Evidence of chromospheric molecular hydrogen emission in a solar flare observed by the IRIS satellite

TL;DR

This study presents the first detailed analysis of molecular hydrogen emission in a solar flare, revealing its excitation mechanism, spectral characteristics, and correlation with Si IV emission, offering insights into the temperature minimum region during flares.

Contribution

It provides the first comprehensive observation of H$_{2}$ emission in a solar flare and confirms fluorescent excitation by Si IV UV radiation as the excitation mechanism.

Findings

H$_{2}$ emission correlates with Si IV brightness during flare phases.

H$_{2}$ lines show non-thermal broadening indicating turbulence.

Optical thinness of Si IV varies spatially during the flare.

Abstract

We have carried out the first comprehensive investigation of enhanced line emission from molecular hydrogen, H$_{2}$ at 1333.79 {\AA}, observed at flare ribbons in SOL2014-04-18T13:03. The cool H$_{2}$ emission is known to be fluorescently excited by Si IV 1402.77 {\AA} UV radiation and provides a unique view of the temperature minimum region (TMR). Strong H$_{2}$ emission was observed when the Si IV 1402.77 {\AA} emission was bright during the flare impulsive phase and gradual decay phase, but it dimmed during the GOES peak. H$_{2}$ line broadening showed non-thermal speeds in the range 7-18 $\rm{km~s}^{-1}$, possibly corresponding to turbulent plasma flows. Small red (blue) shifts, up to 1.8 (4.9) $\rm{km~s}^{-1}$ were measured. The intensity ratio of Si IV 1393.76 {\AA} and Si IV 1402.77 {\AA} confirmed that plasma was optically thin to Si IV (where the ratio = 2) during the impulsive phase of the flare in locations where strong H$_{2}$ emission was observed. In contrast, the ratio differs from optically thin value of 2 in parts of ribbons, indicating a role for opacity effects. A strong spatial and temporal correlation between H$_{2}$ and Si IV emission was evident supporting the notion that fluorescent excitation is responsible.