Modelling Ion Populations in Astrophysical Plasmas: Carbon in the Solar Transition Region

R. P. Dufresne, G. Del Zanna

Published 2019-01-25Version 1

A collisional-radiative model for carbon has been developed to determine ion populations in lower-temperature, higher-density plasmas, such as the solar transition region. These conditions mean the often-used coronal approximation no longer holds for the modelling. The most up-to-date atomic rates have been employed for the processes which influence the populations in these regions. In the absence of level-resolved rates for electron-impact direct ionisation and excitation-autoionisation, new calculations have been made using atomic codes. Comparison with laboratory cross-sections and previous studies, where available, show satisfactory agreement. The ion populations resulting from the modelling are presented, demonstrating the influence each atomic process has as density and temperature vary. An initial investigation into the influence of photo-ionisation has also been investigated. Tests against observations have been made by comparing the ratio of predicted to observed intensities using differential emission measure modelling in the quiet-Sun transition region, showing noticeable improvements and particularly for the anomalous ion, C IV.