Laboratory Calibrations of Fe XII-XIV Line-Intensity Ratios for Electron Density Diagnostics

TL;DR

This study calibrates extreme ultraviolet line ratios for Fe XII-XIV ions using an electron beam ion trap, validating some diagnostics while highlighting discrepancies in Fe XII atomic data that affect solar density measurements.

Contribution

The paper provides laboratory calibration data for Fe XII-XIV line ratios and identifies atomic data issues impacting solar plasma diagnostics.

Findings

Fe XIII and Fe XIV line ratios are reliable for density diagnostics.

Discrepancies found between FAC theory and experimental data for Fe XII ratios.

Atomic data issues likely cause errors in solar coronal density measurements.

Abstract

We have used an electron beam ion trap to measure electron-density-diagnostic line-intensity ratios for extreme ultraviolet lines from F XII, XIII, and XIV at wavelengths of 185-205 255-276 Angstroms. These ratios can be used as density diagnostics for astrophysical spectra and are especially relevant to solar physics. We found that density diagnostics using the Fe XIII 196.53/202.04 and the Fe XIV 264.79/274.21 and 270.52A/274.21 line ratios are reliable using the atomic data calculated with the Flexible Atomic Code. On the other hand, we found a large discrepancy between the FAC theory and experiment for the commonly used Fe XII (186.85 + 186.88)/195.12 line ratio. These FAC theory calculations give similar results to the data tabulated in CHIANTI, which are commonly used to analyze solar observations. Our results suggest that the discrepancies seen between solar coronal density measurements using the Fe XII (186.85 + 186.88)/195.12 and Fe XIII 196.54/202.04 line ratios are likely due to issues with the atomic calculations for Fe XII.