Infrared spectropolarimetry of a C-class solar flare footpoint plasma

I. Spectral features and forward modelling

¹ Astronomical Institute, Slovak Academy of Sciences 05960 Tatranská Lomnica, Slovak Republic
² Evgeni Kharadze Georgian National Astrophysical Observatory Mount Kanobili 0301 Abastumani, Georgia
³ Space Research Centre, Ilia State University Kakutsa Cholokashvili Ave 3/5 0162 Tbilisi, Georgia
⁴ Instituto de Astrofísica de Canarias (IAC) E-38205 La Laguna Tenerife, Spain
⁵ Departamento de Astrofísica, Universidad de La Laguna E-38206 La Laguna Tenerife, Spain
⁶ AG, Institute of Physics, University of Graz Universitätsplatz 5 8010 Graz, Austria
⁷ National Solar Observatory 3665 Discovery Drive Boulder CO 80303, USA
⁸ Leibniz Institute for Astrophysics Potsdam (AIP) An der Sternwarte 16 14482 Potsdam, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 2 October 2025
Accepted: 8 December 2025

Abstract

We performed high-spatial-resolution spectropolarimetric observations of the active region NOAA 13363 during a C-class flare with the Gregor Infrared Spectrograph (GRIS) on 16 July 2023. We examined the coupling between the photosphere and the chromosphere, studying the polarimetric signals during a period that encompasses the decaying phase of a C-class flare and the appearance of a new C-class flare at the same location. We focused on the analysis of various spectral lines. In particular, we studied the Si I 10827 Å, Ca I 10833.4 Å, Na I 10834.9 Å, and Ca I 10838.9 Å photospheric lines, as well as the He I 10830 Å triplet. GRIS data revealed the presence of flare-related red- and blueshifted spectral line components, reaching Doppler velocities of up to ∼90 km s ⁻¹, and complex Si I profiles in which the He I spectral line contribution is blueshifted. In contrast, the photospheric Ca I and Na I transitions remain unchanged, indicating that the flare did not modify the physical conditions of the lower photosphere. We combined that information with simultaneous imaging in the Ca II H line and TiO band with the improved High-resolution Fast Imager (HiFI+), finding that the flare emission did not affect the inverse granulation or nearby plage, in agreement with the results from GRIS. We also complemented the previous studies with a forward modelling computation, concluding that the He I spectral line emission reflects a complex response of the flaring chromosphere. Radiative excitation from coronal EUV irradiation, energy deposition by flare-accelerated electrons, and dynamic field-aligned plasma flows likely act together to produce the observed supersonic downflows and upflows. We plan to expand these findings through inversions of the He I 10830 Å triplet signals in the future.