Si IV Resonance Line Emission During Solar Flares: NLTE radiation transfer simulations

The Interface Region Imaging Spectrograph (IRIS) has routinely observed the strong Si IV resonance lines during solar flares and other heating events. When analyzing quiescent observations of these lines it has typically been assumed that they are optically thin. Approaches that employ an optically thin assumption and do not solve the full radiation transfer (e.g. using the optically thin assumption with the CHIANTI atomic database) can then be used to produce synthetic spectra from simulations with which to compare to observations. However, this assumption has also been applied in the flaring scenario. We used radiation hydrodynamic simulations of solar flares, computed using the RADYN code, to probe the validity of this assumption. Using flare atmospheres we solved the radiation transfer to obtain the non-LTE, non-equilibrium populations, line profiles, and opacities for a 30 level Si atom. Our electron beam driven flare simulations covered a range of beam parameters: energy flux, F = [5x10^8-1x10^11]erg/cm^2/s, low energy cutoffs, Ec = [20,30,40]keV, and spectral indices, delta = [4,6]. All simulations with an injected energy flux F>5x10^10 erg/cm^2/s resulted in optically thick Si IV emission. Lower energy flares also resulted in optically thick Si IV emission, depending on the other beam parameters. Beams that penetrated deeper into the chromosphere were less likely to produce optically thick emission compared to those which strongly heated the upper chromosphere and transition region. We show the some basic plasma properties at the formation region of the resonance lines, commenting on the circumstances that result in optically thick emission. Finally we compare the Si IV emission to the other strong lines observed by IRIS (the Mg II k line, Mg II 2791 line, C II 1334 line, ans the O I 1356 line) in a case study of one strong flare simulation, noting that the lines forming in the uppermost chromosphere are more intense than observed. Since we demonstrate that Si IV can become optically thick in flaring conditions we urge caution when analyzing observations, or when computing synthetic emission assuming optically thin emission.