Deriving the Coronal Hole Electron Temperature: Electron Density Dependent Ionization/Recombination Considerations

TL;DR

This paper refines the estimation of coronal hole electron temperatures by incorporating density-dependent ionization and recombination processes, improving accuracy over traditional methods especially during solar maximum.

Contribution

It introduces a more accurate method for deriving electron temperatures in coronal holes by including metastable level ionization and recombination effects.

Findings

Electron temperature in coronal holes is about 1.04 MK at solar maximum.

The temperature estimate is around 0.82 MK at solar minimum.

Density-dependent effects are negligible in coronal holes but significant during flares.

Abstract

Comparison of appropriate theoretical derived line ratios with observational data can yield estimates of a plasma's physical parameters, such as electron density or temperature. The usual practice in the calculation of the line ratio is the assumption of excitation by electrons/protons followed by radiative decay. Furthermore, it is normal to use the so-called coronal approximation, i.e. one only considers ionization and recombination to and from the ground state. A more accurate treatment is to include the ionization/recombination to and from meta-stable levels. Here, we apply this to two lines from adjacent ionization stages; Mg IX 368A and Mg X 625A, which has been shown to be a very useful temperature diagnostic. At densities typical of coronal hole conditions, the difference between the electron temperature derived assuming the zero density limit compared with the electron density dependent ionization/recombination is small. This however is not the case for flares where the electron density is orders of magnitude larger. The derived temperature for the coronal hole at solar maximum is around 1.04 MK compared to just below 0.82 MK at solar minimum.