Abstract

Splinter: Solar

Evidence of chromospheric molecular hydrogen emission in an IRIS flare

Sargam Mulay, Lyndsay Fletcher
School of Physics & Astronomy, University of Glasgow, UK

We present observations made during a flare of enhanced line emission from molecular hydrogen, H$_{2}$, 1333.79 Angstrom, which is known to be fluorescently excited by Si IV 1402.77 Angstrom UV radiation. The H$_{2}$ emission was mainly observed in flare ribbons and provides a unique view of the temperature minimum region (TMR). The temporal evolution of the H$_{2}$ emission was studied thoroughly using IRIS spectra by measuring plasma parameters such as line intensities, FWHM (0.037-0.084 Angstrom), Doppler (blue-shifts (red-shifts) ~1.8 (3.5) km/s) and nonthermal velocities (7.1-17.8 km/s). By looking at the ratio of Si IV (1393.76/1402.77) we deduce that the plasma is 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 Si IV ratio is between 1.8 and 2.0 during the GOES peak of the flare when H$_{2}$ emission was relatively weak. The variation in the H$_{2}$ and Si IV line intensities and scatter plots confirmed that the ribbon emission at these two wavelengths is strongly correlated, consistent with fluorescent excitation.