Helium line formation and abundance during a C-class flare

TL;DR

This study investigates helium line formation and abundance during a C-class solar flare, using new techniques to analyze EUV radiation and model spectral lines, finding helium abundance close to the photospheric value.

Contribution

The paper introduces a novel method to determine coronal EUV radiation and helium abundance during a solar flare, extending previous analyses to later flare phases.

Findings

Helium abundance A(He)=0.075 +/- 0.010 in the chromosphere and transition region.

Helium abundance is marginally consistent with the photospheric value.

The effect of coronal back-radiation is generally less significant than helium abundance variations.

Abstract

During a coordinated campaign which took place in May 2001, a C-class flare was observed both with SOHO instruments and with the Dunn Solar Telescope of the National Solar Observatory at Sacramento Peak. In two previous papers we have described the observations and discussed some dynamical aspects of the earlier phases of the flare, as well as the helium line formation in the active region prior to the event. Here we extend the analysis of the helium line formation to the later phases of the flare in two different locations of the flaring area. We have devised a new technique, exploiting all available information from various SOHO instruments, to determine the spectral distribution of the photoionizing EUV radiation produced by the corona overlying the two target regions. In order to find semiempirical models matching all of our observables, we analyzed the effect on the calculated helium spectrum both of A(He) (the He abundance) and of the uncertainties in the incident EUV radiation (level and spectral distribution). We found that the abundance has in most cases (but not in all) a larger effect than the coronal back-radiation. The result of our analysis is that, considering the error of the measured lines, and adopting our best estimate for the coronal EUV illumination, the value A(He)=0.075 +/- 0.010 in the chromosphere (for T>6300 K) and transition region yields reasonably good matches for all the observed lines. This value is marginally consistent with the most commonly accepted photospheric value: A(He)=0.085.