Refereed Scientific Papers

@article{2022A&A...666A..21Q,

title = {The European Solar Telescope},

author = {C., Quintero Noda and and 280 co-Authors},

doi = {10.1051/0004-6361/202243867},

year  = {2022},

date = {2022-10-01},

urldate = {2022-10-01},

journal = {Astronomy and Astrophysics},

volume = {666},

pages = {A21},

abstract = {The European Solar Telescope (EST) is a project aimed at studying the magnetic connectivity of the solar atmosphere, from the deep photosphere to the upper chromosphere. Its design combines the knowledge and expertise gathered by the European solar physics community during the construction and operation of state-of-the-art solar telescopes operating in visible and near-infrared wavelengths: the Swedish 1m Solar Telescope, the German Vacuum Tower Telescope and GREGOR, the French Télescope Héliographique pour l'Étude du Magnétisme et des Instabilités Solaires, and the Dutch Open Telescope. With its 4.2 m primary mirror and an open configuration, EST will become the most powerful European ground-based facility to study the Sun in the coming decades in the visible and near-infrared bands. EST uses the most innovative technological advances: the first adaptive secondary mirror ever used in a solar telescope, a complex multi-conjugate adaptive optics with deformable mirrors that form part of the optical design in a natural way, a polarimetrically compensated telescope design that eliminates the complex temporal variation and wavelength dependence of the telescope Mueller matrix, and an instrument suite containing several (etalon-based) tunable imaging spectropolarimeters and several integral field unit spectropolarimeters. This publication summarises some fundamental science questions that can be addressed with the telescope, together with a complete description of its major subsystems. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2022ApJ...936...67R,

title = {Quiet Sun Center to Limb Variation of the Linear Polarization Observed by CLASP2 Across the Mg II h and k Lines},

author = {L. A. Rachmeler and J. Trujillo Bueno and D. E. McKenzie and R. Ishikawa and F. Auch`ere and K. Kobayashi and R. Kano and T. J. Okamoto and C. W. Bethge and D. Song and E. Alsina Ballester and L. Belluzzi and T. del Pino Alemán and A. Asensio Ramos and M. Yoshida and T. Shimizu and A. Winebarger and A. R. Kobelski and G. D. Vigil and B. De Pontieu and N. Narukage and M. Kubo and T. Sakao and H. Hara and Y. Suematsu and J. Štěpán and M. Carlsson and J. Leenaarts},

doi = {10.3847/1538-4357/ac83b8},

year  = {2022},

date = {2022-09-01},

urldate = {2022-09-01},

journal = {Astrophysical Journal},

volume = {936},

number = {1},

pages = {67},

abstract = {The CLASP2 (Chromospheric LAyer Spectro-Polarimeter 2) sounding rocket mission was launched on 2019 April 11. CLASP2 measured the four Stokes parameters of the Mg II h and k spectral region around 2800 Å along a 200″ slit at three locations on the solar disk, achieving the first spatially and spectrally resolved observations of the solar polarization in this near-ultraviolet region. The focus of the work presented here is the center-to-limb variation of the linear polarization across these resonance lines, which is produced by the scattering of anisotropic radiation in the solar atmosphere. The linear polarization signals of the Mg II h and k lines are sensitive to the magnetic field from the low to the upper chromosphere through the Hanle and magneto-optical effects. We compare the observations to theoretical predictions from radiative transfer calculations in unmagnetized semiempirical models, arguing that magnetic fields and horizontal inhomogeneities are needed to explain the observed polarization signals and spatial variations. This comparison is an important step in both validating and refining our understanding of the physical origin of these polarization signatures, and also in paving the way toward future space telescopes for probing the magnetic fields of the solar upper atmosphere via ultraviolet spectropolarimetry.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2022A&A...664A..24C,

title = {Toward fully compressible numerical simulations of stellar magneto-convection with the RAMSES code},

author = {Canivete Cuissa, J. R. and Teyssier, R.},

doi = {10.1051/0004-6361/202142754},

year  = {2022},

date = {2022-08-01},

urldate = {2022-08-01},

journal = {Astronomy and Astrophysics},

volume = {664},

pages = {A24},

abstract = {Context. Numerical simulations of magneto-convection have greatly expanded our understanding of stellar interiors and stellar magnetism. Recently, fully compressible hydrodynamical simulations of full-star models have demonstrated the feasibility of studying the excitation and propagation of pressure and internal gravity waves in stellar interiors, which would allow for a direct comparison with asteroseismological measurements. However, the impact of magnetic fields on such waves has not been taken into account yet in three-dimensional simulations.

Aims: We conduct a proof of concept for the realization of three-dimensional, fully compressible, magneto-hydrodynamical numerical simulations of stellar interiors with the RAMSES code.

Methods: We adapted the RAMSES code to deal with highly subsonic turbulence, typical of stellar convection, by implementing a well-balanced scheme in the numerical solver. We then ran and analyzed three-dimensional hydrodynamical and magneto-hydrodynamical simulations with different resolutions of a plane-parallel convective envelope on a Cartesian grid.

Results: Both hydrodynamical and magneto-hydrodynamical simulations develop a quasi-steady, turbulent convection layer from random density perturbations introduced over the initial profiles. The convective flows are characterized by small-amplitude fluctuations around the hydrodynamical equilibrium of the stellar interior, which is preserved over the whole simulation time. Using our compressible well-balanced scheme, we were able to model flows with Mach numbers as low as ℳ ∼ 10−3, but even lower Mach number flows are possible in principle. In the magneto-hydrodynamical runs, we observe an exponential growth of magnetic energy consistent with the action of a small-scale dynamo. The weak seed magnetic fields are amplified to mean strengths of 37% relative to the kinetic equipartition value in the highest resolution simulation. Since we chose a compressible approach, we see imprints of pressure and internal gravity waves propagating in the stable regions above and beneath the convection zone. In the magneto-hydrodynamical case, we measured a deficit in acoustic and internal gravity wave power with respect to the purely hydrodynamical counterpart of 16% and 13%, respectively.

Conclusions: The well-balanced scheme implemented in RAMSES allowed us to accurately simulate the small-amplitude, turbulent fluctuations of stellar (magneto-)convection. The qualitative properties of the convective flows, magnetic fields, and excited waves are in agreement with previous studies in the literature. The power spectra, profiles, and probability density functions of the main quantities converge with resolution. Therefore, we consider the proof of concept to be successful. The deficit of acoustic power in the magneto-hydrodynamical simulation shows that magnetic fields must be included in the study of pressure waves in stellar interiors. We conclude by discussing future developments. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2022A&A...664A.197B,

title = {Numerical solutions to linear transfer problems of polarized radiation. III. Parallel preconditioned Krylov solver tailored for modeling PRD effects},

author = {Pietro Benedusi and Gioele Janett and Simone Riva and Rolf Krause and Luca Belluzzi},

doi = {10.1051/0004-6361/202243059},

year  = {2022},

date = {2022-08-01},

urldate = {2022-08-01},

journal = {Astronomy and Astrophysics},

volume = {664},

pages = {A197},

abstract = {Context. The polarization signals produced by the scattering of anistropic radiation in strong resonance lines encode important information about the elusive magnetic fields in the outer layers of the solar atmosphere. An accurate modeling of these signals is a very challenging problem from the computational point of view, in particular when partial frequency redistribution (PRD) effects in scattering processes are accounted for with a general angle-dependent treatment.

Aims: We aim at solving the radiative transfer problem for polarized radiation in nonlocal thermodynamic equilibrium conditions, taking angle-dependent PRD effects into account. The problem is formulated for a two-level atomic model in the presence of arbitrary magnetic and bulk velocity fields. The polarization produced by scattering processes and the Zeeman effect is considered.

Methods: The proposed solution strategy is based on an algebraic formulation of the problem and relies on a convenient physical assumption, which allows its linearization. We applied a nested matrix-free GMRES iterative method. Effective preconditioning is obtained in a multifidelity framework by considering the light-weight description of scattering processes in the limit of complete frequency redistribution (CRD).

Results: Numerical experiments for a one-dimensional (1D) atmospheric model show near optimal strong and weak scaling of the proposed CRD-preconditioned GMRES method, which converges in few iterations, independently of the discretization parameters. A suitable parallelization strategy and high-performance computing tools lead to competitive run times, providing accurate solutions in a few minutes.

Conclusions: The proposed solution strategy allows the fast systematic modeling of the scattering polarization signals of strong resonance lines, taking angle-dependent PRD effects into account together with the impact of arbitrary magnetic and bulk velocity fields. Almost optimal strong and weak scaling results suggest that this strategy is applicable to realistic 3D models. Moreover, the proposed strategy is general, and applications to more complex atomic models are possible.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2022A&A...664A..76A,

title = {The transfer of polarized radiation in resonance lines with partial frequency redistribution, J-state interference, and arbitrary magnetic fields. A radiative transfer code and useful approximations},

author = {E. Alsina Ballester and L. Belluzzi and J. Trujillo Bueno},

doi = {10.1051/0004-6361/202142934},

year  = {2022},

date = {2022-08-01},

urldate = {2022-08-01},

journal = {Astronomy and Astrophysics},

volume = {664},

pages = {A76},

abstract = {Aims: We present the theoretical framework and numerical methods we have implemented to solve the problem of the generation and transfer of polarized radiation in spectral lines without assuming local thermodynamical equilibrium, while accounting for scattering polarization, partial frequency redistribution (due to both the Doppler effect and elastic collisions), J-state interference, and hyperfine structure. The resulting radiative transfer code allows one to model the impact of magnetic fields of an arbitrary strength and orientation through the Hanle, incomplete Paschen-Back, and magneto-optical effects. We also evaluate the suitability of a series of approximations for modeling the scattering polarization in the wings of strong resonance lines at a much lower computational cost, which is particularly valuable for the numerically intensive case of three-dimensional radiative transfer.

Methods: We examine the suitability of the considered approximations by using our radiative transfer code to model the Stokes profiles of the Mg II h & k lines and of the H I Lyman-α line in magnetized one-dimensional models of the solar atmosphere.

Results: Neglecting Doppler redistribution in the scattering processes that are unperturbed by elastic collisions (i.e., treating them as coherent in the observer's frame) produces a negligible error in the scattering polarization wings of the Mg II resonance lines and a minor one in the Lyman-α wings, although it is unsuitable to model the cores of these lines. For both lines, the scattering processes that are perturbed by elastic collisions only give a significant contribution to the intensity component of the emissivity. Neglecting collisional as well as Doppler redistribution (so that all scattering processes are coherent) represents a rough but suitable approximation for the wings of the Mg II resonance lines, but a very poor one for the Lyman-α wings. The magnetic sensitivity in the scattering polarization wings of the considered lines can be modeled by accounting for the magnetic field in only the ηI and ρV coefficients of the Stokes-vector transfer equation (i.e., using the zero-field expression for the emissivity).},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2022A&A...662A..46Z,

title = {Hanle rotation signatures in Sr I 4607 Å},

author = {F. Zeuner and L. Belluzzi and N. Guerreiro and R. Ramelli and M. Bianda},

doi = {10.1051/0004-6361/202243350},

year  = {2022},

date = {2022-06-01},

urldate = {2022-06-01},

journal = {Astronomy and Astrophysics},

volume = {662},

pages = {A46},

abstract = { Context. Measuring small-scale magnetic fields and constraining their role in energy transport and dynamics in the solar atmosphere are crucial, albeit challenging, tasks in solar physics. To this aim, observations of scattering polarization and the Hanle effect in various spectral lines are increasingly used to complement traditional magnetic field determination techniques.

Aims: One of the strongest scattering polarization signals in the photosphere is measured in the Sr I line at 4607.3 Å when observed close to the solar limb. Here, we present the first observational evidence of Hanle rotation in the linearly polarized spectrum of this line at several limb distances.

Methods: We used the Zurich IMaging POLarimeter, ZIMPOL at the IRSOL observatory, with exceptionally good seeing conditions and long integration times. We combined the fast-modulating polarimeter with a slow modulator installed in front of the telescope. This combination allows for a high level of precision and unprecedented accuracy in the measurement of spectropolarimetric data.

Results: Fixing the reference direction for positive Stokes Q parallel to the limb, we detected singly peaked U/I signals well above the noise level. We can exclude any instrumental origins for such U/I signals. These signatures are exclusively found in the Sr I line, but not in the adjoining Fe I line, therefore eliminating the Zeeman effect as the mechanism responsible for their appearance. However, we find a clear spatial correlation between the circular polarization produced by the Zeeman effect and the U/I amplitudes. This suggests that the detected U/I signals are the signatures of Hanle rotation caused by a spatially resolved magnetic field.

Conclusions: A novel measurement technique allows for determining the absolute level of polarization with unprecedented precision. Using this technique, high-precision spectropolarimetric observations reveal, for the first time, unambiguous U/I signals attributed to Hanle rotation in the Sr I line.

},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{refId0k,

title = {Methodology for estimating the magnetic Prandtl number and application to solar surface small-scale dynamo simulations},

author = {F. Riva and O. Steiner},

url = {https://doi.org/10.1051/0004-6361/202142644},

doi = {10.1051/0004-6361/202142644},

year  = {2022},

date = {2022-04-23},

urldate = {2022-04-23},

journal = {Astronomy and Astrophysics},

volume = {660},

pages = {A115},

abstract = {Context. A crucial step in the numerical investigation of small-scale dynamos in the solar atmosphere consists of an accurate determination of the magnetic Prandtl number, Prm, stemming from radiative magneto-hydrodynamic (MHD) simulations.

Aims: The aims are to provide a reliable methodology for estimating the effective Reynolds and magnetic Reynolds numbers, Re and Rem, and their ratio Prm = Rem/Re (the magnetic Prandlt number), that characterise MHD simulations and to categorise small-scale dynamo simulations in terms of these dimensionless parameters.

Methods: The methodology proposed for computing Re and Rem is based on the method of projection on proper elements and it relies on a post-processing step carried out using higher order accurate numerical operators than the ones in the simulation code. A number of radiative MHD simulations with different effective viscosities and plasma resistivities were carried out with the CO5BOLD code, and the resulting growth rate of the magnetic energy and saturated magnetic field strengths were characterised in terms of Re and Rem.

Results: Overall, the proposed methodology provides a solid estimate of the dissipation coefficients affecting the momentum and induction equations of MHD simulation codes, and consequently also a reliable evaluation of the magnetic Prandtl number characterising the numerical results. Additionally, it is found that small-scale dynamos are active and can amplify a small seed magnetic field up to significant values in CO5BOLD simulations with a grid spacing smaller than h = 12 km, even at Prm ≃ 0.65. However, it is also evident that it is difficult to categorise dynamo simulations in terms of Prm alone, because it is not only important to estimate the amplitude of the dissipation coefficients, but also at which scales energy dissipation takes place.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2022A&A...659A.179J,

title = {Spectropolarimetric observations of the solar atmosphere in the Halpha 6563 Å line},

author = {Jaume Bestard, J. and Trujillo Bueno, J. and M. Bianda and J. Štěpán and R. Ramelli},

doi = {10.1051/0004-6361/202141834},

year  = {2022},

date = {2022-03-01},

urldate = {2022-03-01},

journal = {Astronomy and Astrophysics},

volume = {659},

pages = {A179},

abstract = {We present novel spectropolarimetric observations of the hydrogen Hα line taken with the Zürich Imaging Polarimeter (ZIMPOL) at the Gregory Coudé Telescope of the Istituto Ricerche Solari Locarno (IRSOL). The linear polarization is clearly dominated by the scattering of anisotropic radiation and the Hanle effect, while the circular polarization is dominated by the Zeeman effect. The observed linear polarization signals show a rich spatial variability, the interpretation of which would open a new window for probing the solar chromosphere. We study their spatial variation within coronal holes, finding a different behaviour for the U/I signals near the north and south solar poles. We identify some spatial patterns, which may facilitate the interpretation of the observations. In close-to-the-limb regions with sizable circular polarization signals, we find similar asymmetric Q/I profiles. We also show examples of net circular polarization profiles (NCP), along with the corresponding linear polarization signals. The application of the weak field approximation to the observed circular polarization signals gives 10 G (40-60 G) close to the limb quiet (plage) regions for the average longitudinal field strength over the spatio-temporal resolution element. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{MARASSI2021,

title = {Trieste CALLISTO Station Setup and Observations of Solar Radio Bursts},

author = {Alessandro Marassi and Christian Monstein},

url = {https://www.sciencedirect.com/science/article/pii/S0273117721009704},

doi = {10.1016/j.asr.2021.12.043},

issn = {0273-1177},

year  = {2021},

date = {2021-12-31},

urldate = {2021-01-01},

journal = {Advances in Space Research},

abstract = {The Trieste CALLISTO station (http://radiosun.oats.inaf.it) was established in 2012 at the Basovizza Observing Station (45°38'37” N, 13°52'34 E”, 398m above MSL) operated by the Italian National Institute for Astrophysics (INAF) - Astronomical Observatory of Trieste (Italy) to study solar radio bursts and the response of the Earth’s ionosphere and geomagnetic field. To date, three ‘Compound Astronomical Low-cost Low frequency Instrument for Spectroscopy and Transportable Observatory’ (CALLISTO) spectrometers have been installed, with the capability of observing in the frequency ranges 45-80 MHz (from 30 December 2014), 220-420 MHz (from 1 June 2012 to 23 October 2012 and from 05 October 2013), 905-1730 MHz (from 30 December 2019). The three receivers are fed respectively by a dipole, log-periodic and cross-dipole antenna. Nominally, frequency spectra are obtained with 4 sweeps per second over in total 600 channels. Here, we describe the Trieste CALLISTO station set-up, the local e-Callisto network digital archive, Trieste CALLISTO Radio Bursts Detection and Visualization System available via web and present dynamic spectra of a sample of Type I, II, III, IV and V radio bursts. As an additional feature, we show also its capability to record lightning strikes.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{angeo-39-945-2021,

title = {Space weather study through analysis of solar radio bursts detected by a single-station CALLISTO spectrometer},

author = {T. Ndacyayisenga and A. C. Umuhire and J. Uwamahoro and C. Monstein},

url = {https://angeo.copernicus.org/articles/39/945/2021/},

doi = {10.5194/angeo-39-945-2021},

year  = {2021},

date = {2021-10-29},

urldate = {2021-01-01},

journal = {Annales Geophysicae},

volume = {39},

number = {5},

pages = {945–959},

abstract = {This article summarises the results of an analysis of solar radio bursts (SRBs) detected by the Compound Astronomical Low-cost Low-frequency Instrument for Spectroscopy and Transportable Observatory (CALLISTO) spectrometer hosted by the University of Rwanda. The data analysed were detected during the first year (2014–2015) of the instrument operation. Using quick plots provided by the e-CALLISTO website, a total of 201 intense and well-separated solar radio bursts detected by the CALLISTO station located in Rwanda, are found consisting of 4 type II, 175 type III and 22 type IV radio bursts. It is found that all analysed type II and ∼ 37 % of type III bursts are associated with impulsive solar flares, while the minority (∼ 13 %) of type IV radio bursts are associated with solar flares. Furthermore, all type II radio bursts are associated with coronal mass ejections (CMEs), ∼ 44 % of type III bursts are associated with CMEs, and the majority (∼ 82 %) of type IV bursts were accompanied by CMEs. With aid of the atmospheric imaging assembly (AIA) images on board the Solar Dynamics Observatory (SDO), the location of open magnetic field lines of non-flare-associated type III radio bursts are shown. The same images are used to show the magnetic loops in the solar corona for type IV radio bursts observed in the absence of solar flares and/or CMEs. Findings from this study indicate that analysis of SRBs that are observed from the ground can provide a significant contribution to the early diagnosis of solar transients phenomena, such as solar flares and CMEs, which are major drivers of potential space weather hazards.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{article,

title = {Reconstruction of the Sunspot Number Source Database and the 1947 Zurich Discontinuity},

author = {Frédéric Clette and Laure Lefèvre and Sabrina Bechet and Renzo Ramelli and Marco Cagnotti},

doi = {10.1007/s11207-021-01882-6},

year  = {2021},

date = {2021-09-15},

urldate = {2021-01-01},

journal = {Solar Physics},

volume = {296},

pages = {137},

abstract = {The recalibration of the sunspot number series, the primary long-term record of the solar cycle, requires the recovery of the entire collection of raw sunspot counts collected by the Zurich Observatory for the production of this index between 1849 and 1980.



Here, we report about the major progresses accomplished recently in the construction of this global digital sunspot number database, and we derive global statistics of all the individual observers and professional observatories who provided sunspot data over more than 130 years.



First, we can announce the full recovery of long-lost source-data tables covering the last 34 years between 1945 and 1979, and we describe the unique information available in those tables. We then also retrace the evolution of the core observing team in Zurich and of the auxiliary stations. In 1947, we find a major disruption in the composition of both the Zurich team and the international network of auxiliary stations.



This sharp transition is unique in the history of the Zurich Observatory and coincides with the main scale-jump found in the original Zurich sunspot number series, the so-called “Waldmeier” jump. This adds key historical evidence explaining why methodological changes introduced progressively in the early 20th century could play a role precisely at that time. We conclude on the remaining steps needed to fully complete this new sunspot data resource.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2021JPCA..125.8132L,

title = {Measurement of Raman Optical Activity with High-Frequency Polarization Modulation},

author = {Carin R. Lightner and Daniel Gisler and Stefan A. Meyer and Hannah Niese and Robert C. Keitel and David J. Norris},

doi = {10.1021/acs.jpca.1c06132},

year  = {2021},

date = {2021-09-01},

urldate = {2021-09-01},

journal = {Journal of Physical Chemistry A},

volume = {125},

number = {36},

pages = {8132-8139},

abstract = {Many chiroptical spectroscopic techniques have been developed to detect chirality in molecular species and probe its role in biological processes. Raman optical activity (ROA) should be one of the most powerful methods, as ROA yields vibrational and chirality information simultaneously and can measure analytes in aqueous and biologically relevant solvents. However, despite its promise, the use of ROA has been limited, largely due to challenges in instrumentation. Here, we report a new approach to ROA that exploits high-frequency polarization modulation. High-frequency polarization modulation, usually implemented with a photoelastic modulator (PEM), has long been the standard technique in other chiroptical spectroscopies. Unfortunately, the need for simultaneous spectral and polarization resolution has precluded the use of PEMs in ROA instruments. We combine a specialized camera system (the Zurich imaging polarimeter, or ZIMPOL) with PEM modulation to perform ROA measurements. We demonstrate performance similar to the current standard in ROA instrumentation while reducing complexity and polarization artifacts. This development should aid researchers in exploiting the full potential of ROA for chemical and biological analysis.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2021PhRvL.127h1101A,

title = {Solving the Paradox of the Solar Sodium D$_1$ Line Polarization},

author = {Alsina Ballester. Ernest and Luca Belluzzi and Trujillo Bueno, Javier},

doi = {10.1103/PhysRevLett.127.081101},

year  = {2021},

date = {2021-08-01},

urldate = {2021-08-01},

journal = {Physical Review Letters},

volume = {127},

number = {8},

pages = {081101},

abstract = {Twenty-five years ago, enigmatic linear polarization signals were 

 discovered in the core of the sodium D$_1$ line. The only 

 explanation that could be found implied that the solar 

 chromosphere is practically unmagnetized, in contradiction with 

 other evidences. This opened a paradox that has challenged 

 physicists for many years. Here we present its solution, 

 demonstrating that these polarization signals can be properly 

 explained in the presence of magnetic fields in the gauss range. 

 This result opens a novel diagnostic window for exploring the 

 elusive magnetism of the solar chromosphere.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2021ExA...tmp...95P,

title = {Magnetic imaging of the outer solar atmosphere (MImOSA)},

author = {H. Peter and E., Alsina Ballester and V. Andretta and F. Auch`ere and L. Belluzzi and A. Bemporad and D. Berghmans and E. Buchlin and A. Calcines and L. P. Chitta and K. Dalmasse and T. del Pino Alemán and A. Feller and C. Froment and R. Harrison and M. Janvier and S. Matthews and S. Parenti and D. Przybylski and S. K. Solanki and J. Štěpán and L. Teriaca and J. Trujillo Bueno},

doi = {10.1007/s10686-021-09774-0},

year  = {2021},

date = {2021-08-01},

urldate = {2021-08-01},

journal = {Experimental Astronomy},

abstract = {The magnetic activity of the Sun directly impacts the Earth and human 

 life. Likewise, other stars will have an impact on the 

 habitability of planets orbiting these host stars. Although the 

 magnetic field at the surface of the Sun is reasonably well 

 characterised by observations, the information on the magnetic 

 field in the higher atmospheric layers is mainly indirect. This 

 lack of information hampers our progress in understanding solar 

 magnetic activity. Overcoming this limitation would allow us to 

 address four paramount long-standing questions: (1) How does the 

 magnetic field couple the different layers of the atmosphere, 

 and how does it transport energy? (2) How does the magnetic 

 field structure, drive and interact with the plasma in the 

 chromosphere and upper atmosphere? (3) How does the magnetic 

 field destabilise the outer solar atmosphere and thus affect the 

 interplanetary environment? (4) How do magnetic processes 

 accelerate particles to high energies? New ground-breaking 

 observations are needed to address these science questions. We 

 suggest a suite of three instruments that far exceed current 

 capabilities in terms of spatial resolution, light-gathering 

 power, and polarimetric performance: (a) A large-aperture UV-to- 

 IR telescope of the 1-3 m class aimed mainly to measure the 

 magnetic field in the chromosphere by combining high spatial 

 resolution and high sensitivity. (b) An extreme-UV-to-IR 

 coronagraph that is designed to measure the large-scale magnetic 

 field in the corona with an aperture of about 40 cm. (c) An 

 extreme-UV imaging polarimeter based on a 30 cm telescope that 

 combines high throughput in the extreme UV with polarimetry to 

 connect the magnetic measurements of the other two instruments. 

 Placed in a near-Earth orbit, the data downlink would be 

 maximised, while a location at L4 or L5 would provide 

 stereoscopic observations of the Sun in combination with Earth- 

 based observatories. This mission to measure the magnetic field 

 will finally unlock the driver of the dynamics in the outer 

 solar atmosphere and thereby will greatly advance our 

 understanding of the Sun and the heliosphere.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2021arXiv210500411K,

title = {A Transient Coronal Sigmoid in Active Region NOAA 11909: Build-up Phase, M-class Eruptive Flare, and Associated Fast Coronal Mass Ejection},

author = {Hema {Kharayat} and Bhuwan {Joshi} and Prabir K {Mitra} and P ~K {Manoharan} and Christian {Monstein}},

url = {https://ui.adsabs.harvard.edu/link_gateway/2021arXiv210500411K/EPRINT_PDF},

doi = {10.1007/s11207-021-01830-4},

year  = {2021},

date = {2021-06-22},

journal = {Solar Physics},

volume = {296},

pages = {99},

abstract = {In this article, we investigate the formation and disruption of a 

 coronal sigmoid from the active region (AR) NOAA 11909 on 07 

 December 2013, by analyzing multi-wavelength and multi- 

 instrument observations. Our analysis suggests that the 

 formation of `transient' sigmoid initiated $approx$1 hour 

 before its eruption through a coupling between two twisted 

 coronal loop systems. A comparison between coronal and 

 photospheric images suggests that the coronal sigmoid was formed 

 over a simple $beta$-type AR which also possessed dispersed 

 magnetic field structure in the photosphere. The line-of-sight 

 photospheric magnetograms also reveal moving magnetic features, 

 small-scale flux cancellation events near the PIL, and overall 

 flux cancellation during the extended pre-eruption phase which 

 suggest the role of tether-cutting reconnection toward the 

 build-up of the flux rope. The disruption of the sigmoid 

 proceeded with a two-ribbon eruptive M1.2 flare 

 (SOL2013-12-07T07:29). In radio frequencies, we observe type III 

 and type II bursts in meter wavelengths during the impulsive 

 phase of the flare. The successful eruption of the flux rope 

 leads to a fast coronal mass ejection (with a linear speed of 

 $approx$1085 km s -1 ) in SOHO/LASCO field-of-view. During the 

 evolution of the flare, we clearly observe typical ``sigmoid-to- 

 arcade'' transformation. Prior to the onset of the impulsive 

 phase of the flare, flux rope undergoes a slow rise ($approx$15 

 km s -1 ) which subsequently transitions into a fast eruption 

 ($approx$110 km s -1 ). The two-phase evolution of the flux 

 rope shows temporal associations with the soft X-ray precursor 

 and impulsive phase emissions of the M-class flare, 

 respectively, thus pointing toward a feedback relationship 

 between magnetic reconnection and early CME dynamics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2021SoPh..296...85J,

title = {Two-Stage Evolution of an Extended C-Class Eruptive Flaring Activity from Sigmoid Active Region NOAA 12734: SDO and Udaipur-CALLISTO Observations},

author = {Bhuwan {Joshi} and Prabir K {Mitra} and R {Bhattacharyya} and Kushagra {Upadhyay} and Divya {Oberoi} and K {Sasikumar Raja} and Christian {Monstein}},

doi = {10.1007/s11207-021-01820-6},

year  = {2021},

date = {2021-06-03},

journal = {Solar Physics},

volume = {296},

number = {6},

pages = {85},

abstract = {In this article, we present a multi-wavelength investigation of a 

 C-class flaring activity that occurred in the active region NOAA 

 12734 on 8 March 2019. The investigation utilizes data from the 

 Atmospheric Imaging Assembly (AIA) and the Helioseismic Magnetic 

 Imager (HMI) on board the Solar Dynamics Observatory (SDO) and 

 the Udaipur-CALLISTO solar radio spectrograph of the Physical 

 Research Laboratory. This low intensity C1.3 event is 

 characterized by typical features of a long-duration event 

 (LDE), viz. extended flare arcade, large-scale two-ribbon 

 structures and twin coronal dimmings. The eruptive event 

 occurred in a coronal sigmoid and displayed two distinct stages 

 of energy release, manifested in terms of temporal and spatial 

 evolution. The formation of twin-dimming regions are consistent 

 with the eruption of a large flux rope with footpoints lying in 

 the western and eastern edges of the coronal sigmoid. The metric 

 radio observations obtained from Udaipur-CALLISTO reveals a 

 broad-band (ensuremathapprox50 -180 MHz), stationary 

 plasma emission for ensuremathapprox7 min during the 

 second stage of the flaring activity that resemble a type IV 

 radio burst. A type III decametre-hectometre radio bursts with 

 starting frequency of ensuremathapprox2.5 MHz precedes the 

 stationary type IV burst observed by Udaipur-CALLISTO by 

 ensuremathapprox5 min. The synthesis of multi-wavelength 

 observations and non-linear force-free field (NLFFF) coronal 

 modeling together with magnetic decay index analysis suggest 

 that the sigmoid flux rope underwent a zipping-like uprooting 

 from its western to eastern footpoints in response to the 

 overlying asymmetric magnetic field confinement. The 

 asymmetrical eruption of the flux rope also accounts for the 

 observed large-scale structures viz. apparent eastward shift of 

 flare ribbons and post-flare loops along the polarity inversion 

 line (PIL), and provides evidence for lateral progression of 

 magnetic reconnection site as the eruption proceeds.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2021ApJ...913...99K,

title = {Coronal Conditions for the Occurrence of Type II Radio Bursts},

author = {Athanasios {Kouloumvakos} and Alexis {Rouillard} and Alexander {Warmuth} and Jasmina {Magdalenic} and Immanuel. C {Jebaraj} and Gottfried {Mann} and Rami {Vainio} and Christian {Monstein}},

doi = {10.3847/1538-4357/abf435},

year  = {2021},

date = {2021-05-28},

journal = {Astrophysical Journal},

volume = {913},

number = {2},

pages = {99},

abstract = {Type II radio bursts are generally observed in association with flare- 

 generated or coronal-mass-ejection-driven shock waves. The exact 

 shock and coronal conditions necessary for the production of 

 type II radio emission are still under debate. Shock waves are 

 important for the acceleration of electrons necessary for the 

 generation of the radio emission. Additionally, the shock 

 geometry and closed field line topology, e.g., quasi- 

 perpendicular shock regions or shocks interacting with 

 streamers, play an important role for the production of the 

 emission. In this study we perform a 3D reconstruction and 

 modeling of a shock wave observed during the 2014 November 5 

 solar event. We determine the spatial and temporal evolution of 

 the shock properties and examine the conditions responsible for 

 the generation and evolution of type II radio emission. Our 

 results suggest that the formation and evolution of a strong, 

 supercritical, quasi-perpendicular shock wave interacting with a 

 coronal streamer were responsible for producing type II radio 

 emission. We find that the shock wave is subcritical before and 

 supercritical after the start of the type II emission. The shock 

 geometry is mostly quasi-perpendicular throughout the event. Our 

 analysis shows that the radio emission is produced in regions 

 where the supercritical shock develops with an oblique to quasi- 

 perpendicular geometry.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{refId0j,

title = {The Alfvénic nature of chromospheric swirls},

author = {Andrea Francesco {Battaglia} and José Roberto {Canivete Cuissa} and Flavio {Calvo} and Aleksi Antoine {Bossart} and Oskar {Steiner}},

url = {https://doi.org/10.1051/0004-6361/202040110

https://arxiv.org/abs/2103.07366},

doi = {10.1051/0004-6361/202040110},

year  = {2021},

date = {2021-05-26},

journal = {A&A},

volume = {649},

pages = {A121},

abstract = {Context. Observations show that small-scale vortical plasma motions are ubiquitous in the quiet solar atmosphere. They have received increasing attention in recent years because they are a viable candidate mechanism for the heating of the outer solar atmospheric layers. However, the true nature and the origin of these swirls, and their effective role in the energy transport, are still unclear.



Aims. We investigate the evolution and origin of chromospheric swirls by analyzing numerical simulations of the quiet solar atmosphere. In particular, we are interested in finding their relation with magnetic field perturbations and in the processes driving their evolution.



Methods. The radiative magnetohydrodynamic code CO5BOLD is used to perform realistic numerical simulations of a small portion of the solar atmosphere, ranging from the top layers of the convection zone to the middle chromosphere. For the analysis, the swirling strength criterion and its evolution equation are applied in order to identify vortical motions and to study their dynamics. As a new criterion, we introduce the magnetic swirling strength, which allows us to recognize torsional perturbations in the magnetic field.



Results. We find a strong correlation between swirling strength and magnetic swirling strength, in particular in intense magnetic flux concentrations, which suggests a tight relation between vortical motions and torsional magnetic field perturbations. Furthermore, we find that swirls propagate upward with the local Alfvén speed as unidirectional swirls driven by magnetic tension forces alone. In the photosphere and low chromosphere, the rotation of the plasma co-occurs with a twist in the upwardly directed magnetic field that is in the opposite direction of the plasma flow. All together, these are clear characteristics of torsional Alfvén waves. Yet, the Alfvén wave is not oscillatory but takes the form of a unidirectional pulse. The novelty of the present work is that these Alfvén pulses naturally emerge from realistic numerical simulations of the solar atmosphere. We also find indications of an imbalance between the hydrodynamic and magnetohydrodynamic baroclinic effects being at the origin of the swirls. At the base of the chromosphere, we find a mean net upwardly directed Poynting flux of 12.8 ± 6.5 kW m−2, which is mainly due to swirling motions. This energy flux is mostly associated with large and complex swirling structures, which we interpret as the superposition of various small-scale vortices.



Conclusions. We conclude that the ubiquitous swirling events observed in numerical simulations are tightly correlated with perturbations of the magnetic field. At photospheric and chromospheric levels, they form Alfvén pulses that propagate upward and may contribute to chromospheric heating.



},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2021AdSpR..67.1425N,

title = {A statistical study of solar radio Type III bursts and space weather implication},

author = {Theogene {Ndacyayisenga} and Jean {Uwamahoro} and K {Sasikumar Raja} and Christian {Monstein}},

url = {https://arxiv.org/abs/2012.01210},

doi = {10.1016/j.asr.2020.11.022},

year  = {2021},

date = {2021-02-01},

urldate = {2021-02-01},

journal = {Advances in Space Research},

volume = {67},

number = {4},

pages = {1425-1435},

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 h 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}

}

@article{2021SciA....7.8406I,

title = {Mapping solar magnetic fields from the photosphere to the base of the corona},

author = {Ryohko Ishikawa and Javier, Trujillo Bueno and Tanausu, Pino Alemán and Takenori J. Okamoto and David E. McKenzie and Frédéric Auch`ere and Ryouhei Kano and Donguk Song and Masaki Yoshida and Laurel A. Rachmeler and Ken Kobayashi and Hirohisa Hara and Masahito Kubo and Noriyuki Narukage and Taro Sakao and Toshifumi Shimizu and Yoshinori Suematsu and Christian Bethge and Bart De Pontieu and Alberto Sainz Dalda and Genevieve D. Vigil and Amy Winebarger and Ernest, Alsina Ballester and Luca Belluzzi and Jiř'i Štěpán and Andrés, Asensio Ramos and Mats Carlsson and Jorrit Leenaarts},

doi = {10.1126/sciadv.abe8406},

year  = {2021},

date = {2021-02-01},

urldate = {2021-02-01},

journal = {Science Advances},

volume = {7},

number = {8},

pages = {eabe8406},

abstract = {Routine ultraviolet imaging of the Sun's upper atmosphere shows the 

 spectacular manifestation of solar activity; yet we remain blind 

 to its main driver, the magnetic field. Here we report 

 unprecedented spectropolarimetric observations of an active 

 region plage and its surrounding enhanced network, showing 

 circular polarization in ultraviolet (Mg II $h$ & $k$ and Mn I) 

 and visible (Fe I) lines. We infer the longitudinal magnetic 

 field from the photosphere to the very upper chromosphere. At 

 the top of the plage chromosphere the field strengths reach more 

 than 300 gauss, strongly correlated with the Mg II $k$ line core 

 intensity and the electron pressure. This unique mapping shows 

 how the magnetic field couples the different atmospheric layers 

 and reveals the magnetic origin of the heating in the plage 

 chromosphere.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@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}

}

@article{refId0c,

title = {Imaging spectropolarimetry for magnetic field diagnostics in solar prominences},

author = {R, {Di Campli} and R Ramelli and M Bianda and I Furno and S, Kumar Dhara and L Belluzzi},

url = {https://doi.org/10.1051/0004-6361/202037931},

doi = {10.1051/0004-6361/202037931},

year  = {2020},

date = {2020-12-01},

urldate = {2020-12-01},

journal = {A&A},

volume = {644},

pages = {A89},

abstract = {Context. Narrowband imaging spectropolarimetry is one of the most powerful tools available to infer information about the intensity and topology of the magnetic fields present in extended plasma structures in the solar atmosphere.



Aims. We describe the instrumental set-up and the observing procedure that we have developed and optimized at the Istituto Ricerche Solari Locarno in order to perform imaging spectropolarimetry. A measurement that highlights the potential of the ensuing observations for magnetic field diagnostics in solar prominences is presented.



Methods. Monochromatic images of solar prominences were obtained by combining a tunable narrowband filter, based on two Fabry-Perot etalons, with a Czerny-Turner spectrograph. Linear and circular polarization were measured at every pixel of the monochromatic image with the Zurich Imaging Polarimeter, ZIMPOL. A wavelength scan was performed across the profile of the considered spectral line. The HAZEL inversion code was applied to the observed Stokes profiles to infer a series of physical properties of the observed structure.



Results. We carried out a spectropolarimetric observation of a prominence, consisting of a set of quasi-monochromatic images across the He I D3 line at 5876 Å in the four Stokes parameters. The map of observed Stokes profiles was inverted with HAZEL, finding magnetic fields with intensities between 15 and 30 G and directed along the spine of the prominence, which is in agreement with the results of previous works.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@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}

}

@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}

}

@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}

}

@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}

}

@article{2020A&A...641A..63C,

title = {Observational indications of magneto-optical effects in the scattering polarization wings of the Ca I 4227 Å line},

author = { Emilia Capozzi and Ernest, Alsina Ballester and Luca Belluzzi and Michele Bianda and Sajal, Kumar Dhara and Renzo Ramelli},

url = {https://arxiv.org/abs/2006.13653},

doi = {10.1051/0004-6361/202038455},

year  = {2020},

date = {2020-09-01},

urldate = {2020-09-01},

journal = {Astronomy and Astrophysics},

volume = {641},

pages = {A63},

abstract = {Context. Several strong resonance lines, such as H I 

 Ly-ensuremathalpha, Mg II k, Ca II K, and Ca I 4227 

 Å, are characterized by deep and broad absorption profiles 

 in the solar intensity spectrum. These resonance lines show 

 conspicuous linear scattering polarization signals when observed 

 in quiet regions close to the solar limb. Such signals show a 

 characteristic triplet-peak structure with a sharp peak in the 

 line core and extended wing lobes. The line core peak is 

 sensitive to the presence of magnetic fields through the Hanle 

 effect, which however is known not to operate in the line wings. 

 Recent theoretical studies indicate that, contrary to what was 

 previously believed, the wing linear polarization signals are 

 also sensitive to the magnetic field through magneto-optical 

 (MO) effects. textbackslash Aims: We search for observational 

 indications of this recently discovered physical mechanism in 

 the scattering polarization wings of the Ca I 4227 Å line. 

 textbackslash Methods: We performed a series of 

 spectropolarimetric observations of this line using the Zurich 

 IMaging POLarimeter camera at the Gregory-Coudé telescope at 

 Istituto Ricerche Solari Locarno in Switzerland and at the 

 GREGOR telescope in Tenerife (Spain). textbackslash Results: 

 Spatial variations of the total linear polarization degree and 

 linear polarization angle are clearly appreciable in the wings 

 of the observed line. We provide a detailed discussion of our 

 observational results, showing that the detected variations 

 always take place in regions in which longitudinal magnetic 

 fields are present, thus supporting the theoretical prediction 

 that they are produced by MO effects.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{refId0b,

title = {Vortices evolution in the solar atmosphere - A dynamical equation for the swirling strength},

author = {José R., Canivete Cuissa and Oskar Steiner},

url = {https://doi.org/10.1051/0004-6361/202038060},

doi = {10.1051/0004-6361/202038060},

year  = {2020},

date = {2020-07-21},

urldate = {2020-07-21},

journal = {Astronomy and Astrophysics},

volume = {639},

pages = {A118},

abstract = {Aims. We study vortex dynamics in the solar atmosphere by employing and deriving the analytical evolution equations of two vortex identification criteria.



Methods. The two criteria used are vorticity and the swirling strength. Vorticity can be biased in the presence of shear flows, but its dynamical equation is well known; the swirling strength is a more precise criterion for the identification of vortical flows, but its evolution equation is not known yet. Therefore, we explore the possibility of deriving a dynamical equation for the swirling strength. We then apply the two equations to analyze radiative magneto-hydrodynamical simulations of the solar atmosphere produced with the CO5BOLD code.



Results. We present a detailed review of the swirling strength criterion and the mathematical derivation of its evolution equation. This equation did not exist in the literature before and it constitutes a novel tool that is suitable for the analysis of a wide range of problems in (magneto-)hydrodynamics. By applying this equation to numerical models, we find that hydrodynamical and magnetic baroclinicities are the driving physical processes responsible for vortex generation in the convection zone and the photosphere. Higher up in the chromosphere, the magnetic terms alone dominate. Moreover, we find that the swirling strength is produced at small scales in a chaotic fashion, especially inside magnetic flux concentrations.



Conclusions. The swirling strength represents an appropriate criterion for the identification of vortices in turbulent flows, such as those in the solar atmosphere. Moreover, its evolution equation, which is derived in this paper, is pivotal for obtaining precise information about the dynamics of these vortices and the physical mechanisms responsible for their production and evolution. Since this equation is available, the swirling strength is now the ideal quantity to study the dynamics of vortices in (magneto-)hydrodynamics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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},

urldate = {2019-08-26},

journal = {Solar Physics},

volume = {294},

number = {8},

publisher = {Springer Science and Business Media LLC},

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 ≳200 MHz originate close to the solar disk center.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@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}

}

@article{2019ApJ...880...85A,

title = {Magnetic Sensitivity in the Wing Scattering Polarization Signals of the Hydrogen Lyman-α Line of the Solar Disk Radiation},

author = {E., {Alsina Ballester} and L. {Belluzzi} and J., {Trujillo Bueno}},

url = {https://arxiv.org/pdf/1901.10994},

doi = {10.3847/1538-4357/ab1e41},

year  = {2019},

date = {2019-08-01},

urldate = {2019-08-01},

journal = {Astrophysical Journal},

volume = {880},

number = {2},

pages = {85},

abstract = {The linear polarization produced by scattering processes in the hydrogen 

 Lyman-α line of the solar disk radiation is a 

 key observable for probing the chromosphere─corona transition 

 region (TR) and the underlying chromospheric plasma. While the 

 line-center signal encodes information on the magnetic field and 

 the three-dimensional structure of the TR, the sizable 

 scattering polarization signals that the joint action of partial 

 frequency redistribution and J-state interference produce in the 

 Lyman-α wings have generally been thought to be 

 sensitive only to the thermal structure of the solar atmosphere. 

 Here we show that the wings of the Q/I and U/I scattering 

 polarization profiles of this line are actually sensitive to the 

 presence of chromospheric magnetic fields, with strengths 

 similar to those that produce the Hanle effect in the line core 

 (i.e., between 5 and 100 G, approximately). In spite of the fact 

 that the Zeeman splitting induced by such weak fields is very 

 small compared to the total width of the line, the magneto- 

 optical effects that couple the transfer equations for Stokes Q 

 and U are actually able to produce sizable changes in the Q/I 

 and U/I wings. We find that magnetic fields with longitudinal 

 components larger than 100 G produce an almost complete 

 depolarization of the wings of the Lyensuremathalpha Q/I 

 profiles within a ensuremathpm5 Å spectral range 

 around the line center, while stronger fields are required for 

 the U/I wing signals to be depolarized to a similar extent. The 

 theoretical results presented here further expand the diagnostic 

 content of the unprecedented spectropolarimetric observations 

 provided by the Chromospheric Lyman-α Spectro-Polarimeter.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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},

urldate = {2018-11-01},

journal = {A&A},

volume = {619},

pages = {A60},

abstract = {Context. Interpreting the Stokes profiles observed in quiet regions of the solar chromosphere is a challenging task. The Stokes Q and U profiles are dominated by the scattering polarisation and the Hanle effect, and these processes can only be correctly quantified if 3D radiative transfer effects are taken into account. Forward-modelling of the intensity and polarisation of spectral lines using a 3D model atmosphere is a suitable approach in order to statistically compare the theoretical and observed line profiles.

Aims: Our aim is to present novel observations of the Ca II 8542 Å line profiles in a quiet region at the centre of the solar disc and to quantitatively compare them with the theoretical Stokes profiles obtained by solving the problem of the generation and transfer of polarised radiation in a 3D model atmosphere. We aim at estimating the reliability of the 3D model atmosphere, excluding its known lack of dynamics and/or insufficient density, using not only the line intensity but the full vector of Stokes parameters.

Methods: We used data obtained with the ZIMPOL instrument at the Istituto Ricerche Solari Locarno (IRSOL) and compared the observations with the theoretical profiles computed with the PORTA radiative transfer code, using as solar model atmosphere a 3D snapshot taken from a radiation-magnetohydrodynamics simulation. The synthetic profiles were degraded to match the instrument and observing conditions.

Results: The degraded theoretical profiles of the Ca II 8542 line are qualitatively similar to the observed ones. We confirm that there is a fundamental difference in the widths of all Stokes profiles: the observed lines are wider than the theoretical lines. We find that the amplitudes of the observed profiles are larger than those of the theoretical ones, which suggests that the symmetry breaking effects in the solar chromosphere are stronger than in the model atmosphere. This means that the isosurfaces of temperature, velocity, and magnetic field strength and orientation are more corrugated in the solar chromosphere than in the currently available 3D radiation-magnetohydrodynamics simulation.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}