Highly Ionized Sodium X-ray Line Emission from the Solar Corona and the Abundance of Sodium

TL;DR

This study uses X-ray observations and atomic data to estimate the sodium to neon abundance ratio in the solar corona, providing insights into the FIP effect and coronal composition.

Contribution

It presents new measurements of the Na/Ne abundance ratio in the solar corona using X-ray spectra and atomic data, clarifying the FIP effect's role in element abundance variations.

Findings

Na/Ne abundance ratio is approximately 0.07.

Line emission mainly from non-flaring active regions.

Low-temperature spectra suggest additional high-temperature components.

Abstract

{The \ion{Na}{x} X-ray lines between 10.9 and 11.2~\AA\ have attracted little attention but are of interest since they enable an estimate of the coronal abundance of Na to be made. This is of great interest in the continuing debate on the nature of the FIP (first ionization potential) effect. } {Observations of the \ion{Na}{x} lines with the Solar Maximum Mission Flat Crystal Spectrometer and a rocket-borne X-ray spectrometer are used to measure the Na/Ne abundance ratio, i.e. the ratio of an element with very low FIP to one with high FIP.} {New atomic data are used to generate synthetic spectra which are compared with the observations, with temperature and the Na/Ne abundance ratio as free parameters.} {Temperature estimates from the observations indicate that the line emission is principally from non-flaring active regions, and that the Na/Ne abundance ratio is $0.07 \pm 50$\%.} {The Na/Ne abundance ratio is close to a coronal value for which the abundances of low-FIP elements (FIP $< 10$~eV) are enhanced by a factor of 3 to 4 over those found in the photosphere. For low-temperature ($T_e \leqslant 1.5$~MK) spectra, the presence of \ion{Fe}{xvii} lines requires that either a higher-temperature component is present or a revision of ionization or recombination rates is needed. }