Behaviour of molecular hydrogen emission in three solar flares

TL;DR

This study analyzes UV emission from molecular hydrogen during three solar flares, revealing fluorescence excitation, dynamic line profiles, and spatial correlations with Si IV emission, providing insights into the flare ribbon environment.

Contribution

It presents the first detailed analysis of H$_{2}$ emission profiles and their relation to flare ribbon structures during solar flares, highlighting fluorescence mechanisms and emission depths.

Findings

H$_{2}$ emission lines are excited by UV fluorescence from Si IV lines.

H$_{2}$ line profiles are broad with non-stationary components and small Doppler shifts.

H$_{2}$ emission originates approximately 1200-1870 km below Si IV source regions.

Abstract

We have systematically investigated ultraviolet (UV) emission from molecular hydrogen (H$_{2}$) using the Interface Region Imaging Spectrometer (IRIS), during three X-ray flares of C5.1, C9.7 and X1.0 classes on Oct. 25, 2014. Significant emission from five H$_{2}$ spectral lines appeared in the flare ribbons, interpreted as photo-excitation (fluorescence) due to the absorption of UV radiation from two Si IV spectral lines. The H$_{2}$ profiles were broad and consisted of two non-stationary components in red and in the blue wings of the line in addition to the stationary component. The red (blue) wing components showed small redshifts (blue shifts) of ~5-15 km s$^{-1}$ (~5-10 km s$^{-1}$). The nonthermal velocities were found to be ~5-15 km s$^{-1}$. The interrelation between intensities of H$_{2}$ lines and their branching ratios confirmed that H$_{2}$ emission formed under optically thin plasma conditions. There is a strong spatial and temporal correlation between Si IV and H$_{2}$ emission, but the H$_{2}$ emission is more extended and diffuse, further suggesting H$_{2}$ fluorescence, and - by analogy with flare ''back-warming'' providing a means to estimate the depth from which the H$_{2}$ emission originates. We find that this is 1871$\pm$157 km and 1207$\pm$112 km below the source of the Si IV emission, in two different ribbon locations.