2023/09/14 by Sargam M. Mulay, Mulay, Sargam M., L. Fletcher +5
Earth and Planetary Sciences · Physics and Astronomy · #Atmospheric Ozone and Climate #FOS: Physical sciences #Ionosphere and magnetosphere dynamics #Solar and Space Plasma Dynamics #Solar and Stellar Astrophysics (astro-ph.SR)
paper · pdf · doi:10.48550/arxiv.2309.07799
openalex publication_date 2023/09/14 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
We have systematically investigated ultraviolet (UV) emission from molecular hydrogen (H2) using the Interface Region Imaging Spectrometer (IRIS), during three X-ray flares of C5.1, C9.7 and X1.0 classes on Oct. 25, 2014. Significant emission from five H2 spectral lines appeared in the flare ribbons, interpreted as photo-excitation (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 (blue) wing components showed small redshifts (blue shifts) of ~5-15 km s-1 (~5-10 km s-1). The nonthermal 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 emission, 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 km and 1207±112 km below the source of the Si IV emission, in two different ribbon locations.