A Si IV/O IV electron density diagnostic for the analysis of IRIS solar spectra

TL;DR

This paper develops a new electron density diagnostic for IRIS solar spectra using Si IV and O IV lines, accounting for high-density effects, and applies it to flare observations revealing densities up to 3×10^{13} cm^{-3}.

Contribution

It introduces a theoretical density diagnostic based on pressure-dependent ionization fractions and applies it to high-density solar flare spectra.

Findings

Densities ranged from 3×10^{12} to 3×10^{13} cm^{-3} during flare.

Stable density around 2×10^{12} cm^{-3} observed in some regions.

Method improves high-density plasma diagnostics in solar spectra.

Abstract

Solar spectra of ultraviolet bursts and flare ribbons from the Interface Region Imaging Spectrograph (IRIS) have suggested high electron densities of $>10^{12}$ cm$^{-3}$ at transition region temperatures of 0.1 MK, based on large intensity ratios of Si IV $\lambda$1402.77 to O IV $\lambda$1401.16. In this work a rare observation of the weak O IV $\lambda$1343.51 line is reported from an X-class flare that peaked at 21:41 UT on 2014 October 24. This line is used to develop a theoretical prediction of the Si IV $\lambda$1402.77 to O IV $\lambda$1401.16 ratio as a function of density that is recommended to be used in the high density regime. The method makes use of new pressure-dependent ionization fractions that take account of the suppression of dielectronic recombination at high densities. It is applied to two sequences of flare kernel observations from the October 24 flare. The first shows densities that vary between $3\times 10^{12}$ to $3 \times 10^{13}$ cm$^{-3}$ over a seven minute period, while the second location shows stable density values of around $2\times 10^{12}$ cm$^{-3}$ over a three minute period.