Solar Flare Ribbon Fronts I: Constraining flare energy deposition with IRIS spectroscopy

TL;DR

This study investigates the physical mechanisms behind the spectral profiles at the leading edge of solar flare ribbons, using IRIS observations and radiative hydrodynamic models to understand energy deposition during flares.

Contribution

It provides the first detailed comparison between IRIS spectral data and radiative hydrodynamic models to constrain flare energy deposition regimes at ribbon fronts.

Findings

Gradual, modest electron fluxes explain IRIS ribbon front spectra.

Stronger, impulsive fluxes are needed for chromospheric evaporation.

Ribbons show weaker transition region emission at the front.

Abstract

Lower atmospheric lines show peculiar profiles at the leading edge of ribbons during solar flares. In particular, increased absorption of the BBSO/GST \hei~10830~\AA\ line \citep[e.g.][]{Xu2016}, as well as broad and centrally reversed profiles in the spectra of the \mgii~and \cii~lines observed by the \iris~satellite \citep[e.g.][]{Panos2018,Panos2021a} have been reported. In this work, we aim to understand the physical origin of the \iris\ ribbon front line profiles, which seem to be common of many, if not all, flares. To achieve this, we quantify the spectral properties of the \iris~\mgii~ribbon front profiles during four large flares and perform a detailed comparison with a grid of radiative hydrodynamic models using the \radynfp~code. We also studied their transition region counterparts, finding that these ribbon front locations are regions where transition region emission and chromospheric evaporation are considerably weaker compared to other parts of the ribbons. Based on our comparison between the \iris~observations and modelling, our interpretation is that there are different heating regimes at play in the leading and trailing regions of the ribbons. More specifically, we suggest that bombardment of the chromosphere by more gradual and modest non-thermal electron energy fluxes can qualitatively explain the \iris~observations at the ribbon front, while stronger and more impulsive energy fluxes are required to drive chromospheric evaporation and more intense TR emission. Our results provide a possible physical origin for the peculiar behaviour of the \iris~chromospheric lines in the ribbon leading edge and new constraints for the flare models.