An Introduction to Photospheric Flare Line Diagnostics

In preparation for solar cycle 25, we present radiative hydrodynamic flare models from the F-CHROMA archive constructed using the RADYN and RH codes. We simulate potential observable perturbations seen in the line-of-sight velocity and heating induced in the photosphere during a flare. Many works have focused on the recreation of chromospheric spectral line profiles as the atmosphere rapidly heats and expands due to the injection of energy from magnetic reconnection. This extreme heating mechanism can permeate through the upper atmosphere and affect the lowest levels of the solar atmosphere. This results in an observable brightening and Doppler shifting of spectral lines formed in the deepest regions, though the extent of this photospheric reaction is still not fully characterised. We investigate how variations in the properties of a solar flare-accelerated electron beam result in discernible asymmetries in the flaring profiles of several deep forming Fe I spectral lines. Through analysis of the contribution functions and response functions of these lines during a flare, we have found an unprecedented level of high velocity chromospheric contribution which significantly alters the emergent profile of these lines. This comparatively weak, high velocity surplus contributing region of the chromosphere can result in false red shifts and/or significantly adds to the flaring Doppler shifted profile, resulting in a greater blue shift of the line core, which had previously been assumed as a purely photospheric velocity diagnostic. Based on this evidence, it is concluded that any future work using these photospheric lines to diagnose properties of the photosphere in a flaring atmosphere must be carefully analysed to mitigate effect of higher forming regions providing a surplus velocity signal.