the Sr I line at different limb distances, from ensuremathmu = 0.2 to ensuremathmu = 0.8, on the solar disk. <BR /> Results:
Spatial variations of the scattering polarization signal in the
Sr I 4607 Å line, with a spatial resolution of about
0.66″, are clearly observed at every ensuremathmu. The
spatial scale of these variations is comparable to the granular
size. A statistical analysis reveals that the linear scattering
polarization amplitude in this Sr I spectral line is positively
correlated with the intensity in the continuum, corresponding to
the granules, at every ensuremathmu.
scattering polarization signals in the visible solar spectrum.
The amplitude of this polarization signal is expected to vary at
granular spatial scales, due to the combined action of the Hanle
effect and the local anisotropy of the radiation field.
Observing these variations would be of great interest because it
would provide precious information on the small-scale activity
of the solar photosphere. At present, few detections of such
spatial variations have been reported. This is due to the
difficulty of these measurements, which require combining high
spatial (̃0.1″), spectral (ensuremathłeq20 mÅ), and
temporal resolution (< 1 min) with increased polarimetric
sensitivity (̃10$^-4$). <BR /> Aims: We aim to detect spatial
variations at granular scales of the scattering polarization
peak of the Sr I 4607 Å line at different limb distances,
and to study the correlation with the continuum intensity. <BR
/> Methods: Using the Zurich IMaging POLarimeter (ZIMPOL) system
mounted at the GREGOR telescope and spectrograph in Tenerife,
Spain, we carried out spectro-polarimetric measurements to
obtain the four Stokes parameters in the Sr I line at different limb distances, from ensuremathmu = 0.2 to ensuremathmu = 0.8, on the solar disk. <BR /> Results:
Spatial variations of the scattering polarization signal in the
Sr I 4607 Å line, with a spatial resolution of about
0.66″, are clearly observed at every ensuremathmu. The
spatial scale of these variations is comparable to the granular
size. A statistical analysis reveals that the linear scattering
polarization amplitude in this Sr I spectral line is positively
correlated with the intensity in the continuum, corresponding to
the granules, at every ensuremathmu.
E., Alsina Ballester; Belluzzi, L.; J., Trujillo Bueno
Magnetic Sensitivity in the Wing Scattering Polarization Signals of the Hydrogen Lyman-α Line of the Solar Disk Radiation Journal Article
In: Astrophysical Journal, vol. 880, no. 2, pp. 85, 2019.
@article{2019ApJ...880...85A,
title = {Magnetic Sensitivity in the Wing Scattering Polarization Signals of the Hydrogen Lyman-α Line of the Solar Disk Radiation},
author = {E., {Alsina Ballester} and L. {Belluzzi} and J., {Trujillo Bueno}},
url = {https://arxiv.org/pdf/1901.10994},
doi = {10.3847/1538-4357/ab1e41},
year = {2019},
date = {2019-08-01},
urldate = {2019-08-01},
journal = {Astrophysical Journal},
volume = {880},
number = {2},
pages = {85},
abstract = {The linear polarization produced by scattering processes in the hydrogen
Lyman-α line of the solar disk radiation is a
key observable for probing the chromosphere─corona transition
region (TR) and the underlying chromospheric plasma. While the
line-center signal encodes information on the magnetic field and
the three-dimensional structure of the TR, the sizable
scattering polarization signals that the joint action of partial
frequency redistribution and J-state interference produce in the
Lyman-α wings have generally been thought to be
sensitive only to the thermal structure of the solar atmosphere.
Here we show that the wings of the Q/I and U/I scattering
polarization profiles of this line are actually sensitive to the
presence of chromospheric magnetic fields, with strengths
similar to those that produce the Hanle effect in the line core
(i.e., between 5 and 100 G, approximately). In spite of the fact
that the Zeeman splitting induced by such weak fields is very
small compared to the total width of the line, the magneto-
optical effects that couple the transfer equations for Stokes Q
and U are actually able to produce sizable changes in the Q/I
and U/I wings. We find that magnetic fields with longitudinal
components larger than 100 G produce an almost complete
depolarization of the wings of the Lyensuremathalpha Q/I
profiles within a ensuremathpm5 Å spectral range
around the line center, while stronger fields are required for
the U/I wing signals to be depolarized to a similar extent. The
theoretical results presented here further expand the diagnostic
content of the unprecedented spectropolarimetric observations
provided by the Chromospheric Lyman-α Spectro-Polarimeter.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The linear polarization produced by scattering processes in the hydrogen
Lyman-α line of the solar disk radiation is a
key observable for probing the chromosphere─corona transition
region (TR) and the underlying chromospheric plasma. While the
line-center signal encodes information on the magnetic field and
the three-dimensional structure of the TR, the sizable
scattering polarization signals that the joint action of partial
frequency redistribution and J-state interference produce in the
Lyman-α wings have generally been thought to be
sensitive only to the thermal structure of the solar atmosphere.
Here we show that the wings of the Q/I and U/I scattering
polarization profiles of this line are actually sensitive to the
presence of chromospheric magnetic fields, with strengths
similar to those that produce the Hanle effect in the line core
(i.e., between 5 and 100 G, approximately). In spite of the fact
that the Zeeman splitting induced by such weak fields is very
small compared to the total width of the line, the magneto-
optical effects that couple the transfer equations for Stokes Q
and U are actually able to produce sizable changes in the Q/I
and U/I wings. We find that magnetic fields with longitudinal
components larger than 100 G produce an almost complete
depolarization of the wings of the Lyensuremathalpha Q/I
profiles within a ensuremathpm5 Å spectral range
around the line center, while stronger fields are required for
the U/I wing signals to be depolarized to a similar extent. The
theoretical results presented here further expand the diagnostic
content of the unprecedented spectropolarimetric observations
provided by the Chromospheric Lyman-α Spectro-Polarimeter.
Lyman-α line of the solar disk radiation is a
key observable for probing the chromosphere─corona transition
region (TR) and the underlying chromospheric plasma. While the
line-center signal encodes information on the magnetic field and
the three-dimensional structure of the TR, the sizable
scattering polarization signals that the joint action of partial
frequency redistribution and J-state interference produce in the
Lyman-α wings have generally been thought to be
sensitive only to the thermal structure of the solar atmosphere.
Here we show that the wings of the Q/I and U/I scattering
polarization profiles of this line are actually sensitive to the
presence of chromospheric magnetic fields, with strengths
similar to those that produce the Hanle effect in the line core
(i.e., between 5 and 100 G, approximately). In spite of the fact
that the Zeeman splitting induced by such weak fields is very
small compared to the total width of the line, the magneto-
optical effects that couple the transfer equations for Stokes Q
and U are actually able to produce sizable changes in the Q/I
and U/I wings. We find that magnetic fields with longitudinal
components larger than 100 G produce an almost complete
depolarization of the wings of the Lyensuremathalpha Q/I
profiles within a ensuremathpm5 Å spectral range
around the line center, while stronger fields are required for
the U/I wing signals to be depolarized to a similar extent. The
theoretical results presented here further expand the diagnostic
content of the unprecedented spectropolarimetric observations
provided by the Chromospheric Lyman-α Spectro-Polarimeter.
Koval, Artem; Chen, Yao; Tsugawa, Takuya; Otsuka, Yuichi; Shinbori, Atsuki; Nishioka, Michi; Brazhenko, Anatoliy; Stanislavsky, Aleksander; Konovalenko, Aleksander; Zhang, Qing-He; Monstein, Christian; Gorgutsa, Roman
Direct Observations of Traveling Ionospheric Disturbances as Focusers of Solar Radiation: Spectral Caustics Journal Article
In: The Astrophysical Journal, vol. 877, no. 2, pp. 98, 2019.
@article{Koval_2019,
title = {Direct Observations of Traveling Ionospheric Disturbances as Focusers of Solar Radiation: Spectral Caustics},
author = {Artem Koval and Yao Chen and Takuya Tsugawa and Yuichi Otsuka and Atsuki Shinbori and Michi Nishioka and Anatoliy Brazhenko and Aleksander Stanislavsky and Aleksander Konovalenko and Qing-He Zhang and Christian Monstein and Roman Gorgutsa},
url = {https://doi.org/10.3847%2F1538-4357%2Fab1b52},
doi = {10.3847/1538-4357/ab1b52},
year = {2019},
date = {2019-05-01},
journal = {The Astrophysical Journal},
volume = {877},
number = {2},
pages = {98},
publisher = {American Astronomical Society},
abstract = {The solar radiation focusing effect is related to the specific phenomenon of propagation of the Sun-emitted HF and VHF waves through terrestrial ionosphere. This natural effect is observed with ground-based radio instruments running within the 10–200 MHz range as distinctive patterns—the spectral caustics (SCs)—on the solar dynamic spectra. It has been suggested that SCs are associated with medium-scale traveling ionospheric disturbances (MSTIDs). In this paper, we present the first direct observations of SCs induced by MSTIDs, using solar dynamic spectra with SCs obtained by different European radio telescopes on 2014 January 8 and simultaneous two-dimensional detrended total electron content (dTEC) maps over Europe. Spatial examination of dTEC maps as well as precise timing analysis of the maps and the dynamic spectra have been performed. First, we found several pairs of one-to-one (TID-SC) correspondences. The study provides strong observational evidence supporting the suggestion that MSTIDs are the cause of SCs.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The solar radiation focusing effect is related to the specific phenomenon of propagation of the Sun-emitted HF and VHF waves through terrestrial ionosphere. This natural effect is observed with ground-based radio instruments running within the 10–200 MHz range as distinctive patterns—the spectral caustics (SCs)—on the solar dynamic spectra. It has been suggested that SCs are associated with medium-scale traveling ionospheric disturbances (MSTIDs). In this paper, we present the first direct observations of SCs induced by MSTIDs, using solar dynamic spectra with SCs obtained by different European radio telescopes on 2014 January 8 and simultaneous two-dimensional detrended total electron content (dTEC) maps over Europe. Spatial examination of dTEC maps as well as precise timing analysis of the maps and the dynamic spectra have been performed. First, we found several pairs of one-to-one (TID-SC) correspondences. The study provides strong observational evidence supporting the suggestion that MSTIDs are the cause of SCs.
Janett, Gioele; Steiner, Oskar; Ballester, Ernest Alsina; Belluzzi, Luca; Mishra, Siddhartha
A novel fourth-order WENO interpolation technique. A possible new tool designed for radiative transfer Journal Article
In: Astronomy and Astrophysics, vol. 624, pp. A104, 2019.
@article{2019A&A...624A.104J,
title = {A novel fourth-order WENO interpolation technique. A possible new tool designed for radiative transfer},
author = {Gioele {Janett} and Oskar {Steiner} and Ernest {Alsina Ballester} and Luca {Belluzzi} and Siddhartha {Mishra}},
url = {http://www.irsol.usi.ch/wp-content/uploads/2019/10/Janett_aa34761-18.pdf},
doi = {10.1051/0004-6361/201834761},
year = {2019},
date = {2019-04-01},
journal = {Astronomy and Astrophysics},
volume = {624},
pages = {A104},
abstract = {Context. Several numerical problems require the interpolation of
discrete data that present at the same time (i) complex smooth
structures and (ii) various types of discontinuities. The
radiative transfer in solar and stellar atmospheres is a typical
example of such a problem. This calls for high-order well-
behaved techniques that are able to interpolate both smooth and
discontinuous data.
Aims: This article expands on
different nonlinear interpolation techniques capable of
guaranteeing high-order accuracy and handling discontinuities in
an accurate and non-oscillatory fashion. The final aim is to
propose new techniques which could be suitable for applications
in the context of numerical radiative transfer.
Methods:
We have proposed and tested two different techniques.
Essentially non-oscillatory (ENO) techniques generate several
candidate interpolations based on different substencils. The
smoothest candidate interpolation is determined from a measure
for the local smoothness, thereby enabling the essentially non-
oscillatory property. Weighted ENO (WENO) techniques use a
convex combination of all candidate substencils to obtain high-
order accuracy in smooth regions while keeping the essentially
non-oscillatory property. In particular, we have outlined and
tested a novel well-performing fourth-order WENO interpolation
technique for both uniform and nonuniform grids.
Results:
Numerical tests prove that the fourth-order WENO interpolation
guarantees fourth-order accuracy in smooth regions of the
interpolated functions. In the presence of discontinuities, the
fourth-order WENO interpolation enables the non-oscillatory
property, avoiding oscillations. Unlike Bézier and monotonic
high-order Hermite interpolations, it does not degenerate to a
linear interpolation near smooth extrema of the interpolated
function. Conclusion. The novel fourth-order WENO interpolation
guarantees high accuracy in smooth regions, while effectively
handling discontinuities. This interpolation technique might be
particularly suitable for several problems, including a number
of radiative transfer applications such as multidimensional
problems, multigrid methods, and formal solutions.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Context. Several numerical problems require the interpolation of
discrete data that present at the same time (i) complex smooth
structures and (ii) various types of discontinuities. The
radiative transfer in solar and stellar atmospheres is a typical
example of such a problem. This calls for high-order well-
behaved techniques that are able to interpolate both smooth and
discontinuous data. <BR /> Aims: This article expands on
different nonlinear interpolation techniques capable of
guaranteeing high-order accuracy and handling discontinuities in
an accurate and non-oscillatory fashion. The final aim is to
propose new techniques which could be suitable for applications
in the context of numerical radiative transfer. <BR /> Methods:
We have proposed and tested two different techniques.
Essentially non-oscillatory (ENO) techniques generate several
candidate interpolations based on different substencils. The
smoothest candidate interpolation is determined from a measure
for the local smoothness, thereby enabling the essentially non-
oscillatory property. Weighted ENO (WENO) techniques use a
convex combination of all candidate substencils to obtain high-
order accuracy in smooth regions while keeping the essentially
non-oscillatory property. In particular, we have outlined and
tested a novel well-performing fourth-order WENO interpolation
technique for both uniform and nonuniform grids. <BR /> Results:
Numerical tests prove that the fourth-order WENO interpolation
guarantees fourth-order accuracy in smooth regions of the
interpolated functions. In the presence of discontinuities, the
fourth-order WENO interpolation enables the non-oscillatory
property, avoiding oscillations. Unlike Bézier and monotonic
high-order Hermite interpolations, it does not degenerate to a
linear interpolation near smooth extrema of the interpolated
function. Conclusion. The novel fourth-order WENO interpolation
guarantees high accuracy in smooth regions, while effectively
handling discontinuities. This interpolation technique might be
particularly suitable for several problems, including a number
of radiative transfer applications such as multidimensional
problems, multigrid methods, and formal solutions.
discrete data that present at the same time (i) complex smooth
structures and (ii) various types of discontinuities. The
radiative transfer in solar and stellar atmospheres is a typical
example of such a problem. This calls for high-order well-
behaved techniques that are able to interpolate both smooth and
discontinuous data. <BR /> Aims: This article expands on
different nonlinear interpolation techniques capable of
guaranteeing high-order accuracy and handling discontinuities in
an accurate and non-oscillatory fashion. The final aim is to
propose new techniques which could be suitable for applications
in the context of numerical radiative transfer. <BR /> Methods:
We have proposed and tested two different techniques.
Essentially non-oscillatory (ENO) techniques generate several
candidate interpolations based on different substencils. The
smoothest candidate interpolation is determined from a measure
for the local smoothness, thereby enabling the essentially non-
oscillatory property. Weighted ENO (WENO) techniques use a
convex combination of all candidate substencils to obtain high-
order accuracy in smooth regions while keeping the essentially
non-oscillatory property. In particular, we have outlined and
tested a novel well-performing fourth-order WENO interpolation
technique for both uniform and nonuniform grids. <BR /> Results:
Numerical tests prove that the fourth-order WENO interpolation
guarantees fourth-order accuracy in smooth regions of the
interpolated functions. In the presence of discontinuities, the
fourth-order WENO interpolation enables the non-oscillatory
property, avoiding oscillations. Unlike Bézier and monotonic
high-order Hermite interpolations, it does not degenerate to a
linear interpolation near smooth extrema of the interpolated
function. Conclusion. The novel fourth-order WENO interpolation
guarantees high accuracy in smooth regions, while effectively
handling discontinuities. This interpolation technique might be
particularly suitable for several problems, including a number
of radiative transfer applications such as multidimensional
problems, multigrid methods, and formal solutions.
Wiehr, E.; Stellmacher, G.; Bianda, M.
Evidence for the Two-fluid Scenario in Solar Prominences Journal Article
In: Astrophysical Journal, vol. 873, pp. 125, 2019.
@article{2019ApJ...873..125W,
title = {Evidence for the Two-fluid Scenario in Solar Prominences},
author = {E. Wiehr and G. Stellmacher and M. Bianda},
url = {http://www.irsol.usi.ch/wp-content/uploads/2019/03/ApJ_873_125.pdf},
doi = {10.3847/1538-4357/ab04a4},
year = {2019},
date = {2019-03-01},
journal = {Astrophysical Journal},
volume = {873},
pages = {125},
abstract = {This paper presents observational evidence of the different dynamical behavior of neutral and ionized species in solar prominences. The analysis of a time-series of Sr II 4078 Å and Na D spectra in a quiescent prominence yields systematically larger Doppler shifts (line-of-sight velocities) for the ions V_LOS(Sr II) = 1.22 x V_LOS(Na D). Both lines show a 30 minute oscillation of good coherence. Sixteen hours later the same prominence underwent marked morphological changes (with a rising dome), and the Sr II velocity excess dropped to V _LOS(Sr II) = 1.11 x V_LOS(Na D). The same excess is found for the line pair Fe II 5018 Å and He I 5015 Å. The widths of the ionic lines, mainly non-thermally broadened, are not related to the macro-velocities. The emission ratio of Na D and Sr II, a measure of the electron density, yields n_e = 4 x 10^10 cm^‑3, shows no relation with the V_LOS variation or with height above the limb, and seems to be reduced 16 hr later during the active phase. We apply a new wavelength reference from aureola spectra, which is independent of photospheric velocity fields. },
keywords = {},
pubstate = {published},
tppubtype = {article}
}
This paper presents observational evidence of the different dynamical behavior of neutral and ionized species in solar prominences. The analysis of a time-series of Sr II 4078 Å and Na D spectra in a quiescent prominence yields systematically larger Doppler shifts (line-of-sight velocities) for the ions V_LOS(Sr II) = 1.22 x V_LOS(Na D). Both lines show a 30 minute oscillation of good coherence. Sixteen hours later the same prominence underwent marked morphological changes (with a rising dome), and the Sr II velocity excess dropped to V _LOS(Sr II) = 1.11 x V_LOS(Na D). The same excess is found for the line pair Fe II 5018 Å and He I 5015 Å. The widths of the ionic lines, mainly non-thermally broadened, are not related to the macro-velocities. The emission ratio of Na D and Sr II, a measure of the electron density, yields n_e = 4 x 10^10 cm^‑3, shows no relation with the V_LOS variation or with height above the limb, and seems to be reduced 16 hr later during the active phase. We apply a new wavelength reference from aureola spectra, which is independent of photospheric velocity fields.
Vigeesh, G; Roth, M; Steiner, O; Jackiewicz, J
Internal Gravity Waves in the Magnetized Solar Atmosphere. II. Energy Transport Journal Article
In: Astrophysical Journal, vol. 872, no. 2, pp. 166, 2019.
@article{2019ApJ...872..166V,
title = {Internal Gravity Waves in the Magnetized Solar Atmosphere. II. Energy Transport},
author = {G {Vigeesh} and M {Roth} and O {Steiner} and J {Jackiewicz}},
url = {https://arxiv.org/pdf/1901.08871},
doi = {10.3847/1538-4357/ab020c},
year = {2019},
date = {2019-02-01},
journal = {Astrophysical Journal},
volume = {872},
number = {2},
pages = {166},
abstract = {In this second paper of the series on internal gravity waves (IGWs), we
present a study of the generation and propagation of IGWs in a
model solar atmosphere with diverse magnetic conditions. A
magnetic field-free and three magnetic models that start with an
initial, vertical, homogeneous field of 10, 50, and 100 G
magnetic flux density, are simulated using the CO$^5$BOLD
code. We find that the IGWs are generated in similar manner in
all four models in spite of the differences in the magnetic
environment. The mechanical energy carried by IGWs is
significantly larger than that of the acoustic waves in the
lower part of the atmosphere, making them an important component
of the total wave energy budget. The mechanical energy flux
(10$^6$-10$^3$ W m$^-2$) is a few orders of magnitude
larger than the Poynting flux (10$^3$-10$^1$ W
m$^-2$). The Poynting fluxes show a downward component in the
frequency range corresponding to the IGWs, which confirm that
these waves do not propagate upward in the atmosphere when the
fields are predominantly vertical and strong. We conclude that,
in the upper photosphere, the propagation properties of IGWs
depend on the average magnetic field strength and therefore
these waves can be potential candidates for magnetic field
diagnostics of these layers. However, their subsequent coupling
to Alfvénic waves is unlikely in a magnetic environment
permeated with predominantly vertical fields, and therefore they
may not directly or indirectly contribute to the heating of
layers above plasma-ensuremathbeta less than 1.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
In this second paper of the series on internal gravity waves (IGWs), we
present a study of the generation and propagation of IGWs in a
model solar atmosphere with diverse magnetic conditions. A
magnetic field-free and three magnetic models that start with an
initial, vertical, homogeneous field of 10, 50, and 100 G
magnetic flux density, are simulated using the CO$^5$BOLD
code. We find that the IGWs are generated in similar manner in
all four models in spite of the differences in the magnetic
environment. The mechanical energy carried by IGWs is
significantly larger than that of the acoustic waves in the
lower part of the atmosphere, making them an important component
of the total wave energy budget. The mechanical energy flux
(10$^6$-10$^3$ W m$^-2$) is a few orders of magnitude
larger than the Poynting flux (10$^3$-10$^1$ W
m$^-2$). The Poynting fluxes show a downward component in the
frequency range corresponding to the IGWs, which confirm that
these waves do not propagate upward in the atmosphere when the
fields are predominantly vertical and strong. We conclude that,
in the upper photosphere, the propagation properties of IGWs
depend on the average magnetic field strength and therefore
these waves can be potential candidates for magnetic field
diagnostics of these layers. However, their subsequent coupling
to Alfvénic waves is unlikely in a magnetic environment
permeated with predominantly vertical fields, and therefore they
may not directly or indirectly contribute to the heating of
layers above plasma-ensuremathbeta less than 1.
present a study of the generation and propagation of IGWs in a
model solar atmosphere with diverse magnetic conditions. A
magnetic field-free and three magnetic models that start with an
initial, vertical, homogeneous field of 10, 50, and 100 G
magnetic flux density, are simulated using the CO$^5$BOLD
code. We find that the IGWs are generated in similar manner in
all four models in spite of the differences in the magnetic
environment. The mechanical energy carried by IGWs is
significantly larger than that of the acoustic waves in the
lower part of the atmosphere, making them an important component
of the total wave energy budget. The mechanical energy flux
(10$^6$-10$^3$ W m$^-2$) is a few orders of magnitude
larger than the Poynting flux (10$^3$-10$^1$ W
m$^-2$). The Poynting fluxes show a downward component in the
frequency range corresponding to the IGWs, which confirm that
these waves do not propagate upward in the atmosphere when the
fields are predominantly vertical and strong. We conclude that,
in the upper photosphere, the propagation properties of IGWs
depend on the average magnetic field strength and therefore
these waves can be potential candidates for magnetic field
diagnostics of these layers. However, their subsequent coupling
to Alfvénic waves is unlikely in a magnetic environment
permeated with predominantly vertical fields, and therefore they
may not directly or indirectly contribute to the heating of
layers above plasma-ensuremathbeta less than 1.
Singh, Dayal; Raja, K Sasikumar; Subramanian, Prasad; Ramesh, R; Monstein, Christian
Automated Detection of Solar Radio Bursts Using a Statistical Method Journal Article
In: Solar Physics, vol. 294, no. 8, pp. 112, 2019.
@article{2019SoPh..294..112S,
title = {Automated Detection of Solar Radio Bursts Using a Statistical Method},
author = {Dayal {Singh} and K {Sasikumar Raja} and Prasad {Subramanian} and R {Ramesh} and Christian {Monstein}},
url = {https://arxiv.org/pdf/1906.11780},
doi = {10.1007/s11207-019-1500-0},
year = {2019},
date = {2019-01-01},
journal = {Solar Physics},
volume = {294},
number = {8},
pages = {112},
abstract = {Radio bursts from the solar corona can provide clues to forecast space-
weather hazards. After recent technology advancements, regular
monitoring of radio bursts has increased and large observational
datasets are produced. Hence, manual identification and
classification of them is a challenging task. In this article,
we describe an algorithm to automatically identify radio bursts
from dynamic solar radio spectrograms using a novel statistical
method. We use e-CALLISTO ( Compound Astronomical Low Cost Low
Frequency Instrument for Spectroscopy and Transportable
Observatory) radio spectrometer data obtained at Gauribidanur
Observatory near Bangalore in India during 2013 - 2014. We have
studied the classifier performance using the receiver operating
characteristics. Further, we analyze type III bursts observed in
the year 2014 and find that 75% of the observed bursts were
below 200 MHz. Our analysis shows that the positions of flare
sites, which are associated with the type III bursts with upper
frequency cutoff ensuremathgtrsim200 MHz originate close
to the solar disk center.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Radio bursts from the solar corona can provide clues to forecast space-
weather hazards. After recent technology advancements, regular
monitoring of radio bursts has increased and large observational
datasets are produced. Hence, manual identification and
classification of them is a challenging task. In this article,
we describe an algorithm to automatically identify radio bursts
from dynamic solar radio spectrograms using a novel statistical
method. We use e-CALLISTO ( Compound Astronomical Low Cost Low
Frequency Instrument for Spectroscopy and Transportable
Observatory) radio spectrometer data obtained at Gauribidanur
Observatory near Bangalore in India during 2013 - 2014. We have
studied the classifier performance using the receiver operating
characteristics. Further, we analyze type III bursts observed in
the year 2014 and find that 75% of the observed bursts were
below 200 MHz. Our analysis shows that the positions of flare
sites, which are associated with the type III bursts with upper
frequency cutoff ensuremathgtrsim200 MHz originate close
to the solar disk center.
weather hazards. After recent technology advancements, regular
monitoring of radio bursts has increased and large observational
datasets are produced. Hence, manual identification and
classification of them is a challenging task. In this article,
we describe an algorithm to automatically identify radio bursts
from dynamic solar radio spectrograms using a novel statistical
method. We use e-CALLISTO ( Compound Astronomical Low Cost Low
Frequency Instrument for Spectroscopy and Transportable
Observatory) radio spectrometer data obtained at Gauribidanur
Observatory near Bangalore in India during 2013 - 2014. We have
studied the classifier performance using the receiver operating
characteristics. Further, we analyze type III bursts observed in
the year 2014 and find that 75% of the observed bursts were
below 200 MHz. Our analysis shows that the positions of flare
sites, which are associated with the type III bursts with upper
frequency cutoff ensuremathgtrsim200 MHz originate close
to the solar disk center.
2018
J.Stepán J. Jurcák, J. Trujillo Bueno
Comparison of theoretical and observed Ca8542 Stokes profiles in quiet regions at the centre of the solar disc Journal Article
In: A&A, vol. 619, pp. A60, 2018.
@article{refId0,
title = {Comparison of theoretical and observed Ca8542 Stokes profiles in quiet regions at the centre of the solar disc},
author = {J. Jurcák, J.Stepán, J.Trujillo Bueno, M. Bianda},
url = {http://www.irsol.usi.ch/wp-content/uploads/2018/11/jurcak-etal-2018-aa32265-17.pdf
https://doi.org/10.1051/0004-6361/201732265},
doi = {10.1051/0004-6361/201732265},
year = {2018},
date = {2018-11-01},
journal = {A&A},
volume = {619},
pages = {A60},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Janett, Gioele; Steiner, Oskar; Belluzzi, Luca
Formal Solutions for Polarized Radiative Transfer. IV. Numerical Performances in Practical Problems Journal Article
In: The Astrophysical Journal, vol. 865, no. 1, pp. 16, 2018.
@article{0004-637X-865-1-16,
title = {Formal Solutions for Polarized Radiative Transfer. IV. Numerical Performances in Practical Problems},
author = {Gioele Janett and Oskar Steiner and Luca Belluzzi},
url = {http://www.irsol.usi.ch/wp-content/uploads/2018/09/Janett_2018_ApJ_865_16.pdf},
year = {2018},
date = {2018-09-17},
journal = {The Astrophysical Journal},
volume = {865},
number = {1},
pages = {16},
abstract = {The numerical computation of reliable and accurate Stokes profiles is of great relevance in solar physics. In the synthesis process, many actors play a relevant role: among them the formal solver, the discrete atmospheric model, and the spectral line. This paper tests the performances of different numerical schemes in the synthesis of polarized spectra for different spectral lines and atmospheric models. The hierarchy between formal solvers is enforced, stressing the peculiarities of high-order and low-order formal solvers. The density of grid points necessary for reaching a given accuracy requirement is quantitatively described for specific situations.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The numerical computation of reliable and accurate Stokes profiles is of great relevance in solar physics. In the synthesis process, many actors play a relevant role: among them the formal solver, the discrete atmospheric model, and the spectral line. This paper tests the performances of different numerical schemes in the synthesis of polarized spectra for different spectral lines and atmospheric models. The hierarchy between formal solvers is enforced, stressing the peculiarities of high-order and low-order formal solvers. The density of grid points necessary for reaching a given accuracy requirement is quantitatively described for specific situations.
Bianda, M; Berdyugina, S; Gisler, D; Ramelli, R; Belluzzi, L; Carlin, E S; Stenflo, J O; Berkefeld, T
Spatial variations of the Sr i 4607 A scattering polarization peak Journal Article
In: A&A, vol. 614, pp. A89, 2018.
@article{2018A-Bianda-et-al,
title = {Spatial variations of the Sr i 4607 A scattering polarization peak},
author = {M {Bianda} and S {Berdyugina} and D {Gisler} and R {Ramelli} and L {Belluzzi} and E S {Carlin} and J O {Stenflo} and T {Berkefeld}},
url = {http://adsabs.harvard.edu/abs/2018arXiv180303531B
},
doi = {10.1051/0004-6361/201731887},
year = {2018},
date = {2018-06-18},
journal = {A&A},
volume = {614},
pages = {A89},
abstract = {Context. The scattering polarization signal observed in the photospheric
Sr i 4607 A line is expected to vary at granular spatial scales.
This variation can be due to changes in the magnetic field intensity and
orientation (Hanle effect), but also to spatial and temporal variations
in the plasma properties. Measuring the spatial variation of such
polarization signal would allow us to study the properties of the
magnetic fields at subgranular scales, but observations are challenging
since both high spatial resolution and high spectropolarimetric
sensitivity are required. Aims. We aim to provide observational evidence
of the polarization peak spatial variations, and to analyze the
correlation they might have with granulation. Methods. Observations
conjugating high spatial resolution and high spectropolarimetric
precision were performed with the Zurich IMaging POLarimeter, ZIMPOL, at
the GREGOR solar telescope, taking advantage of the adaptive optics
system and the newly installed image derotator. Results. Spatial
variations of the scattering polarization in the Sr i 4607 A line
are clearly observed. The spatial scale of these variations is
comparable with the granular size. Small correlations between the
polarization signal amplitude and the continuum intensity indicate that
the polarization is higher at the center of granules than in the
intergranular lanes. },
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Context. The scattering polarization signal observed in the photospheric
Sr i 4607 A line is expected to vary at granular spatial scales.
This variation can be due to changes in the magnetic field intensity and
orientation (Hanle effect), but also to spatial and temporal variations
in the plasma properties. Measuring the spatial variation of such
polarization signal would allow us to study the properties of the
magnetic fields at subgranular scales, but observations are challenging
since both high spatial resolution and high spectropolarimetric
sensitivity are required. Aims. We aim to provide observational evidence
of the polarization peak spatial variations, and to analyze the
correlation they might have with granulation. Methods. Observations
conjugating high spatial resolution and high spectropolarimetric
precision were performed with the Zurich IMaging POLarimeter, ZIMPOL, at
the GREGOR solar telescope, taking advantage of the adaptive optics
system and the newly installed image derotator. Results. Spatial
variations of the scattering polarization in the Sr i 4607 A line
are clearly observed. The spatial scale of these variations is
comparable with the granular size. Small correlations between the
polarization signal amplitude and the continuum intensity indicate that
the polarization is higher at the center of granules than in the
intergranular lanes.
Sr i 4607 A line is expected to vary at granular spatial scales.
This variation can be due to changes in the magnetic field intensity and
orientation (Hanle effect), but also to spatial and temporal variations
in the plasma properties. Measuring the spatial variation of such
polarization signal would allow us to study the properties of the
magnetic fields at subgranular scales, but observations are challenging
since both high spatial resolution and high spectropolarimetric
sensitivity are required. Aims. We aim to provide observational evidence
of the polarization peak spatial variations, and to analyze the
correlation they might have with granulation. Methods. Observations
conjugating high spatial resolution and high spectropolarimetric
precision were performed with the Zurich IMaging POLarimeter, ZIMPOL, at
the GREGOR solar telescope, taking advantage of the adaptive optics
system and the newly installed image derotator. Results. Spatial
variations of the scattering polarization in the Sr i 4607 A line
are clearly observed. The spatial scale of these variations is
comparable with the granular size. Small correlations between the
polarization signal amplitude and the continuum intensity indicate that
the polarization is higher at the center of granules than in the
intergranular lanes.
Salhab, R. G.; Steiner, O.; Berdyugina, S. V.; Freytag, B.; Rajaguru, S. P.; Steffen, M.
Simulation of the small-scale magnetism in main-sequence stellar atmospheres Journal Article
In: Astronomy & Astrophysics, vol. 614, pp. A78, 2018.
@article{salhab+al2018,
title = {Simulation of the small-scale magnetism in main-sequence stellar atmospheres},
author = {R. G. Salhab and O. Steiner and S.V. Berdyugina and B. Freytag and S.P. Rajaguru and M. Steffen},
url = {http://www.irsol.usi.ch/wp-content/uploads/2018/05/aa31945-17.pdf},
doi = {10.1051/0004-6361/201731945},
year = {2018},
date = {2018-06-15},
journal = {Astronomy & Astrophysics},
volume = {614},
pages = {A78},
abstract = {Observations of the Sun tell us that its granular and subgranular small-scale magnetism has significant consequences for global quantities such as the total solar irradiance or convective blueshift of spectral lines. In this paper, properties of the small-scale magnetism of four cool stellar atmospheres, including the Sun, are investigated, and in particular its effects on the radiative intensity and flux. We carried out three-dimensional radiation magnetohydrodynamic simulations with the CO5BOLD code in two different settings: with and without a magnetic field. These are thought to represent states of high and low small-scale magnetic activity of a stellar magnetic cycle. We find that the presence of small-scale magnetism increases the bolometric intensity and flux in all investigated models. The surplus in radiative flux of the magnetic over the magnetic field-free atmosphere increases with increasing effective temperature, Teff, from 0.47 % for spectral type K8V to 1.05 % for the solar model, but decreases for higher effective temperatures than solar. The degree of evacuation of the magnetic flux concentrations monotonically increases with Teff as does their depression of the visible optical surface, that is the Wilson depression. Nevertheless, the strength of the field concentrations on this surface stays remarkably unchanged at ≈ 1560 G throughout the considered range of spectral types. With respect to the surrounding gas pressure, the field strength is close to (thermal) equipartition for the Sun and spectral type F5V but is clearly sub-equipartition for K2V and more so for K8V. The magnetic flux concentrations appear most conspicuous for model K2V owing to their high brightness contrast. For mean magnetic flux densities of approximately 50 G, we expect the small-scale magnetism of stars in the spectral range from F5V to K8V to produce a positive contribution to their bolometric luminosity. The modulation seems to be most effective for early G-type stars.
},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Observations of the Sun tell us that its granular and subgranular small-scale magnetism has significant consequences for global quantities such as the total solar irradiance or convective blueshift of spectral lines. In this paper, properties of the small-scale magnetism of four cool stellar atmospheres, including the Sun, are investigated, and in particular its effects on the radiative intensity and flux. We carried out three-dimensional radiation magnetohydrodynamic simulations with the CO5BOLD code in two different settings: with and without a magnetic field. These are thought to represent states of high and low small-scale magnetic activity of a stellar magnetic cycle. We find that the presence of small-scale magnetism increases the bolometric intensity and flux in all investigated models. The surplus in radiative flux of the magnetic over the magnetic field-free atmosphere increases with increasing effective temperature, Teff, from 0.47 % for spectral type K8V to 1.05 % for the solar model, but decreases for higher effective temperatures than solar. The degree of evacuation of the magnetic flux concentrations monotonically increases with Teff as does their depression of the visible optical surface, that is the Wilson depression. Nevertheless, the strength of the field concentrations on this surface stays remarkably unchanged at ≈ 1560 G throughout the considered range of spectral types. With respect to the surrounding gas pressure, the field strength is close to (thermal) equipartition for the Sun and spectral type F5V but is clearly sub-equipartition for K2V and more so for K8V. The magnetic flux concentrations appear most conspicuous for model K2V owing to their high brightness contrast. For mean magnetic flux densities of approximately 50 G, we expect the small-scale magnetism of stars in the spectral range from F5V to K8V to produce a positive contribution to their bolometric luminosity. The modulation seems to be most effective for early G-type stars.
Calvo, F.; Belluzzi, L.; Steiner, O.
Structure of the Balmer jump. The isolated hydrogen atom Journal Article
In: A&A, vol. 613, pp. A55, 2018.
@article{calvo+al2018,
title = {Structure of the Balmer jump. The isolated hydrogen atom},
author = {F. Calvo and L. Belluzzi and O. Steiner},
url = {http://www.irsol.usi.ch/wp-content/uploads/2018/05/aa31974-17.pdf},
doi = {10.1051/0004-6361/201731974},
year = {2018},
date = {2018-06-01},
journal = {A&A},
volume = {613},
pages = {A55},
abstract = {The spectrum of the hydrogen atom was explained by Bohr more than one century ago. We revisit here some of the aspects of the underlying quantum structure, with a modern formalism, focusing on the limit of the Balmer series. We investigate the behaviour of the absorption coefficient of the isolated hydrogen atom in the neighbourhood of the Balmer limit. We analytically computed the total cross-section arising from bound-bound and bound-free transitions in the isolated hydro- gen atom at the Balmer limit, and established a simplified semi-analytical model for the surroundings of that limit. We worked within the framework of the formalism of Landi Degl’Innocenti & Landolfi (2004), which permits an almost straight-forward generalization of our results to other atoms and molecules, and which is perfectly suitable for including polarization phenomena in the problem. Results. We analytically show that there is no discontinuity at the Balmer limit, even though the concept of a "Balmer jump" is still meaningful. Furthermore, we give a possible definition of the location of the Balmer jump, and we check that this location is dependent on the broadening mechanisms. At the Balmer limit, we compute the cross-section in a fully analytical way. Conclusions. The Balmer jump is produced by a rapid drop of the total Balmer cross-section, yet this variation is smooth and continuous when both bound-bound and bound-free processes are taken into account, and its shape and location is dependent on the broadening mechanisms.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The spectrum of the hydrogen atom was explained by Bohr more than one century ago. We revisit here some of the aspects of the underlying quantum structure, with a modern formalism, focusing on the limit of the Balmer series. We investigate the behaviour of the absorption coefficient of the isolated hydrogen atom in the neighbourhood of the Balmer limit. We analytically computed the total cross-section arising from bound-bound and bound-free transitions in the isolated hydro- gen atom at the Balmer limit, and established a simplified semi-analytical model for the surroundings of that limit. We worked within the framework of the formalism of Landi Degl’Innocenti & Landolfi (2004), which permits an almost straight-forward generalization of our results to other atoms and molecules, and which is perfectly suitable for including polarization phenomena in the problem. Results. We analytically show that there is no discontinuity at the Balmer limit, even though the concept of a "Balmer jump" is still meaningful. Furthermore, we give a possible definition of the location of the Balmer jump, and we check that this location is dependent on the broadening mechanisms. At the Balmer limit, we compute the cross-section in a fully analytical way. Conclusions. The Balmer jump is produced by a rapid drop of the total Balmer cross-section, yet this variation is smooth and continuous when both bound-bound and bound-free processes are taken into account, and its shape and location is dependent on the broadening mechanisms.
Janett, G; Paganini, A
Formal Solutions for Polarized Radiative Transfer. III. Stiffness and Instability Journal Article
In: Astrophysical Journal, vol. 857, pp. 91, 2018.
@article{2018ApJ...857...91J,
title = {Formal Solutions for Polarized Radiative Transfer. III. Stiffness and Instability},
author = {G Janett and A Paganini},
url = {http://www.irsol.usi.ch/wp-content/uploads/2018/05/ApJ_857_91.pdf},
doi = {10.3847/1538-4357/aab3d9},
year = {2018},
date = {2018-03-01},
journal = {Astrophysical Journal},
volume = {857},
pages = {91},
abstract = {Efficient numerical approximation of the polarized radiative transfer equation is challenging because this system of ordinary differential equations exhibits stiff behavior, which potentially results in numerical instability. This negatively impacts the accuracy of formal solvers, and small step-sizes are often necessary to retrieve physical solutions. This work presents stability analyses of formal solvers for the radiative transfer equation of polarized light, identifies instability issues, and suggests practical remedies. In particular, the assumptions and the limitations of the stability analysis of Runge–Kutta methods play a crucial role. On this basis, a suitable and pragmatic formal solver is outlined and tested. An insightful comparison to the scalar radiative transfer equation is also presented. },
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Efficient numerical approximation of the polarized radiative transfer equation is challenging because this system of ordinary differential equations exhibits stiff behavior, which potentially results in numerical instability. This negatively impacts the accuracy of formal solvers, and small step-sizes are often necessary to retrieve physical solutions. This work presents stability analyses of formal solvers for the radiative transfer equation of polarized light, identifies instability issues, and suggests practical remedies. In particular, the assumptions and the limitations of the stability analysis of Runge–Kutta methods play a crucial role. On this basis, a suitable and pragmatic formal solver is outlined and tested. An insightful comparison to the scalar radiative transfer equation is also presented.
Ballester, E Alsina; Belluzzi, L; Bueno, J Trujillo
Magneto-optical Effects in the Scattering Polarization Wings of the Ca I 4227 Å Resonance Line Journal Article
In: Astrophysical Journal, vol. 854, pp. 150, 2018.
@article{2018ApJ...854..150A,
title = {Magneto-optical Effects in the Scattering Polarization Wings of the Ca I 4227 Å Resonance Line},
author = {E {Alsina Ballester} and L {Belluzzi} and J {Trujillo Bueno}},
url = {https://arxiv.org/abs/1711.00372},
doi = {10.3847/1538-4357/aa978a},
year = {2018},
date = {2018-02-01},
journal = {Astrophysical Journal},
volume = {854},
pages = {150},
abstract = {The linear polarization pattern produced by scattering processes in the
Ca I 4227 Å resonance line is a valuable observable for probing
the solar atmosphere. Via the Hanle effect, the very significant Q/I and
U/I line-center signals are sensitive to the presence of magnetic fields
in the lower chromosphere with strengths between 5 and 125 G,
approximately. On the other hand, partial frequency redistribution (PRD)
produces sizable signals in the wings of the Q/I profile, which have
always been thought to be insensitive to the presence of magnetic
fields. Interestingly, novel observations of this line revealed a
surprising behavior: fully unexpected signals in the wings of the U/I
profile and spatial variability in the wings of both Q/I and U/I. We
show that the magneto-optical (MO) terms of the Stokes-vector transfer
equation produce sizable signals in the wings of U/I and a clear
sensitivity of the Q/I and U/I wings to the presence of photospheric
magnetic fields with strengths similar to those that produce the Hanle
effect in the line core. This radiative transfer investigation on the
joint action of scattering processes and the Hanle and Zeeman effects in
the Ca I 4227 Å line should facilitate the development of more
reliable techniques for exploring the magnetism of stellar atmospheres.
To this end, we can now exploit the circular polarization produced by
the Zeeman effect, the magnetic sensitivity caused by the
above-mentioned MO effects in the Q/I and U/I wings, and the Hanle
effect in the line core. },
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The linear polarization pattern produced by scattering processes in the
Ca I 4227 Å resonance line is a valuable observable for probing
the solar atmosphere. Via the Hanle effect, the very significant Q/I and
U/I line-center signals are sensitive to the presence of magnetic fields
in the lower chromosphere with strengths between 5 and 125 G,
approximately. On the other hand, partial frequency redistribution (PRD)
produces sizable signals in the wings of the Q/I profile, which have
always been thought to be insensitive to the presence of magnetic
fields. Interestingly, novel observations of this line revealed a
surprising behavior: fully unexpected signals in the wings of the U/I
profile and spatial variability in the wings of both Q/I and U/I. We
show that the magneto-optical (MO) terms of the Stokes-vector transfer
equation produce sizable signals in the wings of U/I and a clear
sensitivity of the Q/I and U/I wings to the presence of photospheric
magnetic fields with strengths similar to those that produce the Hanle
effect in the line core. This radiative transfer investigation on the
joint action of scattering processes and the Hanle and Zeeman effects in
the Ca I 4227 Å line should facilitate the development of more
reliable techniques for exploring the magnetism of stellar atmospheres.
To this end, we can now exploit the circular polarization produced by
the Zeeman effect, the magnetic sensitivity caused by the
above-mentioned MO effects in the Q/I and U/I wings, and the Hanle
effect in the line core.
Ca I 4227 Å resonance line is a valuable observable for probing
the solar atmosphere. Via the Hanle effect, the very significant Q/I and
U/I line-center signals are sensitive to the presence of magnetic fields
in the lower chromosphere with strengths between 5 and 125 G,
approximately. On the other hand, partial frequency redistribution (PRD)
produces sizable signals in the wings of the Q/I profile, which have
always been thought to be insensitive to the presence of magnetic
fields. Interestingly, novel observations of this line revealed a
surprising behavior: fully unexpected signals in the wings of the U/I
profile and spatial variability in the wings of both Q/I and U/I. We
show that the magneto-optical (MO) terms of the Stokes-vector transfer
equation produce sizable signals in the wings of U/I and a clear
sensitivity of the Q/I and U/I wings to the presence of photospheric
magnetic fields with strengths similar to those that produce the Hanle
effect in the line core. This radiative transfer investigation on the
joint action of scattering processes and the Hanle and Zeeman effects in
the Ca I 4227 Å line should facilitate the development of more
reliable techniques for exploring the magnetism of stellar atmospheres.
To this end, we can now exploit the circular polarization produced by
the Zeeman effect, the magnetic sensitivity caused by the
above-mentioned MO effects in the Q/I and U/I wings, and the Hanle
effect in the line core.
2017
Dhara, S K; Belur, R; Kumar, P; Banyal, R K; Mathew, S K; Joshi, B
Trigger of Successive Filament Eruptions Observed by SDO and STEREO Journal Article
In: Solar Physics, vol. 292, pp. 145, 2017.
@article{2017SoPh..292..145D,
title = {Trigger of Successive Filament Eruptions Observed by SDO and STEREO},
author = {S K {Dhara} and R {Belur} and P {Kumar} and R K {Banyal} and S K {Mathew} and B {Joshi}},
url = {http://adsabs.harvard.edu/abs/2017SoPh..292..145D},
doi = {10.1007/s11207-017-1158-4},
year = {2017},
date = {2017-10-01},
journal = {Solar Physics},
volume = {292},
pages = {145},
abstract = {Using multiwavelength observations from the Solar Dynamics Observatory
(SDO) and the Solar Terrestrial Relations Observatory (STEREO), we
investigate the mechanism of two successive eruptions (F1 and F2) of a
filament in active region NOAA 11444 on 27 March 2012. The filament was
inverse J-shaped and lay along a quasi-circular polarity inversion line
(PIL). The first part of the filament erupted at tilde2$$:$$30 UT on 27
March 2012 (F1), the second part at around 4:20 UT on the same day (F2).
A precursor or preflare brightening was observed below the filament main
axis about 30 min before F1. The brightening was followed by a jet-like
ejection below the filament, which triggered its eruption. Before the
eruption of F2, the filament seemed to be trapped within the overlying
arcade loops for almost 1.5 h before it successfully erupted.
Interestingly, we observe simultaneously contraction (tilde12 km
s^$$-1$$) and expansion (tilde20 km s^$$-1$$) of arcade loops in the
active region before F2. Magnetograms obtained with the Helioseismic and
Magnetic Imager (HMI) show converging motion of the opposite polarities,
which result in flux cancellation near the PIL. We suggest that flux
cancellation at the PIL resulted in a jet-like ejection below the
filament main axis, which triggered F1, similar to the tether-cutting
process. F2 was triggered by removal of the overlying arcade loops via
reconnection. Both filament eruptions produced high-speed (tilde1000
km s^$$-1$$) coronal mass ejections. },
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Using multiwavelength observations from the Solar Dynamics Observatory
(SDO) and the Solar Terrestrial Relations Observatory (STEREO), we
investigate the mechanism of two successive eruptions (F1 and F2) of a
filament in active region NOAA 11444 on 27 March 2012. The filament was
inverse J-shaped and lay along a quasi-circular polarity inversion line
(PIL). The first part of the filament erupted at tilde2$$:$$30 UT on 27
March 2012 (F1), the second part at around 4:20 UT on the same day (F2).
A precursor or preflare brightening was observed below the filament main
axis about 30 min before F1. The brightening was followed by a jet-like
ejection below the filament, which triggered its eruption. Before the
eruption of F2, the filament seemed to be trapped within the overlying
arcade loops for almost 1.5 h before it successfully erupted.
Interestingly, we observe simultaneously contraction (tilde12 km
s^$$-1$$) and expansion (tilde20 km s^$$-1$$) of arcade loops in the
active region before F2. Magnetograms obtained with the Helioseismic and
Magnetic Imager (HMI) show converging motion of the opposite polarities,
which result in flux cancellation near the PIL. We suggest that flux
cancellation at the PIL resulted in a jet-like ejection below the
filament main axis, which triggered F1, similar to the tether-cutting
process. F2 was triggered by removal of the overlying arcade loops via
reconnection. Both filament eruptions produced high-speed (tilde1000
km s^$$-1$$) coronal mass ejections.
(SDO) and the Solar Terrestrial Relations Observatory (STEREO), we
investigate the mechanism of two successive eruptions (F1 and F2) of a
filament in active region NOAA 11444 on 27 March 2012. The filament was
inverse J-shaped and lay along a quasi-circular polarity inversion line
(PIL). The first part of the filament erupted at tilde2$$:$$30 UT on 27
March 2012 (F1), the second part at around 4:20 UT on the same day (F2).
A precursor or preflare brightening was observed below the filament main
axis about 30 min before F1. The brightening was followed by a jet-like
ejection below the filament, which triggered its eruption. Before the
eruption of F2, the filament seemed to be trapped within the overlying
arcade loops for almost 1.5 h before it successfully erupted.
Interestingly, we observe simultaneously contraction (tilde12 km
s^$$-1$$) and expansion (tilde20 km s^$$-1$$) of arcade loops in the
active region before F2. Magnetograms obtained with the Helioseismic and
Magnetic Imager (HMI) show converging motion of the opposite polarities,
which result in flux cancellation near the PIL. We suggest that flux
cancellation at the PIL resulted in a jet-like ejection below the
filament main axis, which triggered F1, similar to the tether-cutting
process. F2 was triggered by removal of the overlying arcade loops via
reconnection. Both filament eruptions produced high-speed (tilde1000
km s^$$-1$$) coronal mass ejections.
Stenflo, J O
History of Solar Magnetic Fields Since George Ellery Hale Journal Article
In: Space Science Reviews, vol. 210, pp. 5-35, 2017.
@article{2017SSRv..210....5S,
title = {History of Solar Magnetic Fields Since George Ellery Hale},
author = {J O {Stenflo}},
url = {http://adsabs.harvard.edu/abs/2017SSRv..210....5S},
doi = {10.1007/s11214-015-0198-z},
year = {2017},
date = {2017-09-01},
journal = {Space Science Reviews},
volume = {210},
pages = {5-35},
abstract = {As my own work on the Sun's magnetic field started exactly 50 years ago
at Crimea in the USSR, I have been a participant in the field during
nearly half the time span since Hale's discovery in 1908 of magnetic
fields in sunspots. The present historical account is accompanied by
photos from my personal slide collection, which show a number of the
leading personalities who advanced the field in different areas:
measurement techniques, from photographic to photoelectric and imaging
methods in spectro-polarimetry; theoretical foundations of MHD and the
origin of cosmic magnetic fields (birth of dynamo theory); the quest for
increased angular resolution from national projects to international
consortia (for instruments both on ground and in space); introduction of
the Hanle effect in astrophysics and the Second Solar Spectrum as its
playground; small-scale nature of the field, the fundamental resolution
limit, and transcending it by resolution-independent diagnostics. },
keywords = {},
pubstate = {published},
tppubtype = {article}
}
As my own work on the Sun's magnetic field started exactly 50 years ago
at Crimea in the USSR, I have been a participant in the field during
nearly half the time span since Hale's discovery in 1908 of magnetic
fields in sunspots. The present historical account is accompanied by
photos from my personal slide collection, which show a number of the
leading personalities who advanced the field in different areas:
measurement techniques, from photographic to photoelectric and imaging
methods in spectro-polarimetry; theoretical foundations of MHD and the
origin of cosmic magnetic fields (birth of dynamo theory); the quest for
increased angular resolution from national projects to international
consortia (for instruments both on ground and in space); introduction of
the Hanle effect in astrophysics and the Second Solar Spectrum as its
playground; small-scale nature of the field, the fundamental resolution
limit, and transcending it by resolution-independent diagnostics.
at Crimea in the USSR, I have been a participant in the field during
nearly half the time span since Hale's discovery in 1908 of magnetic
fields in sunspots. The present historical account is accompanied by
photos from my personal slide collection, which show a number of the
leading personalities who advanced the field in different areas:
measurement techniques, from photographic to photoelectric and imaging
methods in spectro-polarimetry; theoretical foundations of MHD and the
origin of cosmic magnetic fields (birth of dynamo theory); the quest for
increased angular resolution from national projects to international
consortia (for instruments both on ground and in space); introduction of
the Hanle effect in astrophysics and the Second Solar Spectrum as its
playground; small-scale nature of the field, the fundamental resolution
limit, and transcending it by resolution-independent diagnostics.
Janett, Gioele; Steiner, Oskar; Belluzzi, Luca
Formal Solutions for Polarized Radiative Transfer. II. High-order Methods Journal Article
In: The Astrophysical Journal, vol. 845, no. 2, pp. 104, 2017.
@article{0004-637X-845-2-104,
title = {Formal Solutions for Polarized Radiative Transfer. II. High-order Methods},
author = {Gioele Janett and Oskar Steiner and Luca Belluzzi},
url = {http://www.irsol.usi.ch/wp-content/uploads/2017/09/apj_845_2_104o.pdf
http://stacks.iop.org/0004-637X/845/i=2/a=104},
year = {2017},
date = {2017-08-01},
journal = {The Astrophysical Journal},
volume = {845},
number = {2},
pages = {104},
abstract = {When integrating the radiative transfer equation for polarized light, the necessity of high-order numerical methods is well known. In fact, well-performing high-order formal solvers enable higher accuracy and the use of coarser spatial grids. Aiming to provide a clear comparison between formal solvers, this work presents different high-order numerical schemes and applies the systematic analysis proposed by Janett et al., emphasizing their advantages and drawbacks in terms of order of accuracy, stability, and computational cost.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
When integrating the radiative transfer equation for polarized light, the necessity of high-order numerical methods is well known. In fact, well-performing high-order formal solvers enable higher accuracy and the use of coarser spatial grids. Aiming to provide a clear comparison between formal solvers, this work presents different high-order numerical schemes and applies the systematic analysis proposed by Janett et al., emphasizing their advantages and drawbacks in terms of order of accuracy, stability, and computational cost.
Sampoorna, M; Nagendra, K N; Stenflo, J O
Polarized Line Formation in Arbitrary Strength Magnetic Fields Angle-averaged and Angle-dependent Partial Frequency Redistribution Journal Article
In: Astrophysical Journal, vol. 844, pp. 97, 2017.
@article{2017ApJ...844...97S,
title = {Polarized Line Formation in Arbitrary Strength Magnetic Fields Angle-averaged and Angle-dependent Partial Frequency Redistribution},
author = {M {Sampoorna} and K N {Nagendra} and J O {Stenflo}},
url = {http://adsabs.harvard.edu/abs/2017ApJ...844...97S},
doi = {10.3847/1538-4357/aa7a15},
year = {2017},
date = {2017-08-01},
journal = {Astrophysical Journal},
volume = {844},
pages = {97},
abstract = {Magnetic fields in the solar atmosphere leave their fingerprints in the
polarized spectrum of the Sun via the Hanle and Zeeman effects. While
the Hanle and Zeeman effects dominate, respectively, in the weak and
strong field regimes, both these effects jointly operate in the
intermediate field strength regime. Therefore, it is necessary to solve
the polarized line transfer equation, including the combined influence
of Hanle and Zeeman effects. Furthermore, it is required to take into
account the effects of partial frequency redistribution (PRD) in
scattering when dealing with strong chromospheric lines with broad
damping wings. In this paper, we present a numerical method to solve the
problem of polarized PRD line formation in magnetic fields of arbitrary
strength and orientation. This numerical method is based on the concept
of operator perturbation. For our studies, we consider a two-level atom
model without hyperfine structure and lower-level polarization. We
compare the PRD idealization of angle-averaged Hanle-Zeeman
redistribution matrices with the full treatment of angle-dependent PRD,
to indicate when the idealized treatment is inadequate and what kind of
polarization effects are specific to angle-dependent PRD. Because the
angle-dependent treatment is presently computationally prohibitive when
applied to realistic model atmospheres, we present the computed emergent
Stokes profiles for a range of magnetic fields, with the assumption of
an isothermal one-dimensional medium. },
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Magnetic fields in the solar atmosphere leave their fingerprints in the
polarized spectrum of the Sun via the Hanle and Zeeman effects. While
the Hanle and Zeeman effects dominate, respectively, in the weak and
strong field regimes, both these effects jointly operate in the
intermediate field strength regime. Therefore, it is necessary to solve
the polarized line transfer equation, including the combined influence
of Hanle and Zeeman effects. Furthermore, it is required to take into
account the effects of partial frequency redistribution (PRD) in
scattering when dealing with strong chromospheric lines with broad
damping wings. In this paper, we present a numerical method to solve the
problem of polarized PRD line formation in magnetic fields of arbitrary
strength and orientation. This numerical method is based on the concept
of operator perturbation. For our studies, we consider a two-level atom
model without hyperfine structure and lower-level polarization. We
compare the PRD idealization of angle-averaged Hanle-Zeeman
redistribution matrices with the full treatment of angle-dependent PRD,
to indicate when the idealized treatment is inadequate and what kind of
polarization effects are specific to angle-dependent PRD. Because the
angle-dependent treatment is presently computationally prohibitive when
applied to realistic model atmospheres, we present the computed emergent
Stokes profiles for a range of magnetic fields, with the assumption of
an isothermal one-dimensional medium.
polarized spectrum of the Sun via the Hanle and Zeeman effects. While
the Hanle and Zeeman effects dominate, respectively, in the weak and
strong field regimes, both these effects jointly operate in the
intermediate field strength regime. Therefore, it is necessary to solve
the polarized line transfer equation, including the combined influence
of Hanle and Zeeman effects. Furthermore, it is required to take into
account the effects of partial frequency redistribution (PRD) in
scattering when dealing with strong chromospheric lines with broad
damping wings. In this paper, we present a numerical method to solve the
problem of polarized PRD line formation in magnetic fields of arbitrary
strength and orientation. This numerical method is based on the concept
of operator perturbation. For our studies, we consider a two-level atom
model without hyperfine structure and lower-level polarization. We
compare the PRD idealization of angle-averaged Hanle-Zeeman
redistribution matrices with the full treatment of angle-dependent PRD,
to indicate when the idealized treatment is inadequate and what kind of
polarization effects are specific to angle-dependent PRD. Because the
angle-dependent treatment is presently computationally prohibitive when
applied to realistic model atmospheres, we present the computed emergent
Stokes profiles for a range of magnetic fields, with the assumption of
an isothermal one-dimensional medium.
Carlin, E. S.; M.Bianda,
Spatiotemporal Evolution of Hanle and Zeeman Synthetic Polarization in a Chromospheric Spectral Line Journal Article
In: Astrophysical Journal, vol. 843, pp. 64, 2017.
@article{2017ApJ...843...64C,
title = {Spatiotemporal Evolution of Hanle and Zeeman Synthetic Polarization in a Chromospheric Spectral Line},
author = {E. S. Carlin and M.Bianda},
url = {https://arxiv.org/abs/1706.02381},
doi = {10.3847/1538-4357/aa7800},
year = {2017},
date = {2017-07-01},
journal = {Astrophysical Journal},
volume = {843},
pages = {64},
abstract = {Due to the quick evolution of the solar chromosphere, its magnetic field cannot be inferred reliably without accounting for the temporal variations of its polarized light. This has been broadly overlooked in the modeling and interpretation of the polarization, due to technical problems (e.g., lack of temporal resolution or of time-dependent MHD solar models) and/or because many polarization measurements can apparently be explained without dynamics. Here, we show that the temporal evolution is critical for explaining the spectral-line scattering polarization because of its sensitivity to rapidly varying physical quantities and the possibility of signal cancellations and attenuation during extended time integration. For studying the combined effect of time-varying magnetic fields and kinematics, we solved the 1.5D non-LTE problem of the second kind in time-dependent 3D R-MHD solar models and synthesized the Hanle and Zeeman polarization in forward scattering for the chromospheric λ4227 line. We find that the quiet-Sun polarization amplitudes depend on the periodicity and spectral coherence of the signal enhancements produced by kinematics, but that substantially larger linear polarization signals should exist all over the solar disk for short integration times. The spectral morphology of the polarization is discussed as a combination of Hanle, Zeeman, partial redistribution and dynamic effects. We give physical references for observations by degrading and characterizing our slit time series in different spatiotemporal resolutions. The implications of our results for the interpretation of the second solar spectrum and for the investigation of the solar atmospheric heatings are discussed. },
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Due to the quick evolution of the solar chromosphere, its magnetic field cannot be inferred reliably without accounting for the temporal variations of its polarized light. This has been broadly overlooked in the modeling and interpretation of the polarization, due to technical problems (e.g., lack of temporal resolution or of time-dependent MHD solar models) and/or because many polarization measurements can apparently be explained without dynamics. Here, we show that the temporal evolution is critical for explaining the spectral-line scattering polarization because of its sensitivity to rapidly varying physical quantities and the possibility of signal cancellations and attenuation during extended time integration. For studying the combined effect of time-varying magnetic fields and kinematics, we solved the 1.5D non-LTE problem of the second kind in time-dependent 3D R-MHD solar models and synthesized the Hanle and Zeeman polarization in forward scattering for the chromospheric λ4227 line. We find that the quiet-Sun polarization amplitudes depend on the periodicity and spectral coherence of the signal enhancements produced by kinematics, but that substantially larger linear polarization signals should exist all over the solar disk for short integration times. The spectral morphology of the polarization is discussed as a combination of Hanle, Zeeman, partial redistribution and dynamic effects. We give physical references for observations by degrading and characterizing our slit time series in different spatiotemporal resolutions. The implications of our results for the interpretation of the second solar spectrum and for the investigation of the solar atmospheric heatings are discussed.
Janett, Gioele; Carlin, Edgar S.; Steiner, Oskar; Belluzzi, Luca
Formal Solutions for Polarized Radiative Transfer. I. The DELO Family Journal Article
In: The Astrophysical Journal, vol. 840, no. 2, pp. 107, 2017.
@article{0004-637X-840-2-107b,
title = {Formal Solutions for Polarized Radiative Transfer. I. The DELO Family},
author = {Gioele Janett and Edgar S. Carlin and Oskar Steiner and Luca Belluzzi},
url = {http://www.irsol.usi.ch/wp-content/uploads/2017/09/apj_840_2_107o.pdf
http://stacks.iop.org/0004-637X/840/i=2/a=107
},
year = {2017},
date = {2017-05-01},
journal = {The Astrophysical Journal},
volume = {840},
number = {2},
pages = {107},
abstract = {The discussion regarding the numerical integration of the polarized radiative transfer equation is still open and the comparison between the different numerical schemes proposed by different authors in the past is not fully clear. Aiming at facilitating the comprehension of the advantages and drawbacks of the different formal solvers, this work presents a reference paradigm for their characterization based on the concepts of order of accuracy , stability , and computational cost . Special attention is paid to understand the numerical methods belonging to the Diagonal Element Lambda Operator family, in an attempt to highlight their specificities.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The discussion regarding the numerical integration of the polarized radiative transfer equation is still open and the comparison between the different numerical schemes proposed by different authors in the past is not fully clear. Aiming at facilitating the comprehension of the advantages and drawbacks of the different formal solvers, this work presents a reference paradigm for their characterization based on the concepts of order of accuracy , stability , and computational cost . Special attention is paid to understand the numerical methods belonging to the Diagonal Element Lambda Operator family, in an attempt to highlight their specificities.
Kano, R; Bueno, J Trujillo; Winebarger, A; Auchère, F; Narukage, N; Ishikawa, R; Kobayashi, K; Bando, T; Katsukawa, Y; Kubo, M; Ishikawa, S; Giono, G; Hara, H; Suematsu, Y; Shimizu, T; Sakao, T; Tsuneta, S; Ichimoto, K; Goto, M; Belluzzi, L; Stepán, J; Ramos, A Asensio; Sainz, R Manso; Champey, P; Cirtain, J; Pontieu, B De; Casini, R; Carlsson, M
Discovery of Scattering Polarization in the Hydrogen Ly-α Line of the Solar Disk Radiation Journal Article
In: Astrophysical Journal, Letters, vol. 839, pp. L10, 2017.
@article{2017ApJ...839L..10K,
title = {Discovery of Scattering Polarization in the Hydrogen Ly-α Line of the Solar Disk Radiation},
author = {R {Kano} and J {Trujillo Bueno} and A {Winebarger} and F {Auchère} and N {Narukage} and R {Ishikawa} and K {Kobayashi} and T {Bando} and Y {Katsukawa} and M {Kubo} and S {Ishikawa} and G {Giono} and H {Hara} and Y {Suematsu} and T {Shimizu} and T {Sakao} and S {Tsuneta} and K {Ichimoto} and M {Goto} and L {Belluzzi} and J {Stepán} and A {Asensio Ramos} and R {Manso Sainz} and P {Champey} and J {Cirtain} and B {De Pontieu} and R {Casini} and M {Carlsson}},
url = {http://adsabs.harvard.edu/abs/2017ApJ...839L..10K},
doi = {10.3847/2041-8213/aa697f},
year = {2017},
date = {2017-05-01},
journal = {Astrophysical Journal, Letters},
volume = {839},
pages = {L10},
abstract = {There is a thin transition region (TR) in the solar atmosphere where the
temperature rises from 10,000 K in the chromosphere to millions of
degrees in the corona. Little is known about the mechanisms that
dominate this enigmatic region other than the magnetic field plays a key
role. The magnetism of the TR can only be detected by polarimetric
measurements of a few ultraviolet (UV) spectral lines, the Ly$alpha$
line of neutral hydrogen at 121.6 nm (the strongest line of the solar UV
spectrum) being of particular interest given its sensitivity to the
Hanle effect (the magnetic-field-induced modification of the scattering
line polarization). We report the discovery of linear polarization
produced by scattering processes in the Lyα line, obtained with
the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) rocket
experiment. The Stokes profiles observed by CLASP in quiet regions of
the solar disk show that the Q/I and U/I linear polarization signals are
of the order of 0.1% in the line core and up to a few percent in the
nearby wings, and that both have conspicuous spatial variations with
scales of tilde10 arcsec. These observations help constrain theoretical
models of the chromosphere-corona TR and extrapolations of the
magnetic field from photospheric magnetograms. In fact, the observed
spatial variation from disk to limb of polarization at the line core and
wings already challenge the predictions from three-dimensional
magnetohydrodynamical models of the upper solar chromosphere. },
keywords = {},
pubstate = {published},
tppubtype = {article}
}
There is a thin transition region (TR) in the solar atmosphere where the
temperature rises from 10,000 K in the chromosphere to millions of
degrees in the corona. Little is known about the mechanisms that
dominate this enigmatic region other than the magnetic field plays a key
role. The magnetism of the TR can only be detected by polarimetric
measurements of a few ultraviolet (UV) spectral lines, the Ly$alpha$
line of neutral hydrogen at 121.6 nm (the strongest line of the solar UV
spectrum) being of particular interest given its sensitivity to the
Hanle effect (the magnetic-field-induced modification of the scattering
line polarization). We report the discovery of linear polarization
produced by scattering processes in the Lyα line, obtained with
the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) rocket
experiment. The Stokes profiles observed by CLASP in quiet regions of
the solar disk show that the Q/I and U/I linear polarization signals are
of the order of 0.1% in the line core and up to a few percent in the
nearby wings, and that both have conspicuous spatial variations with
scales of tilde10 arcsec. These observations help constrain theoretical
models of the chromosphere-corona TR and extrapolations of the
magnetic field from photospheric magnetograms. In fact, the observed
spatial variation from disk to limb of polarization at the line core and
wings already challenge the predictions from three-dimensional
magnetohydrodynamical models of the upper solar chromosphere.
temperature rises from 10,000 K in the chromosphere to millions of
degrees in the corona. Little is known about the mechanisms that
dominate this enigmatic region other than the magnetic field plays a key
role. The magnetism of the TR can only be detected by polarimetric
measurements of a few ultraviolet (UV) spectral lines, the Ly$alpha$
line of neutral hydrogen at 121.6 nm (the strongest line of the solar UV
spectrum) being of particular interest given its sensitivity to the
Hanle effect (the magnetic-field-induced modification of the scattering
line polarization). We report the discovery of linear polarization
produced by scattering processes in the Lyα line, obtained with
the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) rocket
experiment. The Stokes profiles observed by CLASP in quiet regions of
the solar disk show that the Q/I and U/I linear polarization signals are
of the order of 0.1% in the line core and up to a few percent in the
nearby wings, and that both have conspicuous spatial variations with
scales of tilde10 arcsec. These observations help constrain theoretical
models of the chromosphere-corona TR and extrapolations of the
magnetic field from photospheric magnetograms. In fact, the observed
spatial variation from disk to limb of polarization at the line core and
wings already challenge the predictions from three-dimensional
magnetohydrodynamical models of the upper solar chromosphere.
Giono, G; Ishikawa, R; Narukage, N; Kano, R; Katsukawa, Y; Kubo, M; Ishikawa, S; Bando, T; Hara, H; Suematsu, Y; Winebarger, A; Kobayashi, K; Auchère, F; Bueno, J Trujillo; Tsuneta, S; Shimizu, T; Sakao, T; Cirtain, J; Champey, P; Ramos, A Asensio; v Stv epán, J; Belluzzi, L; Sainz, R Manso; Pontieu, B De; Ichimoto, K; Carlsson, M; Casini, R; Goto, M
In: Solar Physics, vol. 292, pp. 57, 2017.
@article{2017SoPh..292...57G,
title = {Polarization Calibration of the Chromospheric Lyman-Alpha SpectroPolarimeter for a 0.1% Polarization Sensitivity in the VUV Range. Part II: In-Flight Calibration},
author = {G {Giono} and R {Ishikawa} and N {Narukage} and R {Kano} and Y {Katsukawa} and M {Kubo} and S {Ishikawa} and T {Bando} and H {Hara} and Y {Suematsu} and A {Winebarger} and K {Kobayashi} and F {Auchère} and J {Trujillo Bueno} and S {Tsuneta} and T {Shimizu} and T {Sakao} and J {Cirtain} and P {Champey} and A {Asensio Ramos} and J {{v S}t{v e}pán} and L {Belluzzi} and R {Manso Sainz} and B {De Pontieu} and K {Ichimoto} and M {Carlsson} and R {Casini} and M {Goto}},
url = {http://adsabs.harvard.edu/abs/2017SoPh..292...57G},
doi = {10.1007/s11207-017-1062-y},
year = {2017},
date = {2017-04-01},
journal = {Solar Physics},
volume = {292},
pages = {57},
abstract = {The Chromospheric Lyman-Alpha SpectroPolarimeter is a sounding rocket
instrument designed to measure for the first time the linear
polarization of the hydrogen Lyman-$$$alpha$$$ line (121.6 nm). The
instrument was successfully launched on 3 September 2015 and
observations were conducted at the solar disc center and close to the
limb during the five-minutes flight. In this article, the disc center
observations are used to provide an in-flight calibration of the
instrument spurious polarization. The derived in-flight spurious
polarization is consistent with the spurious polarization levels
determined during the pre-flight calibration and a statistical analysis
of the polarization fluctuations from solar origin is conducted to
ensure a 0.014% precision on the spurious polarization. The combination
of the pre-flight and the in-flight polarization calibrations provides a
complete picture of the instrument response matrix, and a proper error
transfer method is used to confirm the achieved polarization accuracy.
As a result, the unprecedented 0.1% polarization accuracy of the
instrument in the vacuum ultraviolet is ensured by the polarization
calibration. },
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The Chromospheric Lyman-Alpha SpectroPolarimeter is a sounding rocket
instrument designed to measure for the first time the linear
polarization of the hydrogen Lyman-$$$alpha$$$ line (121.6 nm). The
instrument was successfully launched on 3 September 2015 and
observations were conducted at the solar disc center and close to the
limb during the five-minutes flight. In this article, the disc center
observations are used to provide an in-flight calibration of the
instrument spurious polarization. The derived in-flight spurious
polarization is consistent with the spurious polarization levels
determined during the pre-flight calibration and a statistical analysis
of the polarization fluctuations from solar origin is conducted to
ensure a 0.014% precision on the spurious polarization. The combination
of the pre-flight and the in-flight polarization calibrations provides a
complete picture of the instrument response matrix, and a proper error
transfer method is used to confirm the achieved polarization accuracy.
As a result, the unprecedented 0.1% polarization accuracy of the
instrument in the vacuum ultraviolet is ensured by the polarization
calibration.
instrument designed to measure for the first time the linear
polarization of the hydrogen Lyman-$$$alpha$$$ line (121.6 nm). The
instrument was successfully launched on 3 September 2015 and
observations were conducted at the solar disc center and close to the
limb during the five-minutes flight. In this article, the disc center
observations are used to provide an in-flight calibration of the
instrument spurious polarization. The derived in-flight spurious
polarization is consistent with the spurious polarization levels
determined during the pre-flight calibration and a statistical analysis
of the polarization fluctuations from solar origin is conducted to
ensure a 0.014% precision on the spurious polarization. The combination
of the pre-flight and the in-flight polarization calibrations provides a
complete picture of the instrument response matrix, and a proper error
transfer method is used to confirm the achieved polarization accuracy.
As a result, the unprecedented 0.1% polarization accuracy of the
instrument in the vacuum ultraviolet is ensured by the polarization
calibration.