Refereed Scientific Papers

@article{2023A&A...678A..58K,

title = {Peering into the tilted heart of Cyg X-1 with high-precision optical polarimetry},

author = {Vadim Kravtsov and Alexandra Veledina and Andrei V. Berdyugin and Andrzej A. Zdziarski and Gary D. Henson and Vilppu Piirola and Takeshi Sakanoi and Masato Kagitani and Svetlana V. Berdyugina and Juri Poutanen},

doi = {10.1051/0004-6361/202346932},

year  = {2023},

date = {2023-10-01},

urldate = {2023-10-01},

journal = {Astronomy and Astrophysics},

volume = {678},

pages = {A58},

abstract = {We present high-precision optical polarimetric observations of the black 

 hole X-ray binary Cygnus X-1 that span several cycles of its 

 5.6-day orbital period. The week-long observations on two 

 telescopes located in opposite hemispheres allowed us to track 

 the evolution of the polarization within one orbital cycle with 

 the highest temporal resolution to date. Using the field stars, 

 we determined the interstellar polarization in the source 

 direction and subsequently its intrinsic polarization P$_int$ = 0.82%ensuremathpm0.15% with a polarization angle ensuremaththeta$_int$ = 155textdegree 

 ensuremathpm5textdegree. The optical polarization angle 

 is aligned with that in the X-rays recently obtained with the 

 Imaging X-ray Polarimetry Explorer. Furthermore, it is 

 consistent within the uncertainties with the position angle of 

 the radio ejections. We show that the intrinsic polarization 

 degree is variable with the orbital period with an amplitude of 

 ensuremathsim0.2% and discuss various sites of its 

 production. Assuming that the polarization arises from a single 

 Thomson scattering of the primary star radiation by the matter 

 that follows the black hole in its orbital motion, we 

 constrained the inclination of the binary orbit i > 

 120textdegree and its eccentricity e < 0.08. The asymmetric 

 shape of the orbital profiles of the Stokes parameters also 

 implies the asymmetry of the scattering matter distribution in 

 the orbital plane, which may arise from the tilted accretion 

 disk. We compared our data to the polarimetric observations made 

 in 1975-1987 and find good agrement within 1textdegree 

 between the intrinsic polarization angles. On the other hand, 

 the polarization degree decreased by 0.4% over half a century, 

 suggesting secular changes in the geometry of the accreting 

 matter.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2023arXiv230912880J,

title = {The impact of angle-dependent partial frequency redistribution on the scattering polarization of the solar Na i D lines},

author = {Gioele Janett and Ernest Alsina Ballester and Luca Belluzzi and Tanaus'u Pino Alemán and Javier Trujillo Bueno},

doi = {10.48550/arXiv.2309.12880},

year  = {2023},

date = {2023-09-01},

urldate = {2023-09-01},

journal = {arXiv e-prints},

pages = {arXiv:2309.12880},

abstract = {The long-standing paradox of the linear polarization signal of the Na i 

 D1 line was recently resolved by accounting for the atom's 

 hyperfine structure and the detailed spectral structure of the 

 incident radiation field. That modeling relied on the 

 simplifying angle-averaged (AA) approximation for partial 

 frequency redistribution (PRD) in scattering, which potentially 

 neglects important angle-frequency couplings. This work aims at 

 evaluating the suitability of a PRD-AA modeling for the D1 and 

 D2 lines through comparisons with general angle-dependent (AD) 

 PRD calculations, both in the absence and presence of magnetic 

 fields. We solved the radiative transfer problem for polarized 

 radiation in a one-dimensional semi-empirical atmospheric model 

 with microturbulent and isotropic magnetic fields, accounting 

 for PRD effects, comparing PRD-AA and PRD-AD modelings. The D1 

 and D2 lines are modeled separately as two-level atomic system 

 with hyperfine structure. The numerical results confirm that a 

 spectrally structured radiation field induces linear 

 polarization in the D1 line. However, the PRD-AA approximation 

 greatly impacts the Q/I shape, producing an antisymmetric 

 pattern instead of the more symmetric PRD-AD one, while 

 presenting a similar sensitivity to magnetic fields between 10 

 and 200 G. Under the PRD-AA approximation, the Q/I profile of 

 the D2 line presents an artificial dip in its core, which is not 

 found for the PRD-AD case. We conclude that accounting for PRD- 

 AD effects is essential to suitably model the scattering 

 polarization of the Na i D lines. These results bring us closer 

 to exploiting the full diagnostic potential of these lines for 

 the elusive chromospheric magnetic fields.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Makela2023Ground,

title = {Ground-based infrastructure for improved space weather specification at low latitudes},

author = {Jonathan Makela and Qian Wu and Christian Monstein and John Bosco Habarulema and Keith Groves and Norbert Jakowski and Cristine Amory},

year  = {2023},

date = {2023-07-01},

urldate = {2023-07-01},

journal = {Bulletin of the AAS},

volume = {55},

number = {3},

note = {https://baas.aas.org/pub/2023n3i259},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2023SoPh..298...82B,

title = {Automatic Burst Detection in Solar Radio Spectrograms Using Deep Learning: deARCE Method},

author = {Javier Bussons Gordo and Mario Fernández Ruiz and Manuel Prieto Mateo and Jorge Alvarado D'iaz and Francisco Chávez de la O and J. Ignacio Hidalgo and Christian Monstein},

doi = {10.1007/s11207-023-02171-0},

year  = {2023},

date = {2023-06-01},

urldate = {2023-06-01},

journal = {Solar Physics},

volume = {298},

number = {6},

pages = {82},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2023JCoPh.47912013B,

title = {Scalable matrix-free solver for 3D transfer of polarized radiation in stellar atmospheres},

author = {Pietro Benedusi and Simone Riva and Patrick Zulian and Jiř'i Štěpán and Luca Belluzzi and Rolf Krause},

doi = {10.1016/j.jcp.2023.112013},

year  = {2023},

date = {2023-04-01},

urldate = {2023-04-01},

journal = {Journal of Computational Physics},

volume = {479},

pages = {112013},

abstract = {We present an efficient and massively parallel solution strategy for the 

 transfer problem of polarized radiation, for a 3D stationary 

 medium out of local thermodynamic equilibrium. Scattering 

 processes are included accounting for partial frequency 

 redistribution effects. Such a setting is one of the most 

 challenging ones in radiative transfer modeling. The problem is 

 formulated for a two-level atomic model, which allows 

 linearization. The discrete ordinate method alongside an 

 exponential integrator are used for discretization. Efficient 

 solution is obtained with a Krylov method equipped with a 

 tailored physics-based preconditioner. A matrix-free approach 

 results in a lightweight implementation, suited for tackling 

 large problems. Near-optimal strong and weak scalability are 

 obtained with two complementary decompositions of the 

 computational domain. The presented approach made it possible to 

 perform simulations for the Ca I line at 4227 Å with more 

 than 10$^9$ degrees of freedom in less than half an hour on 

 massively parallel machines, always converging in a few 

 iterations for the proposed tests.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2023ApJ...947...71A,

title = {The Potential of the Wavelength-integrated Scattering Polarization of the Hydrogen Lyensuremathalpha Line for Probing the Solar Chromosphere},

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

doi = {10.3847/1538-4357/acc186},

year  = {2023},

date = {2023-04-01},

urldate = {2023-04-01},

journal = {Astrophysical Journal},

volume = {947},

number = {2},

pages = {71},

abstract = {The intensity and the linear scattering polarization profiles of the 

 hydrogen Lyensuremathalpha line encode valuable 

 information on the thermodynamic and magnetic structure of the 

 upper layers of the solar chromosphere. The Chromospheric Lyman- 

 Alpha Spectro-Polarimeter (CLASP) sounding rocket experiment 

 provided unprecedented spectropolarimetric data of this line, as 

 well as two-dimensional broadband images in intensity and linear 

 polarization. We theoretically investigate the potential of the 

 Lyensuremathalpha broadband polarimetric signals for 

 probing the solar chromosphere and its magnetic fields. We 

 analyze the synthetic Stokes profiles obtained from a series of 

 radiative transfer (RT) calculations out of local thermodynamic 

 equilibrium, considering semi-empirical one-dimensional models 

 of the solar atmosphere. The wavelength-integrated linear 

 polarization signal is found to be dominated by the contribution 

 from the wings when considering a Gaussian weighting function 

 with a FWHM that corresponds to the CLASP slit-jaw broadband 

 filter. These broadband linear polarization signals are strongly 

 sensitive to magnetic fields of strengths on the order of 50 G, 

 via the action of magneto-optical (MO) effects, and are expected 

 to encode information on the middle-upper chromosphere. The two- 

 dimensional broadband intensity and linear polarization images 

 observed by CLASP can be suitably mimicked using synthetic 

 wavelength-integrated signals obtained considering atmospheric 

 models and magnetic fields that are representative of solar 

 regions with different levels of activity, provided that the 

 impact of MO effects is taken into account. Despite the 

 limitations of a one-dimensional RT modeling, this work 

 illustrates the diagnostic potential of filter-polarimetric 

 Lyensuremathalpha signals for probing the solar 

 chromosphere and its magnetism.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2023ApJ...945..125I,

title = {Evidence for the Operation of the Hanle and Magneto-optical Effects in the Scattering Polarization Signals Observed by CLASP2 across the Mg II h and k Lines},

author = {Ryohko Ishikawa and Javier Trujillo Bueno and Ernest Alsina Ballester and Luca Belluzzi and Tanaus'u Pino Alemán and David E. McKenzie and Frédéric Auch`ere and Ken Kobayashi and Takenori J. Okamoto and Laurel A. Rachmeler and Donguk Song},

doi = {10.3847/1538-4357/acb64e},

year  = {2023},

date = {2023-03-01},

urldate = {2023-03-01},

journal = {Astrophysical Journal},

volume = {945},

number = {2},

pages = {125},

abstract = {Radiative transfer investigations of the solar Mg II h and k resonance 

 lines around 280 nm have shown that, while their circular 

 polarization (Stokes V) signals arise from the Zeeman effect, 

 the linear polarization profiles (Stokes Q and U) are dominated 

 by the scattering of anisotropic radiation and the Hanle and 

 magneto-optical (MO) effects. Using the unprecedented 

 observations of the Mg II and Mn I resonance lines obtained by 

 the Chromospheric LAyer Spectro-Polarimeter (CLASP2), here we 

 investigate how the linear polarization signals at different 

 wavelengths (i.e., at the center, and at the near and far wings 

 of the k line) vary with the longitudinal component of the 

 magnetic field (B $_ L $) at their approximate height of 

 formation. The B $_ L $ is estimated from the V signals in the 

 aforementioned spectral lines. Particular attention is given to 

 the following quantities that are expected to be influenced by 

 the presence of magnetic fields through the Hanle and MO 

 effects: the sign of the U signals, the total linear 

 polarization amplitude (LP) and its direction 

 (ensuremathchi) with respect to a reference direction. We 

 find that at the center and near wings of the k line, the 

 behavior of these quantities is significantly different in the 

 observed quiet and plage regions, and that both LP and 

 ensuremathchi seem to depend on B $_ L $. These 

 observational results are indicative of the operation of the 

 Hanle effect at the center of the k line and of the MO effects 

 at the near wings of the k line.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{universe9020103,

title = {Cosmological Constant from Boundary Condition and Its Implications beyond the Standard Model},

author = {Jan O. Stenflo},

url = {https://www.mdpi.com/2218-1997/9/2/103},

doi = {10.3390/universe9020103},

issn = {2218-1997},

year  = {2023},

date = {2023-02-17},

urldate = {2023-01-01},

journal = {Universe},

volume = {9},

number = {2},

abstract = {Standard cosmology has long been plagued by a number of persistent problems. The origin of the apparent acceleration of the cosmic expansion remains enigmatic. The cosmological constant has been reintroduced as a free parameter with a value in energy density units that “happens” to be of the same order as the present matter energy density. There is an internal inconsistency with regards to the Hubble constant, the so-called H0 tension. The derived value of H0 depends on the type of data that is used. With supernovae as standard candles, one gets a H0 that is 4–5 σ larger than the value that one gets from CMB (Cosmic Microwave Background) data for the early universe. Here we show that these problems are related and can be solved if the cosmological constant represents a covariant integration constant that arises from a spatial boundary condition, instead of being a new type of hypothetical physical field, “dark energy”, as assumed by standard cosmology. The boundary condition only applies to the bounded 3D subspace that represents the observable universe, the hypersurface of the past light cone.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2023A&A...670A..56B,

title = {Identifying the energy release site in a solar microflare with a jet},

author = {Andrea Francesco Battaglia and Wen Wang and Jonas Saqri and Tatiana Podladchikova and Astrid M. Veronig and Hannah Collier and Ewan C. M. Dickson and Olena Podladchikova and Christian Monstein and Alexander Warmuth and Frédéric Schuller and Louise Harra and Säm Krucker},

doi = {10.1051/0004-6361/202244996},

year  = {2023},

date = {2023-02-01},

urldate = {2023-02-01},

journal = {Astronomy and Astrophysics},

volume = {670},

pages = {A56},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2023SSRv..219....1T,

title = {Vortex Motions in the Solar Atmosphere},

author = {K. Tziotziou and E. Scullion and S. Shelyag and O. Steiner and E. Khomenko and G. Tsiropoula and J. R. Canivete Cuissa and S. Wedemeyer and I. Kontogiannis and N. Yadav and I. N. Kitiashvili and S. J. Skirvin and I. Dakanalis and A. G. Kosovichev and V. Fedun},

doi = {10.1007/s11214-022-00946-8},

year  = {2023},

date = {2023-02-01},

urldate = {2023-02-01},

journal = {Space Science Reviews},

volume = {219},

number = {1},

pages = {1},

abstract = {Vortex flows, related to solar convective turbulent dynamics at granular 

 scales and their interplay with magnetic fields within 

 intergranular lanes, occur abundantly on the solar surface and 

 in the atmosphere above. Their presence is revealed in high- 

 resolution and high-cadence solar observations from the ground 

 and from space and with state-of-the-art magnetoconvection 

 simulations. Vortical flows exhibit complex characteristics and 

 dynamics, excite a wide range of different waves, and couple 

 different layers of the solar atmosphere, which facilitates the 

 channeling and transfer of mass, momentum and energy from the 

 solar surface up to the low corona. Here we provide a 

 comprehensive review of documented research and new developments 

 in theory, observations, and modelling of vortices over the past 

 couple of decades after their observational discovery, including 

 recent observations in Hensuremathalpha , innovative 

 detection techniques, diverse hydrostatic modelling of waves and 

 forefront magnetohydrodynamic simulations incorporating effects 

 of a non-ideal plasma. It is the first systematic overview of 

 solar vortex flows at granular scales, a field with a plethora 

 of names for phenomena that exhibit similarities and differences 

 and often interconnect and rely on the same physics. With the 

 advent of the 4-m Daniel K. Inouye Solar Telescope and the 

 forthcoming European Solar Telescope, the ongoing Solar Orbiter 

 mission, and the development of cutting-edge simulations, this 

 review timely addresses the state-of-the-art on vortex flows and 

 outlines both theoretical and observational future research 

 directions.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2023A&A...670A...2B,

title = {Discovery of magnetic fields in five DC white dwarfs},

author = {Andrei V. Berdyugin and Vilppu Piirola and Stefano Bagnulo and John D. Landstreet and Svetlana V. Berdyugina},

doi = {10.1051/0004-6361/202245149},

year  = {2023},

date = {2023-02-01},

urldate = {2023-02-01},

journal = {Astronomy and Astrophysics},

volume = {670},

pages = {A2},

abstract = {About half of white dwarfs (WDs) evolve to the DC state as they cool; 

 the others become DQ or (temporarily?) DZ WDs. The recent 

 magnetic survey of the local 20 pc volume has established a high 

 frequency of magnetic fields among WDs older than 2-3 Gyr, 

 demonstrating that in low- and average-mass WDs, the effects of 

 magnetism become more common as they age, and the fields on 

 average become stronger. However, the available statistics of 

 WDs older than about 5 Gyr do not clearly establish how fields 

 evolve beyond this age. We are carrying out a survey to clarify 

 the occurrence of magnetism in DC-type WDs in order to better 

 understand this late evolution. We use broadband filter 

 polarimetry, arguably the most efficient way to detect magnetic 

 fields in featureless WDs via continuum circular polarization. 

 Here we report the discovery of a magnetic field in five DC WDs 

 (of 23 observed), almost doubling the total sample of known 

 magnetic WDs belonging to the DC spectral class.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{egusphere-2023-201,

title = {An Overview of Solar Radio Type II Bursts through analysis of associated solar and near Earth space weather features during Ascending phase of SC 25},

author = {T. Ndacyayisenga and J. Uwamahoro and J. C. Uwamahoro and R. Babatunde and D. Okoh and K. Sasikumar Raja and C. Kwisanga and C. Monstein},

url = {https://egusphere.copernicus.org/preprints/egusphere-2023-201/},

doi = {10.5194/egusphere-2023-201},

year  = {2023},

date = {2023-01-01},

urldate = {2023-01-01},

journal = {EGUsphere},

volume = {2023},

pages = {1–22},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{refId0g,

title = {The effects of solar radio bursts on frequency bands utilised by the aviation industry in Sub-Saharan Africa},

author = {Sarah Ruth McKee and Pierre Johannes Cilliers and Stefan Lotz and Christian Monstein},

url = {https://doi.org/10.1051/swsc/2023001},

doi = {10.1051/swsc/2023001},

year  = {2023},

date = {2023-01-01},

urldate = {2023-01-01},

journal = {J. Space Weather Space Clim.},

volume = {13},

pages = {4},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{POHJOLAINEN2023,

title = {Separating the effects of earthside and far side solar events. A case study},

author = {Silja Pohjolainen and Nasrin Talebpour Shesvan and Christian Monstein},

url = {https://www.sciencedirect.com/science/article/pii/S0273117723007317},

doi = {https://doi.org/10.1016/j.asr.2023.09.009},

issn = {0273-1177},

year  = {2023},

date = {2023-01-01},

urldate = {2023-01-01},

journal = {Advances in Space Research},

abstract = {On 8 November 2013 a halo-type coronal mass ejection (CME) was observed, together with flares and type II radio bursts, but the association between the flares, radio bursts, and the CME was not clear. Our aim is to identify the origin of the CME and its direction of propagation, and to exclude features that were not connected to it. On the Earth-facing side, a GOES C5.7 class flare occurred close to the estimated CME launch time, followed by an X1.1 class flare. The latter flare was associated with an EUV wave and metric type II bursts. On the far side of the Sun, a filament eruption, EUV dimmings, and ejected CME loops were observed by imaging instruments onboard the Solar TErrestrial RElations Observatory (STEREO) spacecraft that were viewing the backside of the Sun. The STEREO radio instruments observed an interplanetary (IP) type II radio burst at decameter-hectometric wavelengths, which was not observed by the radio instrument onboard the Wind spacecraft located at L1 near Earth. We show that the halo CME originated from the eruption on the far side of the Sun, and that the IP type II burst was created by a shock wave ahead of the halo CME. The radio burst remained unobserved from the earthside, even at heliocentric source heights larger than 9 solar radii. During the CME propagation, the X-class flare eruption caused a small plasmoid ejection earthward, the material of which was superposed on the earlier CME structures observed in projection. The estimated heights of the metric type II burst match well with the EUV wave launched by the X-class flare. As this radio emission did not continue to lower frequencies, we conclude that the shock wave did not propagate any further. Either the shock driver died out, as a blast wave, or the driver speed no longer exceeded the local Alfven speed.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2023RASTI...2..148R,

title = {The effect of stellar contamination on low-resolution transmission spectroscopy: needs identified by NASA's Exoplanet Exploration Program Study Analysis Group 21},

author = {Benjamin V. Rackham and Néstor Espinoza and Svetlana V. Berdyugina and Heidi Korhonen and Ryan J. MacDonald and Benjamin T. Montet and Brett M. Morris and Mahmoudreza Oshagh and Alexander I. Shapiro and Yvonne C. Unruh and Elisa V. Quintana and Robert T. Zellem and Dániel Apai and Thomas Barclay and Joanna K. Barstow and Giovanni Bruno and Ludmila Carone and Sarah L. Casewell and Heather M. Cegla and Serena Criscuoli and Catherine Fischer and Damien Fournier and Mark S. Giampapa and Helen Giles and Aishwarya Iyer and Greg Kopp and Nadiia M. Kostogryz and Natalie Krivova and Matthias Mallonn and Chima McGruder and Karan Molaverdikhani and Elisabeth R. Newton and Mayukh Panja and Sarah Peacock and Kevin Reardon and Rachael M. Roettenbacher and Gaetano Scandariato and Sami Solanki and Keivan G. Stassun and Oskar Steiner and Kevin B. Stevenson and Jeremy Tregloan-Reed and Adriana Valio and Sven Wedemeyer and Luis Welbanks and Jie Yu and Munazza K. Alam and James R. A. Davenport and Drake Deming and Chuanfei Dong and Elsa Ducrot and Chloe Fisher and Emily Gilbert and Veselin Kostov and Mercedes López-Morales and Mike Line and Teo Močnik and Susan Mullally and Rishi R. Paudel and Ignasi Ribas and Jeff A. Valenti},

doi = {10.1093/rasti/rzad009},

year  = {2023},

date = {2023-01-01},

urldate = {2023-01-01},

journal = {RAS Techniques and Instruments},

volume = {2},

number = {1},

pages = {148-206},

abstract = {Study Analysis Group 21 (SAG21) of NASA's Exoplanet Exploration Program 

 Analysis Group was organized to study the effect of stellar 

 contamination on space-based transmission spectroscopy, a method 

 for studying exoplanetary atmospheres by measuring the 

 wavelength-dependent radius of a planet as it transits its star. 

 Transmission spectroscopy relies on a precise understanding of 

 the spectrum of the star being occulted. However, stars are not 

 homogeneous, constant light sources but have temporally evolving 

 photospheres and chromospheres with inhomogeneities like spots, 

 faculae, plages, granules, and flares. This SAG brought together 

 an interdisciplinary team of more than 100 scientists, with 

 observers and theorists from the heliophysics, stellar 

 astrophysics, planetary science, and exoplanetary atmosphere 

 research communities, to study the current research needs that 

 can be addressed in this context to make the most of transit 

 studies from current NASA facilities like Hubble Space Telescope 

 and JWST. The analysis produced 14 findings, which fall into 

 three science themes encompassing (i) how the Sun is used as our 

 best laboratory to calibrate our understanding of stellar 

 heterogeneities ('The Sun as the Stellar Benchmark'), (ii) how 

 stars other than the Sun extend our knowledge of heterogeneities 

 ('Surface Heterogeneities of Other Stars'), and (iii) how to 

 incorporate information gathered for the Sun and other stars 

 into transit studies ('Mapping Stellar Knowledge to Transit 

 Studies'). In this invited review, we largely reproduce the 

 final report of SAG21 as a contribution to the peer-reviewed 

 literature.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2022Ap&SS.367..122W,

title = {UV spectropolarimetry with Polstar: protoplanetary disks},

author = {John P. Wisniewski and Andrei V. Berdyugin and Svetlana V. Berdyugina and William C. Danchi and Ruobing Dong and René D. Oudmaijer and Vladimir S. Airapetian and Sean D. Brittain and Ken Gayley and Richard Ignace and Maud Langlois and Kellen D. Lawson and Jamie R. Lomax and Evan A. Rich and Motohide Tamura and Jorick S. Vink and Paul A. Scowen},

doi = {10.1007/s10509-022-04125-7},

year  = {2022},

date = {2022-12-01},

urldate = {2022-12-01},

journal = {Astrophysics and Space Science},

volume = {367},

number = {12},

pages = {122},

abstract = {Polstar is a proposed NASA MIDEX mission that carries a high resolution 

 UV spectropolarimeter capable of measure all four Stokes 

 parameters onboard a 60 cm telescope. The mission has been 

 designed to pioneer the field of time-domain UV 

 spectropolarimetry. Time domain UV spectropolarimetry offers the 

 best resource to determine the geometry and physical conditions 

 of protoplanetary disks from the stellar surface to <5 AU. We 

 detail two key objectives that a dedicated time domain UV 

 spectropolarimetry survey, such as that enabled by Polstar or a 

 similar mission concept, could achieve: 1) Test the hypothesis 

 that magneto-accretion operating in young planet-forming disks 

 around lower-mass stars transitions to boundary layer accretion 

 in planet-forming disks around higher mass stars; and 2) 

 Discriminate whether transient events in the innermost regions 

 of planet-forming disks of intermediate mass stars are caused by 

 inner disk mis-alignments or from stellar or disk emissions.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2022Sci...378..650K,

title = {Polarized x-rays constrain the disk-jet geometry in the black hole x-ray binary Cygnus X-1},

author = {Henric Krawczynski and Fabio Muleri and Michal Dovčiak and Alexandra Veledina and Nicole Rodriguez Cavero and Jiri Svoboda and Adam Ingram and Giorgio Matt and Javier A. Garcia and Vladislav Loktev and Michela Negro and Juri Poutanen and Takao Kitaguchi and Jakub Podgorn'y and John Rankin and Wenda Zhang and Andrei Berdyugin and Svetlana V. Berdyugina and Stefano Bianchi and Dmitry Blinov and Fiamma Capitanio and Niccol`o Di Lalla and Paul Draghis and Sergio Fabiani and Masato Kagitani and Vadim Kravtsov and Sebastian Kiehlmann and Luca Latronico and Alexander A. Lutovinov and Nikos Mandarakas and Frédéric Marin and Andrea Marinucci and Jon M. Miller and Tsunefumi Mizuno and Sergey V. Molkov and Nicola Omodei and Pierre-Olivier Petrucci and Ajay Ratheesh and Takeshi Sakanoi and Andrei N. Semena and Raphael Skalidis and Paolo Soffitta and Allyn F. Tennant and Phillipp Thalhammer and Francesco Tombesi and Martin C. Weisskopf and Joern Wilms and Sixuan Zhang and Iván Agudo and Lucio A. Antonelli and Matteo Bachetti and Luca Baldini and Wayne H. Baumgartner and Ronaldo Bellazzini and Stephen D. Bongiorno and Raffaella Bonino and Alessandro Brez and Niccol`o Bucciantini and Simone Castellano and Elisabetta Cavazzuti and Stefano Ciprini and Enrico Costa and Alessandra De Rosa and Ettore Del Monte and Laura Di Gesu and Alessandro Di Marco and Immacolata Donnarumma and Victor Doroshenko and Steven R. Ehlert and Teruaki Enoto and Yuri Evangelista and Riccardo Ferrazzoli and Shuichi Gunji and Kiyoshi Hayashida and Jeremy Heyl and Wataru Iwakiri and Svetlana G. Jorstad and Vladimir Karas and Jeffery J. Kolodziejczak and Fabio La Monaca and Ioannis Liodakis and Simone Maldera and Alberto Manfreda and Alan P. Marscher and Herman L. Marshall and Ikuyuki Mitsuishi and Chi-Yung Ng and Stephen L. O’Dell and Chiara Oppedisano and Alessandro Papitto and George G. Pavlov and Abel L. Peirson and Matteo Perri and Melissa Pesce-Rollins and Maura Pilia and Andrea Possenti and Simonetta Puccetti and Brian D. Ramsey and Roger W. Romani and Carmelo Sgr`o and Patrick Slane and Gloria Spandre and Toru Tamagawa and Fabrizio Tavecchio and Roberto Taverna and Yuzuru Tawara and Nicholas E. Thomas and Alessio Trois and Sergey Tsygankov and Roberto Turolla and Jacco Vink and Kinwah Wu and Fei Xie and Silvia Zane},

doi = {10.1126/science.add5399},

year  = {2022},

date = {2022-11-01},

urldate = {2022-11-01},

journal = {Science},

volume = {378},

number = {6620},

pages = {650-654},

abstract = {A black hole x-ray binary (XRB) system forms when gas is stripped from a 

 normal star and accretes onto a black hole, which heats the gas 

 sufficiently to emit x-rays. We report a polarimetric 

 observation of the XRB Cygnus X-1 using the Imaging X-ray 

 Polarimetry Explorer. The electric field position angle aligns 

 with the outflowing jet, indicating that the jet is launched 

 from the inner x-ray–emitting region. The 

 polarization degree is 4.01 ensuremathpm 0.20% at 2 to 8 

 kiloelectronvolts, implying that the accretion disk is viewed 

 closer to edge-on than the binary orbit. These observations 

 reveal that hot x-ray–emitting plasma is spatially 

 extended in a plane perpendicular to, not parallel to, the jet 

 axis.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{refId0f,

title = {Innovative and automated method for vortex identification - I. Description of the SWIRL algorithm},

author = {J. R. Canivete Cuissa and O. Steiner},

url = {https://doi.org/10.1051/0004-6361/202243740},

doi = {10.1051/0004-6361/202243740},

year  = {2022},

date = {2022-10-07},

urldate = {2022-10-07},

journal = {A&A},

volume = {668},

pages = {A118},

abstract = {Context. As a universally accepted definition of a vortex has not yet been established, the community lacks an unambiguous and rigorous method for identifying vortices in fluid flows. Such a method would be useful for conducting robust statistical studies on vortices in highly dynamical and turbulent systems such as the solar atmosphere.



Aims. We aim to develop an innovative and robust automated methodology for the identification of vortices based on local and global characteristics of the flow, while avoiding the use of a threshold that could potentially prevent the detection of weak vortices in the process.



Methods. We present a new method that combines the rigor of mathematical criteria with the global perspective of morphological techniques. The core of the method consists of an estimation of the center of rotation for every point of the flow that presents some degree of curvature in its neighborhood. For this purpose, we employed the Rortex criterion and combined it with morphological considerations of the velocity field. We then identified coherent vortical structures based on clusters of estimated centers of rotation.



Results. We demonstrate that the Rortex is a more reliable criterion than the swirling strength and the vorticity for the extraction of physical information from vortical flows, because it measures the rigid-body rotational part of the flow alone and is not biased by the presence of pure or intrinsic shears. We show that the method performs well in the context of a simplistic test case composed of two Lamb-Oseen vortices. We combined the proposed method with a state-of-the-art clustering algorithm to build an automated vortex identification algorithm. The algorithm was applied to an artificial flow composed of multiple Lamb–Oseen vortices, with a random noisy background, and to the turbulent flow of a simulated magneto-hydrodynamical Orszag-Tang vortex test. The results demonstrate the reliability and accuracy of the method.



Conclusions. The present automated vortex identification method can be considered a new tool for the detection and study of vortices in dynamical and turbulent (magneto)hydrodynamical flows. By applying the implemented algorithm to numerical simulations and observational data, as well as comparing it to existing detection methods, we seek to successively improve the reliability of the detections and, ultimately, our knowledge on swirling motions in the solar, stellar, and planetary atmospheres.



},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@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}

}