Highly Ionized Potassium Lines in Solar X-ray Spectra and the Abundance of Potassium

TL;DR

This study measures potassium abundance in solar flare X-ray spectra, finding it higher than photospheric levels and suggesting FIP effect enrichment occurs in the low chromosphere, with implications for solar plasma composition models.

Contribution

It provides the first detailed analysis of highly ionized potassium lines in solar X-ray spectra, combining observations with synthesized spectra to determine potassium abundance.

Findings

Potassium abundance in solar corona is about 5.5 times higher than photospheric levels.

The observed spectra match synthesized models, confirming temperature predictions.

Enrichment of low-FIP elements likely occurs in the low chromosphere.

Abstract

The abundance of potassium is derived from X-ray lines observed during flares by the RESIK instrument on the solar mission CORONAS-F between 3.53 A and 3.57 A. The lines include those emitted by He-like K and Li-like K dielectronic satellites, which have been synthesized using the CHIANTI atomic code and newly calculated atomic data. There is good agreement of observed and synthesized spectra, and the theoretical behavior of the spectra with varying temperature estimated from the ratio of the two GOES channels is correctly predicted. The observed fluxes of the He-like K resonance line per unit emission measure gives log A(K) = 5.86 (on a scale log A(H) = 12), with a total range of a factor 2.9. This is higher than photospheric abundance estimates by a factor 5.5, a slightly greater enhancement than for other elements with first ionization potential (FIP) less than about 10 eV. There is, then, the possibility that enrichment of low-FIP elements in coronal plasmas depends weakly on the value of the FIP which for K is extremely low (4.34 eV). Our work also suggests that fractionation of elements to form the FIP effect occurs in the low chromosphere rather than higher up, as in some models.