Behaviour of molecular hydrogen emission in three solar flares

We have systematically investigated ultraviolet (UV) emission from molecular hydrogen (H-2) 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 H-2 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 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-wing (blue-wing) components showed small redshifts (blueshifts) of similar to 5-15 km s(-1) (similar to 5-10 km s(-1)). The non-thermal velocities were found to be similar to 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 emissions, 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 +/- 157 and 1207 +/- 112 km below the source of the Si IV emission, in two different ribbon locations.