Infrared spectropolarimetry of a C-class solar flare footpoint plasma

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 <ns0:sc>I</ns0:sc> 10827 Å, Ca <ns0:sc>I</ns0:sc> 10833.4 Å, Na <ns0:sc>I</ns0:sc> 10834.9 Å, and Ca <ns0:sc>I</ns0:sc> 10838.9 Å photospheric lines, as well as the He <ns0:sc>I</ns0:sc> 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 <ns0:sc>I</ns0:sc> profiles in which the He <ns0:sc>I</ns0:sc> spectral line contribution is blueshifted. In contrast, the photospheric Ca <ns0:sc>I</ns0:sc> and Na <ns0:sc>I</ns0:sc> 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 <ns0:sc>II</ns0:sc> 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 <ns0:sc>I</ns0:sc> 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 <ns0:sc>I</ns0:sc> 10830 Å triplet signals in the future.