SI IV Resonance Line Emission during Solar Flares: Non-LTE, Nonequilibrium, Radiation Transfer Simulations

The Interface Region Imaging Spectrograph routinely observes the Si IV resonance lines. When analyzing quiescent observations of these lines, it has typically been assumed that they form under optically thin conditions. This is likely valid for the quiescent Sun, but this assumption has also been applied to the more extreme flaring scenario. We used 36 electron-beam-driven radiation hydrodynamic solar flare simulations, computed using the RADYN code, to probe the validity of this assumption. Using these simulated atmospheres, we solved the radiation transfer equations to obtain the non-LTE, nonequilibrium populations, line profiles, and opacities for a model silicon atom, including charge exchange processes. This was achieved using the "minority species" version of RADYN. The inclusion of charge exchange resulted in a substantial fraction of Si IV at cooler temperatures than those predicted by ionization equilibrium. All simulations with an injected energy flux F > 5 x 10(10) erg cm(-2) s(-1) resulted in optical depth effects on the Si IV emission, with differences in both intensity and line shape compared to the optically thin calculation. Weaker flares (down to F approximate to 5 x 10(9) erg cm(-2) s(-1)) also resulted in Si IV emission forming under optically thick conditions, depending on the other beam parameters. When opacity was significant, the atmospheres generally had column masses in excess of 5 x 10(-6) g cm(-2) over the temperature range 40-100 kK, and the Si IV formation temperatures were between 30 and 60 kK. We urge caution when analyzing Si IV flare observations, or when computing synthetic emission without performing a full radiation transfer calculation.