Behaviour of molecular hydrogen emission in three solar flares

Abstract

ABSTRACT We have systematically investigated ultraviolet (UV) emission from molecular hydrogen (H2) using the Interface Region Imaging Spectrograph (IRIS), during three X-ray flares of C5.1, C9.7, and X1.0 classes on 2014 October 25. Significant emission from five H2 spectral lines appeared in the flare ribbons, interpreted as photoexcitation (fluorescence) due to the absorption of UV radiation from two Si iv spectral lines. The H2 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-wing (blue-wing) components showed small redshifts (blueshifts) of ∼5–15 km s−1 (∼5–10 km s−1). The non-thermal velocities were found to be ∼5–15 km s−1. The interrelation between intensities of H2 lines and their branching ratios confirmed that H2 emission formed under optically thin plasma conditions. There is a strong spatial and temporal correlation between Si iv and H2 emissions, but the H2 emission is more extended and diffuse, further suggesting H2 fluorescence, and – by analogy with flare ‘back-warming’ – providing a means to estimate the depth from which the H2 emission originates. We find that this is 1871 ± 157 and 1207 ± 112 km below the source of the Si iv emission, in two different ribbon locations.