Modelling ion populations in astrophysical plasmas: carbon in the solar transition region

Abstract

The aim of this work is to improve the modelling of ion populations in higher density, lower temperature astrophysical plasmas, of the type commonly found in lower solar and stellar atmospheres. Ion population models for these regions frequently employ the coronal approximation, which assumes conditions more suitable to the upper solar atmosphere, where high temperatures and lower densities prevail. Using the coronal approximation for modelling the solar transition region gives theoretical lines intensities for the Li-like and Na-like isoelectronic sequences which can be factors of 2-5 times lower than observed. The works of Burgess & Summers (1969) and Nussbaumer & Storey (1975) showed the important part ions in excited levels play when included in the modelling. Their models, however, used approximations for the atomic rates to determine the ion balance. Presented here is the first stage in updating these earlier models of carbon by using rates from up-to-date atomic calculations and more recent photo-ionising radiances for the quiet Sun. Where such atomic rates are not readily available, in the case of electron-impact direct ionisation and excitation--auto-ionisation, new calculations have been made and compared to theoretical and experimental studies. The effects each atomic process has on the ion populations as density changes is demonstrated, and final results from the modelling are compared to the earlier works. Lastly, the new results for ion populations are used to predict line intensities for the solar transition region in the quiet Sun, and these are compared with predictions from coronal-approximation modelling and with observations. Significant improvements in the predicted line intensities are seen in comparison to those obtained from zero-density modelling of carbon.

Draft accepted by A&A, 13 pages, 15 figures

References in corpus (7)

• CHIANTI - an atomic database for emission lines - Paper XV: Version 9, improvements for the X-ray satellite lines
• Radiative recombination data for modelling dynamic finite-density plasmas
• Non-Equilibrium Processes in the Solar Corona, Transition Region, Flares, and Solar Wind \textit{(Invited Review)}
• Density diagnostics derived from the OIV and SIV intercombination lines observed by IRIS
• The EUV spectrum of the Sun: quiet and active Sun irradiances and chemical composition
• Electron-Impact Multiple Ionization Cross Sections for Atoms and Ions of Helium through Zinc
• Elemental composition in quiescent prominences

Cited by in corpus (8)

• CHIANTI -- an atomic database for emission lines -- Paper XVI: Version 10, further extensions
• Linking the Sun to the Heliosphere Using Composition Data and Modelling. A Test Case with a Coronal Jet
• Modelling low charge ions in the solar atmosphere
• Effects of density on the oxygen ionisation equilibrium in collisional plasmas
• Scientific Objectives of the Hot Universe Baryon Surveyor (HUBS) Mission
• The influence of photo-induced processes and charge transfer on carbon and oxygen in the lower solar atmosphere
• A benchmark study of atomic models for the transition region against quiet Sun observations
• CHIANTI -- an atomic database for emission lines -- Paper XVIII. Version 11, advanced ionization equilibrium models: density and charge transfer effects