The Influence of the Solar Coronal Radiation on Coronal Plasma Structures, I: Determination of the Incident Coronal Radiation

Gerrard M Brown,Nicolas Labrosse

doi:10.1007/s11207-018-1255-z

Abstract

Coronal structures receive radiation not only from the solar disc, but also from the corona. This height-dependent incident radiation plays a crucial role in the excitation and the ionisation of the illuminated plasma. The aim of this article is to present a method for computing the detailed incident radiation coming from the solar corona, which is perceived at a point located at an arbitrary height. The coronal radiation is calculated by integrating the radiation received at a point in the corona over all of the corona visible from this point. The emission from the corona at all wavelengths of interest is computed using atomic data provided by CHIANTI. We obtain the spectrum illuminating points located at varying heights in the corona at wavelengths between 100 and 912 Å when photons can ionise H or He atoms and ions in their ground states. As expected, individual spectral lines will contribute most at the height within the corona where the local temperature is closest to their formation temperature. As there are many spectral lines produced by many ions, the coronal intensity cannot be assumed to vary in the same way at all wavelengths and so must be calculated for each separate height that is to be considered. This code can be used to compute the spectrum from the corona illuminating a point at any given height above the solar surface. This brings a necessary improvement to models where an accurate determination of the excitation and ionisation states of coronal plasma structures is crucial.

Highlights

Structures located in the Sun’s corona, such as prominences, loops, streamers, or spicules, receive light directly from the solar disc and from the surrounding coronal plasma
In order to assess the influence of the radiation coming from the solar corona on the radiative processes within coronal structures, it is necessary to determine the characteristics of the incident radiation from the corona incident on a point at an arbitrary height above the solar surface
We present a method for computing the coronal spectrum illuminating a point located in the quiet solar corona at an arbitrary height, based on atomic data provided by CHIANTI (v7: Dere et al, 1997; Landi et al, 2012)

Summary

Introduction

Structures located in the Sun’s corona, such as prominences, loops, streamers, or spicules, receive light directly from the solar disc and from the surrounding coronal plasma. In the case of prominences, for instance, it has been well known since the work of Hirayama (1963) that the incident radiation is crucial for determining the excitation and ionisation of the prominence plasma, at wavelengths below the H ionisation threshold 35 Page 2 of 12. In order to assess the influence of the radiation coming from the solar corona on the radiative processes within coronal structures, it is necessary to determine the characteristics of the incident radiation from the corona incident on a point at an arbitrary height above the solar surface. The coronal spectrum incident on a structure located in the solar corona must be computed using our best knowledge of atomic data. As far as we are aware, this has not been done for an arbitrary height inside the corona before

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