Modelling low charge ions in the solar atmosphere

TL;DR

This paper extends ion modelling in the solar atmosphere to include effects like metastable levels, photo-ionisation, and charge transfer, showing significant differences from the coronal approximation in ion balances and line emissions.

Contribution

It introduces a comprehensive ion modelling approach for the solar transition region, incorporating additional physical processes not considered in the coronal approximation.

Findings

Ion balances differ significantly from the coronal approximation.

Metastable levels influence ionisation and recombination rates.

Line emission predictions are affected by the included processes.

Abstract

Extensions have been made recently to the coronal approximation for the purpose of modelling line emission from carbon and oxygen in the lower solar atmosphere. The same modelling is used here for other elements routinely observed in the solar transition region: N, Ne, Mg, Si and S. The modelling includes the effects of higher densities suppressing dielectronic recombination and populating long-lived, metastable levels; the presence of metastable levels typically causes effective ionisation rates to increase and recombination rates to decrease. Processes induced by the radiation field, namely photo-ionisation and photo-excitation, have been included, along with charge transfer, which occurs when electrons are exchanged during atom-ion and ion-ion collisions. The resulting ion balances are shown, and indicate significant changes compared to the frequently-employed coronal approximation. The effect on level populations within ions caused by photo-excitation is also assessed. To give an illustration of how line emission could be altered by these processes, selected line contribution functions are presented at the end.