Fe I 6173 Å Emission During White-Light Solar Flares

Abstract: Between 2017 and 2024, the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory has observed numerous white-light solar flares (WLFs). HMI spectro-polarimetric observations of certain WLFs, in particular the X9.3 flare of September 6, 2017, reveal one or more locations within the umbra or along the umbra/penumbra boundary of the flaring active region where the Fe\,I 6173\,\AA~line briefly goes into full emission, indicating significant heating of the photosphere and lower chromosphere. For five flares featuring Fe\,I 6173\,\AA~line-core emission, we perform spectro-polarimetric analysis using HMI 90\,s cadence Stokes data. For all investigated flares, line-core emission is observed to last for a single 90\,s frame and is either concurrent with or followed by an increase in the line continuum intensity lasting one to two frames (90\,s\,--\,180\,s). Additionally, permanent changes to the Stokes Q, U, and/or V profiles were observed, indicating long-lasting non-transient changes to the photospheric magnetic field. These emissions coincided with local maxima in hard X-ray emission observed by Konus-Wind, as well as local maxima in the time derivative of soft X-ray emission observed by GOES 16-18. Comparison of the Fe\,I 6173\,\AA~line profile synthesis for the ad-hoc heating of the initial empirical VAL-S umbra model and quiescent Sun (VAL-C-like) model indicates that the Fe\,I 6173\,\AA~line emission in the white-light flare kernels could be explained by the strong heating of initially cool photospheric regions.