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

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A rich spectrum of molecular hydrogen (H2) 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 Active Region 11861. Based on this observation, we have determined 37 new line identifications by comparing synthetic spectra produced using 1D modeling of H2 fluorescence. To avoid misidentification of the H2 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 H2 emission that occurs during the flares and find the following: (1) in spatially resolved observations the H2 line ratios may show optically thick line formation, contrary to previous results; (2) comparison of the spatial distribution of H2 Doppler velocities with those measured from other species reveals that H2 remote sensing probes an intermediate depth in the atmosphere between the photosphere and chromosphere, consistent with expectations from modeling; (3) the relationship between H2 line intensity and the observed intensity of its exciter is related to the atmospheric stratification; however, (4) H2 fluorescence can sometimes occur in response to radiation from distant sources many megameters away across the solar surface.