Solar flares are explosive events which produce their signatures at all layers of the solar atmosphere. The energy deposition occurs at the lower layers, chromosphere and temperature minimum region (TMR). The turbulent nature of plasma at these layers during flares could be studied using spectroscopic observations of cool lines.
We carried out a thorough investigation of activity at the TMR during flares using high cadence (~5 sec) spectroscopic observations of optically thin UV emission from molecular hydrogen, H2 using the Interface Region Imaging Spectrometer (IRIS). Five H2 spectral lines are formed by photo-excitation (fluorescence) due to the absorption of ultraviolet (UV) radiation from two Si IV spectral lines. Three X-ray flares of C5.1, C9.7 and X1.0 classes occurred on Oct. 25, 2014, and we studied the behaviour of various H2 lines at three flare ribbons during the systematic increase in X-ray activity.
The H2 lines were broad and needed multiple Gaussian to fit the lines. In addition to a stationary component in five H2 spectral profiles, we observed the presence of red wing and blue wing components indicating the downflows and upflows respectively. Each component was fitted independently with a single Gaussian. The Doppler velocities were measured to be 0-20 km/s (red-shifts) and -10 to -20 km/s (blue-shifts). The spectral line broadening and nonthermal velocity of H2 of 10-20 km/s showed evidence of turbulent plasma flows at TMR. In addition, we studied how the intensities of H2 lines belonging to the same upper-level transition are related to their branching ratios. We obtained evidence of the strong spatial and temporal correlation between Si IV and H2 emission observed at three ribbons during the X1.0 flare. We confirmed that the photo-excitation (fluorescence) process is responsible for strong H2 emission. The diffuse H2 emission observed around the flare ribbons might be the heating effect due to radiative backwarming as an observational driver for 3D modelling.
