arXiv subjects

M. Kriginsky

Publications and source records attributed to M. Kriginsky.

6 recordsLinked to original sources

arXiv2026

Chromospheric heating and magnetic topology above the shared penumbra of a delta-spot: Multi-line inversions and multi-height magnetic-field extrapolations

We analysed observations of the Fe I 617.3 nm, Ca II 854.2 nm, and Ca II H lines obtained with CRISP and CHROMIS at the SST. Spatially coupled non-LTE inversions constrained the chromospheric atmosphere, while the WFA provided estimates of the chromospheric line-of-sight magnetic field. We combined these photospheric and chromospheric constraints with an HMI magnetogram as input to a multi-height field extrapolation. We characterised the reconstructed topology using the twist number, squashing factor, current density, and field-line connectivity. Results. The Ca II H magnetic signal is concentrated mainly above the strongest photospheric field concentrations, whereas Ca II 854.2 nm yields stronger and more spatially extended line-of-sight fields. The chromosphere above the shared penumbra is approximately 300 K hotter than nearby quiet regions. The selected brightening follows a chromospheric loop, with enhanced temperature and a transition from blueshift to redshift along the structure. The extrapolation recovers field strengths broadly consistent with the inversions and reveals a left-handed, flux-rope-like core following the polarity inversion line. Enhanced currents and connectivity gradients occur near parts of its boundary, where field lines connect the twisted structure to overarching loops. Conclusions. The temperature and velocity patterns and magnetic topology are consistent with reconnection between the twisted polarity-inversion-line field and the surrounding loops, depositing energy in the chromosphere and driving plasma along reconfigured field lines. These signatures do not uniquely establish reconnection, but show that combining high-resolution spectropolarimetric inversions with multi-height extrapolations can relate chromospheric energy release to the local three-dimensional magnetic structure.

astro-ph.SR↗

arXiv2026

The magnetic sensitivity of the Ca II H and K lines

The solar chromosphere is a transition layer between the cool, dense photosphere and the hot, rarefied corona. This boundary region plays a key role in regulating energy transport and structuring the magnetic field throughout the solar atmosphere. Understanding its thermodynamic and magnetic properties is essential to model and interpret solar phenomena. This study investigates the theoretical properties and diagnostic potential of the polarisation signals in the Ca II H and K lines, with particular emphasis on their capability to probe magnetic fields in the upper chromosphere with the CHROMIS instrument at the Swedish 1-m solar telescope. We combine semi-empirical atmospheric models with high-resolution solar observations to model the formation of the Ca II H and K lines using non-local thermodynamic equilibrium radiative transfer calculations. The sensitivity of the lines to the magnetic field is examined through response functions and synthetic inversions, enabling an assessment of their diagnostic performance under realistic chromospheric conditions. For typical chromospheric field strengths, the linear polarisation of the Ca ii H & K lines is less than 1.7%, below the expected detection threshold of CHROMIS. However, their circular polarisation reaches more than 10% in strong-field regions, which is detectable by CHROMIS. Both lines are sensitive to magnetic fields in the upper chromosphere, with the K line forming slightly higher due to its larger opacity, and the H line exhibiting a somewhat stronger Zeeman sensitivity owing to its higher effective Lande factor and longer wavelength. Using the weak-field approximation, the line of sight magnetic field can be reliably inferred. These results confirm that the Ca II H & K lines constitute powerful diagnostics for studying the magnetic structure of the upper solar chromosphere.

astro-ph.SR↗

arXiv2024

Thermal and kinetic coronal rain diagnostics with MgII h & k lines

We performed the temperature diagnostics using inversions of data from nine spectroscopic observations obtained with the IRIS spectrograph in the MgII h & k lines. The sensitivity to the temperature of the emission peaks of these lines was exploited to determine the temperature of the coronal rain plasma using inversions of the spectroscopic profiles. Additional relationships between different spectral features of these lines, derived from the use of 3D radiative transfer line synthesis applied to simulations, were employed in order to derive the line-of-sight (LoS) velocities in different parts of the coronal rain plasma. For the first time, spectroscopic inversions of coronal rain were successfully performed. Temperatures derived from the inversions yield coronal rain clump temperatures at the formation height of the emission peaks of the MgII h & k lines in the range between 5000 and 7000 K. This narrow range of values remains consistent among all the different observations used in this work. We obtained LoS velocities of up to 40 km/s, which are consistent with the motion of the plasma being mostly constrained to the plane of the sky, as the coronal rain was mostly detected shortly after its formation and the observations took place in the disc. Furthermore, velocity diagnostics led to the detection of larger velocities at higher layers of the coronal rain plasma in some cases. This increased velocity seems to indicate that at some point (at least) during the fall of coronal rain clumps towards the chromosphere, the material in the upper part of the coronal rain plasma is falling with greater velocity than the material below it. The conditions of the temperature and density of the coronal rain plasma where the Mg II h line forms appear to be slightly different that those of the Mg II k line, with the former found at slightly colder and denser parts of the plasma.

astro-ph.SR↗

arXiv2023

Temperature diagnostics of chromospheric fibrils

Context. Chromospheric fibrils are thin and elongated structures that connect nearby photospheric magnetic field concentrations of opposite polarities. Aims. We assess the possibilities and drawbacks related to the use of current instrumentation and inversion techniques to infer the thermodynamic structure of chromospheric fibrils. Methods. We employed spectroscopic observations obtained in the Ca ii 854.2 nm line with the CRISP instrument at the Swedish 1-m Solar Telescope and in coordination with observations in the ultraviolet Mg ii h & k lines taken with the IRIS satellite. We studied the temperature sensitivity of these chromospheric lines to properly invert their spectral profiles with the Stockholm inversion Code and determine the temperature, line-of-sight velocity, and microturbulent velocity of manually traced chromospheric fibrils present in the field of view. Results. Fibril-like structures show a very particular dependence of their temperature as a function of the position along their length. Their temperatures at the detected footpoints are, on average, 300 K higher than the temperature at the midpoint. The temperature variation appears to be almost symmetrical in shape, with partially traced fibrils showing a similar trend for the temperature variation. Additionally, the response of the Ca ii 854.2 nm line core to variations of the temperature for the inverted models of the atmosphere in fibril areas seems to be insufficient to properly resolve the aforementioned temperature structure. Only the addition of more temperature sensitive lines such as the Mg ii h & k lines would make it possible to properly infer the thermodynamic properties of chromospheric fibrils. Comparisons between the results obtained here and in previous studies focused on bright Ca ii K fibrils yield great similarities between these structures in terms of their temperature.

astro-ph.SR↗

arXiv2021

Magnetic field inference in active region coronal loops using coronal rain clumps

Aims. We aim to infer information about the magnetic field in the low solar corona from coronal rain clumps using high-resolution spectropolarimetric observations in the Ca ii 8542 A line obtained with the Swedish 1-m Solar Telescope. Methods. The weak-field approximation (WFA) provides a simple tool to obtain the line-of-sight component of the magnetic field from spectropolarimetric observations. We adapted a method developed in a previous paper in order to assess the different conditions that must be satisfied in order to properly use the WFA for the data at hand. We also made use of velocity measurements in order to estimate the plane-of-the-sky magnetic field component, so that the magnetic field vector could be inferred. Results. We have inferred the magnetic field vector from a data set totalling 100 spectral scans in the Ca ii 8542 A line, containing an off-limb view of the lower portion of catastrophically cooled coronal loops in an active region. Our results, albeit limited by the cadence and signal-to-noise ratio of the data, suggest that magnetic field strengths of hundreds of Gauss, even reaching up to 1000 G, are omnipresent at coronal heights below 9 Mm from the visible limb. Our results are also compatible with the presence of larger magnetic field values such as those reported by previous works. However, for large magnetic fields, the Doppler width from coronal rain is not that much larger than the Zeeman width, thwarting the application of the WFA. Furthermore, we have determined the temperature, T , and microturbulent velocity, $\xi$, of coronal rain clumps and off-limb spicules present in the same data set, and we have found that the former ones have narrower T and $\xi$ distributions, their average temperature is similar, and coronal rain has microturbulent velocities smaller than those of spicules

astro-ph.SR↗

arXiv2020

Ubiquitous hundred-Gauss magnetic fields in solar spicules

Aims. We use high-resolution spectropolarimetric observations in the Ca ii 8542 A line obtained with the SST to study the magnetic field in solar spicules. Methods. The equations that result from the application of the Weak Field Approximation (WFA) to the radiative transfer equations are used to infer the LOS component of the magnetic field (BLOS). Two restrictive conditions are imposed on the Stokes I and V profiles at each pixel before they can be used in a Bayesian inversion to compute its BLOS. Results. The LOS magnetic field component has been inferred in six data sets totalling 448 spectral scans in the Ca ii 8542 A line and containing both active region and quiet Sun areas, with values of hundreds of G being abundantly inferred. There seems to be no difference, from the statistical point of view, between the magnetic field strength of spicules in the quiet Sun or near an active region. On the other hand, the BLOS distributions present smaller values on the disk than off-limb, a fact that can be explained by the effect of superposition on the chromosphere of on-disk structures. We find that spicules in the vicinity of a sunspot have a magnetic field polarity (i.e. north or south) equal to that of the sunspot. This paper also contains an analysis of the effect of off-limb overlapping structures on the observed Stokes I and V parameters and the BLOS obtained from the WFA. It is found that this value is equal to or smaller than the largest LOS magnetic field components of the two structures. In addition, using random BLOS, Doppler velocities and line intensities of these two structures leads in ~ 50% of the cases to Stokes I and V parameters unsuitable to be used with the WFA. Conclusions. Our results present a scarcity of LOS magnetic field components smaller than some 50 G, which must not be taken as evidence against the existence of these magnetic field strengths in spicules.

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