2021 |
Paganini, A; Hashemi, B; Alsina Ballester, E; Belluzzi, L Fast and accurate approximation of the angle-averaged redistribution function for polarized radiation Journal Article In: Astronomy and Astrophysics, 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$^-2$]. |
2020 |
Ndacyayisenga, Theogene; Uwamahoro, Jean; Raja, Sasikumar K; Monstein, Christian Solar activity, Solar Radio Type III Bursts, e-CALLISTO, Space weather Journal Article In: Advances in Space Research, (0273-1177), 2020, ISSN: 0273-1177. @article{NDACYAYISENGA2020d, title = {Solar activity, Solar Radio Type III Bursts, e-CALLISTO, Space weather}, author = {Theogene Ndacyayisenga and Jean Uwamahoro and Sasikumar K Raja and Christian Monstein}, url = {http://www.sciencedirect.com/science/article/pii/S027311772030822X}, doi = {10.1016/j.asr.2020.11.022}, issn = {0273-1177}, year = {2020}, date = {2020-12-08}, journal = {Advances in Space Research}, number = {0273-1177}, abstract = {Solar radio bursts (SRBs) are the signatures of various phenomenon that happen in the solar corona and interplanetary medium (IPM). In this article, we have studied occurrence of Type III bursts and their association with the Sunspot number. This study confirms that occurrence of Type III bursts correlate well with Sunspot number. Further, using the data obtained using e-CALLISTO network, we have investigated drift rates of isolated Type III bursts and duration of the group of Type III bursts. Since Type II, Type III and Type IV bursts are signatures of solar flares and/or CMEs, we can use the radio observations to predict space weather hazards. In this article, we have discussed two events that have caused near Earth radio blackouts. Since e-CALLISTO comprises more than 152 stations at different longitudes, we can use it to monitor the radio emissions from the solar corona 24 hours a day. Such observations play a crucial role in monitoring and predicting space weather hazards within few minutes to hours of time.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Solar radio bursts (SRBs) are the signatures of various phenomenon that happen in the solar corona and interplanetary medium (IPM). In this article, we have studied occurrence of Type III bursts and their association with the Sunspot number. This study confirms that occurrence of Type III bursts correlate well with Sunspot number. Further, using the data obtained using e-CALLISTO network, we have investigated drift rates of isolated Type III bursts and duration of the group of Type III bursts. Since Type II, Type III and Type IV bursts are signatures of solar flares and/or CMEs, we can use the radio observations to predict space weather hazards. In this article, we have discussed two events that have caused near Earth radio blackouts. Since e-CALLISTO comprises more than 152 stations at different longitudes, we can use it to monitor the radio emissions from the solar corona 24 hours a day. Such observations play a crucial role in monitoring and predicting space weather hazards within few minutes to hours of time. |
Di Campli, R; Ramelli, R; Bianda, M; Furno, I; Dhara, Kumar S; Belluzzi, L Imaging spectropolarimetry for magnetic field diagnostics in solar prominences Journal Article In: A&A, 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}, 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. |
Mahender, Aroori; Sasikumar Raja, K; Ramesh, R; Panditi, Vemareddy; Monstein, Christian; Ganji, Yellaiah A Statistical Study of Low-Frequency Solar Radio Type III Bursts Journal Article In: Solar Physics, 295 (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, 903 (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. |
Stenflo, Jan O Cosmological constant caused by observer-induced boundary condition Journal Article In: Journal of Physics Communications, 4 (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. |
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, 498 (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; Ballester, Ernest Alsina; Belluzzi, Luca; Bianda, Michele; Dhara, Sajal Kumar; 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, 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}, 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. |
José R. Canivete Cuissa, Oskar Steiner Vortices evolution in the solar atmosphere - A dynamical equation for the swirling strength Journal Article In: Astronomy and Astrophysics, 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, Oskar Steiner}, url = {https://doi.org/10.1051/0004-6361/202038060}, doi = {10.1051/0004-6361/202038060}, year = {2020}, date = {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. |
Nagendra, K N; Sowmya, K; Sampoorna, M; Stenflo, J O; Anusha, L S In: Astrophysical Journal, 898 (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. |
Wan Mokhtar, W Z A; 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 ; Manso Sainz, Rafael ; Feller, Alex ; van Noort, Michiel ; Solanki, Sami K; Iglesias, Francisco A; Reardon, Kevin ; Martinez Pillet, Valentin Solar Disk Center Shows Scattering Polarization in the Sr I 4607 Å Line Journal Article In: Astrophysical Journal, Letters, 893 (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. |
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, 295 (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, 295 (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, 883 (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. |
Stenflo, J O Origin of the cosmological constant Journal Article In: Astrophysics and Space Science, 364 (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. |
Singh, Dayal; Raja, Sasikumar K; Subramanian, Prasad; Ramesh, R; Monstein, Christian Automated Detection of Solar Radio Bursts Using a Statistical Method Journal Article In: Solar Physics, 294 (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 ; Biand a, Michele ; 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, 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. |
Alsina Ballester, E; Belluzzi, L; Trujillo Bueno, J Magnetic Sensitivity in the Wing Scattering Polarization Signals of the Hydrogen Lyman-α Line of the Solar Disk Radiation Journal Article In: Astrophysical Journal, 880 (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}, 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. |
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, 877 (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 ; Alsina Ballester, Ernest ; 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, 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. |
Wiehr, E; Stellmacher, G; Bianda, M Evidence for the Two-fluid Scenario in Solar Prominences Journal Article In: Astrophysical Journal, 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, 872 (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. |
Singh, Dayal ; Sasikumar Raja, K; Subramanian, Prasad ; Ramesh, R; Monstein, Christian Automated Detection of Solar Radio Bursts Using a Statistical Method Journal Article In: Solar Physics, 294 (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. |
2018 |
J. Jurcák J.Stepán, J.Trujillo Bueno Bianda M Comparison of theoretical and observed Ca8542 Stokes profiles in quiet regions at the centre of the solar disc Journal Article In: A&A, 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, 865 (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, 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. |
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, 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, 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, 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. |
Alsina Ballester, E; Belluzzi, L; Trujillo Bueno, J Magneto-optical Effects in the Scattering Polarization Wings of the Ca I 4227 Å Resonance Line Journal Article In: Astrophysical Journal, 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. |
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, 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. |
Stenflo, J O History of Solar Magnetic Fields Since George Ellery Hale Journal Article In: Space Science Reviews, 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. |
Janett, Gioele; Steiner, Oskar; Belluzzi, Luca Formal Solutions for Polarized Radiative Transfer. II. High-order Methods Journal Article In: The Astrophysical Journal, 845 (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, 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. |
Carlin, E S; M.Bianda, Spatiotemporal Evolution of Hanle and Zeeman Synthetic Polarization in a Chromospheric Spectral Line Journal Article In: Astrophysical Journal, 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, 840 (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; Trujillo Bueno, J; 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; Asensio Ramos, A; Manso Sainz, R; Champey, P; Cirtain, J; De Pontieu, B; 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, 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. |
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; Trujillo Bueno, J; Tsuneta, S; Shimizu, T; Sakao, T; Cirtain, J; Champey, P; Asensio Ramos, A; v e}pán, J {{v S}t; Belluzzi, L; Manso Sainz, R; De Pontieu, B; Ichimoto, K; Carlsson, M; Casini, R; Goto, M In: Solar Physics, 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. |
Jafarzadeh, S; Solanki, S K; Stangalini, M; Steiner, O; Cameron, R H; Danilovic, S High-frequency Oscillations in Small Magnetic Elements Observed with Sunrise/SuFI Journal Article In: Astrophysical Journal, Supplement, 229 , pp. 10, 2017. @article{2017ApJS..229...10J, title = {High-frequency Oscillations in Small Magnetic Elements Observed with Sunrise/SuFI}, author = {S {Jafarzadeh} and S K {Solanki} and M {Stangalini} and O {Steiner} and R H {Cameron} and S {Danilovic}}, doi = {10.3847/1538-4365/229/1/10}, year = {2017}, date = {2017-04-01}, journal = {Astrophysical Journal, Supplement}, volume = {229}, pages = {10}, abstract = {We characterize waves in small magnetic elements and investigate their propagation in the lower solar atmosphere from observations at high spatial and temporal resolution. We use the wavelet transform to analyze oscillations of both horizontal displacement and intensity in magnetic bright points found in the 300 nm and the Ca II H 396.8 nm passbands of the filter imager on board the Sunrise balloon-borne solar observatory. Phase differences between the oscillations at the two atmospheric layers corresponding to the two passbands reveal upward propagating waves at high frequencies (up to 30 mHz). Weak signatures of standing as well as downward propagating waves are also obtained. Both compressible and incompressible (kink) waves are found in the small-scale magnetic features. The two types of waves have different, though overlapping, period distributions. Two independent estimates give a height difference of approximately 450 plusmn 100 km between the two atmospheric layers sampled by the employed spectral bands. This value, together with the determined short travel times of the transverse and longitudinal waves provide us with phase speeds of 29 plusmn 2 km s$^-1$ and 31 plusmn 2 km s$^-1$, respectively. We speculate that these phase speeds may not reflect the true propagation speeds of the waves. Thus, effects such as the refraction of fast longitudinal waves may contribute to an overestimate of the phase speed. }, keywords = {}, pubstate = {published}, tppubtype = {article} } We characterize waves in small magnetic elements and investigate their propagation in the lower solar atmosphere from observations at high spatial and temporal resolution. We use the wavelet transform to analyze oscillations of both horizontal displacement and intensity in magnetic bright points found in the 300 nm and the Ca II H 396.8 nm passbands of the filter imager on board the Sunrise balloon-borne solar observatory. Phase differences between the oscillations at the two atmospheric layers corresponding to the two passbands reveal upward propagating waves at high frequencies (up to 30 mHz). Weak signatures of standing as well as downward propagating waves are also obtained. Both compressible and incompressible (kink) waves are found in the small-scale magnetic features. The two types of waves have different, though overlapping, period distributions. Two independent estimates give a height difference of approximately 450 plusmn 100 km between the two atmospheric layers sampled by the employed spectral bands. This value, together with the determined short travel times of the transverse and longitudinal waves provide us with phase speeds of 29 plusmn 2 km s$^-1$ and 31 plusmn 2 km s$^-1$, respectively. We speculate that these phase speeds may not reflect the true propagation speeds of the waves. Thus, effects such as the refraction of fast longitudinal waves may contribute to an overestimate of the phase speed. |
Ballester, E A; Belluzzi, L; Trujillo Bueno, J The Transfer of Resonance Line Polarization with Partial Frequency Redistribution in the General Hanlen-Zeeman Regime Journal Article In: Astrophysical Journal, 836 , pp. 6, 2017. @article{2017ApJ...836....6B, title = {The Transfer of Resonance Line Polarization with Partial Frequency Redistribution in the General Hanlen-Zeeman Regime}, author = {Ballester, E.A. and Belluzzi, L. and Trujillo Bueno, J.}, url = {https://arxiv.org/pdf/1609.05723}, doi = {10.3847/1538-4357/836/1/6}, year = {2017}, date = {2017-02-01}, journal = {Astrophysical Journal}, volume = {836}, pages = {6}, abstract = {The spectral line polarization encodes a wealth of information about the thermal and magnetic properties of the solar atmosphere. Modeling the Stokes profiles of strong resonance lines is, however, a complex problem both from a theoretical and computational point of view, especially when partial frequency redistribution (PRD) effects need to be taken into account. In this work, we consider a two-level atom in the presence of magnetic fields of arbitrary intensity (HanlendashZeeman regime) and orientation, both deterministic and micro-structured. Working within the framework of a rigorous PRD theoretical approach, we have developed a numerical code that solves the full non-LTE radiative transfer problem for polarized radiation, in one-dimensional models of the solar atmosphere, accounting for the combined action of the Hanle and Zeeman effects, as well as for PRD phenomena. After briefly discussing the relevant equations, we describe the iterative method of solution of the problem and the numerical tools that we have developed and implemented. We finally present some illustrative applications to two resonance lines that form at different heights in the solar atmosphere, and provide a detailed physical interpretation of the calculated Stokes profiles. We find that magneto-optical effects have a strong impact on the linear polarization signals that PRD effects produce in the wings of strong resonance lines. We also show that the weak-field approximation has to be used with caution when PRD effects are considered. }, keywords = {}, pubstate = {published}, tppubtype = {article} } The spectral line polarization encodes a wealth of information about the thermal and magnetic properties of the solar atmosphere. Modeling the Stokes profiles of strong resonance lines is, however, a complex problem both from a theoretical and computational point of view, especially when partial frequency redistribution (PRD) effects need to be taken into account. In this work, we consider a two-level atom in the presence of magnetic fields of arbitrary intensity (HanlendashZeeman regime) and orientation, both deterministic and micro-structured. Working within the framework of a rigorous PRD theoretical approach, we have developed a numerical code that solves the full non-LTE radiative transfer problem for polarized radiation, in one-dimensional models of the solar atmosphere, accounting for the combined action of the Hanle and Zeeman effects, as well as for PRD phenomena. After briefly discussing the relevant equations, we describe the iterative method of solution of the problem and the numerical tools that we have developed and implemented. We finally present some illustrative applications to two resonance lines that form at different heights in the solar atmosphere, and provide a detailed physical interpretation of the calculated Stokes profiles. We find that magneto-optical effects have a strong impact on the linear polarization signals that PRD effects produce in the wings of strong resonance lines. We also show that the weak-field approximation has to be used with caution when PRD effects are considered. |
Vigeesh, G; Jackiewicz, J; Steiner, O Internal Gravity Waves in the Magnetized Solar Atmosphere. I. Magnetic Field Effects Journal Article In: Astrophysical Journal, 835 , pp. 148, 2017. @article{2017ApJ...835..148V, title = {Internal Gravity Waves in the Magnetized Solar Atmosphere. I. Magnetic Field Effects}, author = {Vigeesh, G. and Jackiewicz, J. and Steiner, O.}, url = {https://arxiv.org/abs/1612.04729}, doi = {10.3847/1538-4357/835/2/148}, year = {2017}, date = {2017-02-01}, journal = {Astrophysical Journal}, volume = {835}, pages = {148}, abstract = {Observations of the solar atmosphere show that internal gravity waves are generated by overshooting convection, but are suppressed at locations of magnetic flux, which is thought to be the result of mode conversion into magnetoacoustic waves. Here, we present a study of the acoustic-gravity wave spectrum emerging from a realistic, self-consistent simulation of solar (magneto)convection. A magnetic field free, hydrodynamic simulation and a magnetohydrodynamic (MHD) simulation with an initial, vertical, homogeneous field of 50 G flux density were carried out and compared with each other to highlight the effect of magnetic fields on the internal gravity wave propagation in the Sunrsquos atmosphere. We find that the internal gravity waves are absent or partially reflected back into the lower layers in the presence of magnetic fields and argue that the suppression is due to the coupling of internal gravity waves to slow magnetoacoustic waves still within the high-$beta$ region of the upper photosphere. The conversion to Alfv'en waves is highly unlikely in our model because there is no strongly inclined magnetic field present. We argue that the suppression of internal waves observed within magnetic flux concentrations may also be due to nonlinear breaking of internal waves due to vortex flows that are ubiquitously present in the upper photosphere and the chromosphere. }, keywords = {}, pubstate = {published}, tppubtype = {article} } Observations of the solar atmosphere show that internal gravity waves are generated by overshooting convection, but are suppressed at locations of magnetic flux, which is thought to be the result of mode conversion into magnetoacoustic waves. Here, we present a study of the acoustic-gravity wave spectrum emerging from a realistic, self-consistent simulation of solar (magneto)convection. A magnetic field free, hydrodynamic simulation and a magnetohydrodynamic (MHD) simulation with an initial, vertical, homogeneous field of 50 G flux density were carried out and compared with each other to highlight the effect of magnetic fields on the internal gravity wave propagation in the Sunrsquos atmosphere. We find that the internal gravity waves are absent or partially reflected back into the lower layers in the presence of magnetic fields and argue that the suppression is due to the coupling of internal gravity waves to slow magnetoacoustic waves still within the high-$beta$ region of the upper photosphere. The conversion to Alfv'en waves is highly unlikely in our model because there is no strongly inclined magnetic field present. We argue that the suppression of internal waves observed within magnetic flux concentrations may also be due to nonlinear breaking of internal waves due to vortex flows that are ubiquitously present in the upper photosphere and the chromosphere. |
Trujillo Bueno, J; Landi Degl'Innocenti, E; Belluzzi, L The Physics and Diagnostic Potential of Ultraviolet Spectropolarimetry Journal Article In: Space Science Reviews, 210 , pp. 183-226, 2017. @article{2017SSRv..210..183T, title = {The Physics and Diagnostic Potential of Ultraviolet Spectropolarimetry}, author = {J {Trujillo Bueno} and E {Landi Degl'Innocenti} and L {Belluzzi}}, doi = {10.1007/s11214-016-0306-8}, year = {2017}, date = {2017-01-01}, journal = {Space Science Reviews}, volume = {210}, pages = {183-226}, abstract = {The empirical investigation of the magnetic field in the outer solar atmosphere is a very important challenge in astrophysics. To this end, we need to identify, measure and interpret observable quantities sensitive to the magnetism of the upper chromosphere, transition region and corona. This paper provides an overview of the physics and diagnostic potential of spectropolarimetry in permitted spectral lines of the ultraviolet solar spectrum, such as the Mg ii h and k lines around 2800 AA, the hydrogen Lyman-$alpha$ line at 1216 AA, and the Lyman-$alpha$ line of He ii at 304 AA. The outer solar atmosphere is an optically pumped vapor and the linear polarization of such spectral lines is dominated by the atomic level polarization produced by the absorption and scattering of anisotropic radiation. Its modification by the action of the Hanle and Zeeman effects in the inhomogeneous and dynamic solar atmosphere needs to be carefully understood because it encodes the magnetic field information. The circular polarization induced by the Zeeman effect in some ultraviolet lines (e.g., Mg ii h amp k) is also of diagnostic interest, especially for probing the outer solar atmosphere in plages and more active regions. The few (pioneering) observational attempts carried out so far to measure the ultraviolet spectral line polarization produced by optically pumped atoms in the upper chromosphere, transition region and corona are also discussed. We emphasize that ultraviolet spectropolarimetry is a key gateway to the outer atmosphere of the Sun and of other stars. }, keywords = {}, pubstate = {published}, tppubtype = {article} } The empirical investigation of the magnetic field in the outer solar atmosphere is a very important challenge in astrophysics. To this end, we need to identify, measure and interpret observable quantities sensitive to the magnetism of the upper chromosphere, transition region and corona. This paper provides an overview of the physics and diagnostic potential of spectropolarimetry in permitted spectral lines of the ultraviolet solar spectrum, such as the Mg ii h and k lines around 2800 AA, the hydrogen Lyman-$alpha$ line at 1216 AA, and the Lyman-$alpha$ line of He ii at 304 AA. The outer solar atmosphere is an optically pumped vapor and the linear polarization of such spectral lines is dominated by the atomic level polarization produced by the absorption and scattering of anisotropic radiation. Its modification by the action of the Hanle and Zeeman effects in the inhomogeneous and dynamic solar atmosphere needs to be carefully understood because it encodes the magnetic field information. The circular polarization induced by the Zeeman effect in some ultraviolet lines (e.g., Mg ii h amp k) is also of diagnostic interest, especially for probing the outer solar atmosphere in plages and more active regions. The few (pioneering) observational attempts carried out so far to measure the ultraviolet spectral line polarization produced by optically pumped atoms in the upper chromosphere, transition region and corona are also discussed. We emphasize that ultraviolet spectropolarimetry is a key gateway to the outer atmosphere of the Sun and of other stars. |
2016 |
Kubo, M; Katsukawa, Y; Suematsu, Y; Kano, R; Bando, T; Narukage, N; Ishikawa, R; Hara, H; Giono, G; Tsuneta, S; Ishikawa, S; Shimizu, T; Sakao, T; Winebarger, A; Kobayashi, K; Cirtain, J; Champey, P; Auch`ere, F; Trujillo Bueno, J; Asensio Ramos, A; v Stv ep'an, J; Belluzzi, L; Manso Sainz, R; De Pontieu, B; Ichimoto, K; Carlsson, M; Casini, R; Goto, M Discovery of Ubiquitous Fast-Propagating Intensity Disturbances by the Chromospheric Lyman α Spectropolarimeter (CLASP) Journal Article In: Astrophysical Journal, 832 , pp. 141, 2016. @article{2016ApJ...832..141K, title = {Discovery of Ubiquitous Fast-Propagating Intensity Disturbances by the Chromospheric Lyman α Spectropolarimeter (CLASP)}, author = {Kubo, M. and Katsukawa, Y. and Suematsu, Y. and Kano, R. and Bando, T. and Narukage, N. and Ishikawa, R. and Hara, H. and Giono, G. and Tsuneta, S. and Ishikawa, S. and Shimizu, T. and Sakao, T. and Winebarger, A. and Kobayashi, K. and Cirtain, J. and Champey, P. and Auch`ere, F. and Trujillo Bueno, J. and Asensio Ramos, A. and v Stv ep'an, J. and Belluzzi, L. and Manso Sainz, R. and De Pontieu, B. and Ichimoto, K. and Carlsson, M. and Casini, R. and Goto, M.}, url = {http://adsabs.harvard.edu/abs/2016ApJ...832..141K}, doi = {10.3847/0004-637X/832/2/141}, year = {2016}, date = {2016-12-01}, journal = {Astrophysical Journal}, volume = {832}, pages = {141}, abstract = {High-cadence observations by the slit-jaw (SJ) optics system of the sounding rocket experiment known as the Chromospheric Lyman Alpha Spectropolarimeter (CLASP) reveal ubiquitous intensity disturbances that recurrently propagate in either the chromosphere or the transition region or both at a speed much higher than the speed of sound. The CLASP/SJ instrument provides a time series of two-dimensional images taken with broadband filters centered on the Lyα line at a 0.6 s cadence. The multiple fast-propagating intensity disturbances appear in the quiet Sun and in an active region, and they are clearly detected in at least 20 areas in a field of view of 527″ × 527″ during the 5 minute observing time. The apparent speeds of the intensity disturbances range from 150 to 350 km s-1, and they are comparable to the local Alfvén speed in the transition region. The intensity disturbances tend to propagate along bright elongated structures away from areas with strong photospheric magnetic fields. This suggests that the observed fast-propagating intensity disturbances are related to the magnetic canopy structures. The maximum distance traveled by the intensity disturbances is about 10″, and the widths are a few arcseconds, which are almost determined by a pixel size of 1.″03. The timescale of each intensity pulse is shorter than 30 s. One possible explanation for the fast-propagating intensity disturbances observed by CLASP is magnetohydrodynamic fast-mode waves. }, keywords = {}, pubstate = {published}, tppubtype = {article} } High-cadence observations by the slit-jaw (SJ) optics system of the sounding rocket experiment known as the Chromospheric Lyman Alpha Spectropolarimeter (CLASP) reveal ubiquitous intensity disturbances that recurrently propagate in either the chromosphere or the transition region or both at a speed much higher than the speed of sound. The CLASP/SJ instrument provides a time series of two-dimensional images taken with broadband filters centered on the Lyα line at a 0.6 s cadence. The multiple fast-propagating intensity disturbances appear in the quiet Sun and in an active region, and they are clearly detected in at least 20 areas in a field of view of 527″ × 527″ during the 5 minute observing time. The apparent speeds of the intensity disturbances range from 150 to 350 km s-1, and they are comparable to the local Alfvén speed in the transition region. The intensity disturbances tend to propagate along bright elongated structures away from areas with strong photospheric magnetic fields. This suggests that the observed fast-propagating intensity disturbances are related to the magnetic canopy structures. The maximum distance traveled by the intensity disturbances is about 10″, and the widths are a few arcseconds, which are almost determined by a pixel size of 1.″03. The timescale of each intensity pulse is shorter than 30 s. One possible explanation for the fast-propagating intensity disturbances observed by CLASP is magnetohydrodynamic fast-mode waves. |
Cortesi, S; Cagnotti, M; Bianda, M; Ramelli, R; Manna, A Sunspot Observations and Counting at Specola Solare Ticinese in Locarno Since 1957 Journal Article In: Solar Physics, 291 , pp. 3075-3080, 2016. @article{2016SoPh..291.3075C, title = {Sunspot Observations and Counting at Specola Solare Ticinese in Locarno Since 1957}, author = {Cortesi, S. and Cagnotti, M. and Bianda, M. and Ramelli, R. and Manna, A.}, url = {https://arxiv.org/pdf/1602.07998v1 http://adsabs.harvard.edu/abs/2016SoPh..291.3075C}, doi = {10.1007/s11207-016-0872-7}, year = {2016}, date = {2016-11-01}, journal = {Solar Physics}, volume = {291}, pages = {3075-3080}, keywords = {}, pubstate = {published}, tppubtype = {article} } |
Alsina Ballester, E; Belluzzi, L; Trujillo Bueno, J The Magnetic Sensitivity of the Mg ii k Line to the Joint Action of Hanle, Zeeman, and Magneto-optical Effects Journal Article In: The Astrophysical Journal Letters, 831 , pp. L15, 2016. @article{2016ApJ...831L..15A, title = {The Magnetic Sensitivity of the Mg ii k Line to the Joint Action of Hanle, Zeeman, and Magneto-optical Effects}, author = {Alsina Ballester, E. and Belluzzi, L. and Trujillo Bueno, J.}, url = {https://arxiv.org/abs/1610.00649}, doi = {10.3847/2041-8205/831/2/L15}, year = {2016}, date = {2016-11-01}, journal = {The Astrophysical Journal Letters}, volume = {831}, pages = {L15}, abstract = {We highlight the main results of a radiative transfer investigation on the magnetic sensitivity of the solar Mg ii k resonance line at 2795.5 Å, accounting for the joint action of the Hanle and Zeeman effects as well as partial frequency redistribution phenomena. We confirm that at the line center, the linear polarization signals produced by scattering processes are measurable, and that they are sensitive, via the Hanle effect, to magnetic fields with strengths between 5 and 50 G, approximately. We also show that the Zeeman effect produces conspicuous circular polarization signals, especially for longitudinal fields stronger than 50 G, which can be used to estimate the magnetization of the solar chromosphere via the familiar magnetograph formula. The most novel result is that magneto-optical effects produce, in the wings of the line, a decrease of the Q/I scattering polarization pattern and the appearance of U/I signals (i.e., a rotation of the plane of linear polarization). This sensitivity of the Q/I and U/I wing signals to both weak (∼5 G) and stronger magnetic fields expands the scientific interest of the Mg ii k line for probing the chromosphere in quiet and active regions of the Sun. }, keywords = {}, pubstate = {published}, tppubtype = {article} } We highlight the main results of a radiative transfer investigation on the magnetic sensitivity of the solar Mg ii k resonance line at 2795.5 Å, accounting for the joint action of the Hanle and Zeeman effects as well as partial frequency redistribution phenomena. We confirm that at the line center, the linear polarization signals produced by scattering processes are measurable, and that they are sensitive, via the Hanle effect, to magnetic fields with strengths between 5 and 50 G, approximately. We also show that the Zeeman effect produces conspicuous circular polarization signals, especially for longitudinal fields stronger than 50 G, which can be used to estimate the magnetization of the solar chromosphere via the familiar magnetograph formula. The most novel result is that magneto-optical effects produce, in the wings of the line, a decrease of the Q/I scattering polarization pattern and the appearance of U/I signals (i.e., a rotation of the plane of linear polarization). This sensitivity of the Q/I and U/I wing signals to both weak (∼5 G) and stronger magnetic fields expands the scientific interest of the Mg ii k line for probing the chromosphere in quiet and active regions of the Sun. |
Kato, Y; Steiner, O; Hansteen, V; Gudiksen, B; Wedemeyer, S; Carlsson, M Chromospheric and Coronal Wave Generation in a Magnetic Flux Sheath Journal Article In: Astrophysical Journal, 827 , pp. 7, 2016. @article{2016ApJ...827....7K, title = {Chromospheric and Coronal Wave Generation in a Magnetic Flux Sheath}, author = {Kato, Y. and Steiner, O. and Hansteen, V. and Gudiksen, B. and Wedemeyer, S. and Carlsson, M.}, url = {https://arxiv.org/abs/1606.08826}, doi = {10.3847/0004-637X/827/1/7}, year = {2016}, date = {2016-11-01}, journal = {Astrophysical Journal}, volume = {827}, pages = {7}, abstract = {Using radiation magnetohydrodynamic simulations of the solar atmospheric layers from the upper convection zone to the lower corona, we investigate the self-consistent excitation of slow magneto-acoustic body waves (slow modes) in a magnetic flux concentration. We find that the convective downdrafts in the close surroundings of a two-dimensional flux slab łdquopumprdquo the plasma inside it in the downward direction. This action produces a downflow inside the flux slab, which encompasses ever higher layers, causing an upwardly propagating rarefaction wave. The slow mode, excited by the adiabatic compression of the downflow near the optical surface, travels along the magnetic field in the upward direction at the tube speed. It develops into a shock wave at chromospheric heights, where it dissipates, lifts the transition region, and produces an offspring in the form of a compressive wave that propagates further into the corona. In the wake of downflows and propagating shock waves, the atmosphere inside the flux slab in the chromosphere and higher tends to oscillate with a period of $nu$ ap 4 mHz. We conclude that this process of łdquomagnetic pumpingrdquo is a most plausible mechanism for the direct generation of longitudinal chromospheric and coronal compressive waves within magnetic flux concentrations, and it may provide an important heat source in the chromosphere. It may also be responsible for certain types of dynamic fibrils. }, keywords = {}, pubstate = {published}, tppubtype = {article} } Using radiation magnetohydrodynamic simulations of the solar atmospheric layers from the upper convection zone to the lower corona, we investigate the self-consistent excitation of slow magneto-acoustic body waves (slow modes) in a magnetic flux concentration. We find that the convective downdrafts in the close surroundings of a two-dimensional flux slab łdquopumprdquo the plasma inside it in the downward direction. This action produces a downflow inside the flux slab, which encompasses ever higher layers, causing an upwardly propagating rarefaction wave. The slow mode, excited by the adiabatic compression of the downflow near the optical surface, travels along the magnetic field in the upward direction at the tube speed. It develops into a shock wave at chromospheric heights, where it dissipates, lifts the transition region, and produces an offspring in the form of a compressive wave that propagates further into the corona. In the wake of downflows and propagating shock waves, the atmosphere inside the flux slab in the chromosphere and higher tends to oscillate with a period of $nu$ ap 4 mHz. We conclude that this process of łdquomagnetic pumpingrdquo is a most plausible mechanism for the direct generation of longitudinal chromospheric and coronal compressive waves within magnetic flux concentrations, and it may provide an important heat source in the chromosphere. It may also be responsible for certain types of dynamic fibrils. |
Stenflo, J ~O Transition of the Sunspot Number from Zurich to Brussels in 1980: A Personal Perspective Journal Article In: Solar Physics, 291 , pp. 2487-2492, 2016. @article{2016SoPh..291.2487S, title = {Transition of the Sunspot Number from Zurich to Brussels in 1980: A Personal Perspective}, author = {Stenflo, J.~O.}, url = {https://arxiv.org/abs/1512.06229}, doi = {10.1007/s11207-015-0837-2}, year = {2016}, date = {2016-11-01}, journal = {Solar Physics}, volume = {291}, pages = {2487-2492}, abstract = {The Swiss Federal Observatory, which had been founded in 1863 by Rudolf Wolf, was dissolved in connection with the retirement of Max Waldmeier in 1979. The determination of the Zurich sunpot number, which had been a cornerstone activity of the observatory, was then discontinued by ETH Zurich. A smooth transition of the responsibility for the sunspot number from Zurich to Brussels was achieved in 1980, however, through which it was possible to avoid a discontinuity in this important time series. Here we describe the circumstances that led to the termination in Zurich, how Brussels was chosen for the succession, and how the transfer was accomplished. }, keywords = {}, pubstate = {published}, tppubtype = {article} } The Swiss Federal Observatory, which had been founded in 1863 by Rudolf Wolf, was dissolved in connection with the retirement of Max Waldmeier in 1979. The determination of the Zurich sunpot number, which had been a cornerstone activity of the observatory, was then discontinued by ETH Zurich. A smooth transition of the responsibility for the sunspot number from Zurich to Brussels was achieved in 1980, however, through which it was possible to avoid a discontinuity in this important time series. Here we describe the circumstances that led to the termination in Zurich, how Brussels was chosen for the succession, and how the transfer was accomplished. |
Carlin, E S; Bianda, M The Key Role of Solar Dynamics in the Chromospheric Hanle Polarization Journal Article In: The Astrophysical Journal Letters, 831 (1), pp. L5, 2016. @article{2041-8205-831-1-L5, title = {The Key Role of Solar Dynamics in the Chromospheric Hanle Polarization}, author = {E. S. Carlin and M. Bianda}, url = {http://www.irsol.usi.ch/wp-content/uploads/2016/10/apjl_831_1_L5o-1.pdf}, year = {2016}, date = {2016-10-25}, journal = {The Astrophysical Journal Letters}, volume = {831}, number = {1}, pages = {L5}, abstract = {The quantum theory of polarized light allows one to model scattering in the solar atmosphere for inferring its properties. This powerful approach has revealed two key long-standing problems in solar physics: the puzzling dilemmas between theory and observations in several anomalously polarized spectral lines and the need for inferring the ubiquitous weak chromospheric magnetic fields, which requires discriminating the Hanle effect in dynamic optically thick plasmas. However, the ever-present dynamics, i.e., the temporal evolution of heatings and macroscopic motions, has been widely disregarded when modeling and interpreting the scattering polarization. This has hindered a consistent theoretical solution to the puzzle while falsifying the Hanle diagnosis. Here, we show that the dynamical evolution is a keystone for solving both problems because its systematic impact allows an explanation of the observations from “anomalous” instantaneous polarization signals. Evolution accounted for, we reproduce amplitudes and (spectral and spatial) shapes of the Ca i 4227 Å polarization at solar disk center, identifying a restrictive arrangement of magnetic fields, kinematics, heatings, and spatio-temporal resolution. We find that the joint action of dynamics, Hanle effect, and low temporal resolutions mimics Zeeman linear polarization profiles, the true weak-field Zeeman signals being negligible. Our results allow reinterpretation of many polarization signals of the solar spectra and support time-dependent scattering polarization as a powerful tool for deciphering the spatio-temporal distribution of chromospheric heatings and fields. This approach may be a key aid in developing the Hanle diagnosis for the solar atmosphere.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The quantum theory of polarized light allows one to model scattering in the solar atmosphere for inferring its properties. This powerful approach has revealed two key long-standing problems in solar physics: the puzzling dilemmas between theory and observations in several anomalously polarized spectral lines and the need for inferring the ubiquitous weak chromospheric magnetic fields, which requires discriminating the Hanle effect in dynamic optically thick plasmas. However, the ever-present dynamics, i.e., the temporal evolution of heatings and macroscopic motions, has been widely disregarded when modeling and interpreting the scattering polarization. This has hindered a consistent theoretical solution to the puzzle while falsifying the Hanle diagnosis. Here, we show that the dynamical evolution is a keystone for solving both problems because its systematic impact allows an explanation of the observations from “anomalous” instantaneous polarization signals. Evolution accounted for, we reproduce amplitudes and (spectral and spatial) shapes of the Ca i 4227 Å polarization at solar disk center, identifying a restrictive arrangement of magnetic fields, kinematics, heatings, and spatio-temporal resolution. We find that the joint action of dynamics, Hanle effect, and low temporal resolutions mimics Zeeman linear polarization profiles, the true weak-field Zeeman signals being negligible. Our results allow reinterpretation of many polarization signals of the solar spectra and support time-dependent scattering polarization as a powerful tool for deciphering the spatio-temporal distribution of chromospheric heatings and fields. This approach may be a key aid in developing the Hanle diagnosis for the solar atmosphere. |
Privitera, G; Meynet, G; Eggenberger, P; Vidotto, A A; Villaver, E; Bianda, M Star-planet interactions. II. Is planet engulfment the origin of fast rotating red giants? Journal Article In: Astronomy and Astrophysics, 593 , pp. A128, 2016. @article{2016A&A...593A.128P, title = {Star-planet interactions. II. Is planet engulfment the origin of fast rotating red giants?}, author = {Privitera, G. and Meynet, G. and Eggenberger, P. and Vidotto, A.A. and Villaver, E. and Bianda, M.}, url = {http://www.aanda.org/articles/aa/abs/2016/09/aa28758-16/aa28758-16.html}, doi = {10.1051/0004-6361/201628758}, year = {2016}, date = {2016-09-01}, journal = {Astronomy and Astrophysics}, volume = {593}, pages = {A128}, abstract = {Context. Fast rotating red giants in the upper part of the red giant branch have surface velocities that cannot be explained by single star evolution. Aims: We check whether tides between a star and a planet followed by planet engulfment can indeed accelerate the surface rotation of red giants for a sufficiently long time to produce these fast rotating red giants. Methods: We studied how the surface rotation velocity at the stellar surface evolves using rotating stellar models, accounting for the redistribution of the angular momentum inside the star by different transport mechanisms, the exchanges of angular momentum between the planet orbit and the star before the engulfment, and for the deposition of angular momentum inside the star at the engulfment. We considered different situations with masses of stars in the range between 1.5 and 2.5 M$_⊙$, masses of the planets between 1 and 15 M$_J$ (Jupiter mass), and initial semimajor axis between 0.5 and 1.5 au. The metallicity Z for our stellar models is 0.02. Results: We show that the surface velocities reached at the end of the orbital decay due to tidal forces and planet engulfment can be similar to values observed for fast rotating red giants. This surface velocity then decreases when the star evolves along the red giant branch but at a sufficiently slow pace to allowing stars to be detected with such a high velocity. More quantitatively, star-planet interaction can produce a rapid acceleration of the surface of the star, above values equal to 8 km s$^-1$, for periods lasting up to more than 30% the red giant branch phase. As found already by previous works, the changes of the surface carbon isotopic ratios produced by the dilution of the planetary material into the convective envelope is modest. The increase of the lithium abundance due to this effect might be much more important, however lithium may be affected by many different, still uncertain, processes. Thus any lithium measurement can hardly be taken as a support or argument against any star-planet interaction. Conclusions: The acceleration of the stellar surface to rotation velocities above limits that depend on the surface gravity does appear at the moment to be the clearest signature of a star-planet interaction.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Context. Fast rotating red giants in the upper part of the red giant branch have surface velocities that cannot be explained by single star evolution. <BR /> Aims: We check whether tides between a star and a planet followed by planet engulfment can indeed accelerate the surface rotation of red giants for a sufficiently long time to produce these fast rotating red giants. <BR /> Methods: We studied how the surface rotation velocity at the stellar surface evolves using rotating stellar models, accounting for the redistribution of the angular momentum inside the star by different transport mechanisms, the exchanges of angular momentum between the planet orbit and the star before the engulfment, and for the deposition of angular momentum inside the star at the engulfment. We considered different situations with masses of stars in the range between 1.5 and 2.5 M$_⊙$, masses of the planets between 1 and 15 M$_J$ (Jupiter mass), and initial semimajor axis between 0.5 and 1.5 au. The metallicity Z for our stellar models is 0.02. <BR /> Results: We show that the surface velocities reached at the end of the orbital decay due to tidal forces and planet engulfment can be similar to values observed for fast rotating red giants. This surface velocity then decreases when the star evolves along the red giant branch but at a sufficiently slow pace to allowing stars to be detected with such a high velocity. More quantitatively, star-planet interaction can produce a rapid acceleration of the surface of the star, above values equal to 8 km s$^-1$, for periods lasting up to more than 30% the red giant branch phase. As found already by previous works, the changes of the surface carbon isotopic ratios produced by the dilution of the planetary material into the convective envelope is modest. The increase of the lithium abundance due to this effect might be much more important, however lithium may be affected by many different, still uncertain, processes. Thus any lithium measurement can hardly be taken as a support or argument against any star-planet interaction. <BR /> Conclusions: The acceleration of the stellar surface to rotation velocities above limits that depend on the surface gravity does appear at the moment to be the clearest signature of a star-planet interaction. |
Istituto Ricerche Solari Locarno
