Infrared spectropolarimetry of a C-class solar flare footpoint plasma. I. Spectral features and forward modelling

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 10827 Å, 10833.4 Å, 10834.9 Å, and 10838.9 Å photospheric lines, as well as the Si i Ca i Na i Ca i 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 profiles in which the spectral line contribution is blueshifted. In contrast, the photospheric and 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 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 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 10830 Å triplet signals in the future. Si i He i Ca i Na i Ca ii He i He i