Modeling Mg ii during Solar Flares. II. Nonequilibrium Effects

Abstract

To extract the information that the Mg ii NUV spectra (observed by the Interface Region Imaging Spectrograph; IRIS), carries about the chromosphere during solar flares, and to validate models of energy transport via model-data comparison, forward modelling is required. The assumption of statistical equilibrium is typically used to obtain the atomic level populations from snapshots of flare atmospheres, due to computational necessity. However it is possible that relying on statistical equilibrium could lead to spurious results. We compare solving the atomic level populations via statistical equilibrium versus a non-equilibrium timedependent approach. This was achieved using flare simulations from RADYN alongside the minority species version, MS RADYN, from which the time-dependent Mg ii atomic level populations and radiation transfer were computed in complete frequency redistribution. The impacts on the emergent profiles, lightcurves, line ratios, and formation heights are discussed. In summary we note that non-equilibrium effects during flares are typically important only in the initial stages and for a short period following the cessation of the energy injection. An analysis of the timescales of ionisation equilibrium reveals that for most of the duration of the flare, when the temperatures and densities are sufficiently enhanced, the relaxation timescales are short (τ relax < 0.1 s), so that the equilibrium solution is an adequate approximation. These effects vary with the size of the flare, however. In weaker flares effects can be more pronounced. We recommend that non-equilibrium effects be considered when possible, but that statistical equilibrium is sufficient at most stages of the flare.

Modeling the IRIS Lines During a Flare. I. The Blue-wing Enhancement in the Mg II k Line

“…We use a 10-level-plus-continuum Mg II atom that is the same as in Leenaarts et al (2013a). Nevertheless, RH solves the radiative transfer equation under the assumption of statistical equilibrium instead of non-equilibrium ionization, which can change the ionization fraction and lead to a different line formation height (Kerr et al 2019c). In order to mitigate this effect, we adopt the values of the electron density and hydrogen density from RADYN that conform to the non-equilibrium ionization and fix them in RH calculations, as done in Rubio da Costa & Kleint (2017) and Kerr et al (2019b).…”

“…In order to mitigate this effect, we adopt the values of the electron density and hydrogen density from RADYN that conform to the non-equilibrium ionization and fix them in RH calculations, as done in Rubio da Costa & Kleint (2017) and Kerr et al (2019b). We do note that for those weaker flares in our simulations (the f9 and f10 models), there will be non-equilibrium effects in the main heating phase (Kerr et al 2019c). However, for strong flares, the difference in the Mg II resonance line profiles between statistical equilibrium and non-equilibrium ionization is smaller than the difference between PRD and CRD in the main heating phase (Kerr et al 2019c).…”

Exploring mutual information between IRIS spectral lines. I. Correlations between spectral lines during solar flares and within the quiet Sun

“…The methods used in this paper can easily be extended and applied to the analysis of smaller flares, which would greatly extend the statistical validity of these results. It would also be of interest to investigate whether these correlations vary between smaller and larger flares, since there may exist fundamental differences in ionization equilibrium timescales (Kerr et al 2019a). Additionally, the MINE network represents a highly versatile tool, which could also be used to examine the mutual information between two different data types, such as images and spectra.…”

Exploring mutual information between IRIS spectral lines. II. Calculating the most probable response in all spectral windows

“…Kerr et al (2019a) showed that PRD effects are still important even in the presence of increased collisional rates from enhanced densities. Furthermore, the assumption of statistical equilibrium appears to hold for strong flares (Kerr et al 2019b). The Mg II k-line also has a companion of triplets at 2791.60, 2798.75 and 2798.82 Å, with the two mixed subordinate lines in the red wing going into emission during flares.…”