Molecular Hydrogen Line Identifications in Solar Flares Observed by IRIS: Lower Atmospheric Structure from Radiometric Analysis

A rich spectrum of molecular hydrogen (H$_2$) emission lines is seen in sensitive observations from the far ultraviolet (FUV) channels of the Interface Region Imaging Spectrograph (IRIS) during flare activity in solar active region NOAA 11861. Based on this observation we have determined 37 new line identifications by comparing synthetic spectra produced using 1D modeling of H$_2$ fluorescence. To avoid misidentification of the H$_2$ lines, we have also compiled a complete list of atomic line identifications for the IRIS FUV bandpasses from previous work. We carry out analysis of the spatially resolved H$_2$ emission that occurs during the flares and find that: (1) in spatially resolved observations the H$_2$ line ratios may show optically thick line formation, contrary to previous results; (2) comparison of the spatial distribution of H$_2$ Doppler velocities with those measured from other species reveals that H$_2$ remote sensing probes an intermediate depth in the atmosphere between the photosphere and chromosphere, consistent with expectations from modeling; (3) the relationship between H$_2$ line intensity and the observed intensity of its exciter is related to the atmospheric stratification; however, (4) H$_2$ fluorescence can sometimes occur in response to radiation from distant sources many Mm away across the solar surface.