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

We have carried out the first comprehensive investigation of enhanced line emission from molecular hydrogen, H$_{2}$ at 1333.79 Å, 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 Å 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 Å 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 Å and Si IV 1402.77 Å 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.