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

Abstract

The Interface Region Imaging Spectrograph (IRIS) routinely observes the Si iv resonance lines. When analyzing observations of these lines it has typically been assumed 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, non-equilibrium 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 ionisation equilibrium. All simulations with an injected energy flux F > 5 × 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 ≈ 5 × 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 5×10 −6 g cm −2 over the temperature range 40 to 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.

Radiation hydrodynamics in simulations of the solar atmosphere

“…For most lines, and for most regions on the sun, the TR is optically thin. In solar flares this is not necessarily the case: Kerr et al (2019) showed that the TR can have appreciable optical thickness in the Si iv 140 nm lines.…”

Free electron Compton scattering produces the illusion of the universe expansion

“…The FEC and the Doppler effects were used to simulate the spectral line velocity in flare AR 12205. The value of K was roughly estimated around -28 2 2 10 m  and the electron density of the flare was in the range of 12 14 -3 10 10 cm : 31,32 . Since there is no accurate value for electron density, its scale, and the particle velocity, K can only be roughly estimated.…”

Evidence of chromospheric molecular hydrogen emission in a solar flare observed by the IRIS satellite

“…5 show histograms of the Si ⇤⌅ intensity ratios for the two ribbons, showing a greater tendency for the ratio to be less than 2 in R1 and greater than 2 in R2. This will be discussed in Section 3, particularly in the context of flare-specific simulations by Kerr et al (2019).…”