The influence of photo-induced processes and charge transfer on carbon and oxygen in the lower solar atmosphere

R P Dufresne,G Del Zanna,N R Badnell

doi:10.1093/mnras/stab514

Abstract

ABSTRACT To predict line emission in the solar atmosphere requires models that are fundamentally different depending on whether the emission is from the chromosphere or the corona. At some point between the two regions, there must be a change between the two modelling regimes. Recent extensions to the coronal modelling for carbon and oxygen lines in the solar transition region have shown improvements in the emission of singly and doubly charged ions, along with Li-like ions. However, discrepancies still remain, particularly for singly charged ions and intercombination lines. The aim of this work is to explore additional atomic processes that could further alter the charge-state distribution and the level populations within ions, in order to resolve some of the discrepancies. To this end, excitation and ionization caused by both the radiation field and by atom–ion collisions have been included, along with recombination through charge transfer. The modelling is carried out using conditions which would be present in the quiet Sun. This allows an assessment of the part atomic processes play in changing coronal modelling, separately from dynamic and transient events taking place in the plasma. The effect the processes have on the fractional ion populations are presented, as well as the change in level populations brought about by the new excitation mechanisms. Contribution functions of selected lines from low-charge states are also shown, to demonstrate the extent to which line emission in the lower atmosphere could be affected by the new modelling.

Highlights

The transition region in the solar atmosphere bridges the lower temperature, higher density chromosphere at the base with the tenuous, high temperature plasma in the corona
For the rest of the work, the results using the modelling of Chianti will be referred as the coronal approximation; the CR models which only include the processes given in Sect. 2.1.1 will be referred to as
To aid in the understanding of the reasons for the effects seen from the new processes, a list of ionisation and recombination rates for the low charge states for each atomic process are given in Tabs 1 and 2

Summary

Introduction

The transition region in the solar atmosphere bridges the lower temperature, higher density chromosphere at the base with the tenuous, high temperature plasma in the corona. The transition region (TR) undergoes a change in temperature from around 25000 K at the bottom to about 600000 K at the interface with the corona. These changes take place in a region only a few thousand kilometres thick. For predicting chromospheric line emission nothing short of time-dependent modelling with radiative transfer and (magneto-)hydrodynamics, along with atomic models featuring multiple levels, electron collisional, atomic collisional and photo-induced processes, will suffice This is demonstrated by such works as Kerr et al (2019), Lin et al (2017) and Judge et al (2003).

Objectives

Methods

Results

Discussion

Conclusion