On the Fine Structure Splitting of the 3p43d 4D5/2 and 3p43d 4D7/2 Levels of Fe X

TL;DR

This study estimates the fine structure splitting of specific Fe X energy levels using UV spectra from SKYLAB, revealing a small splitting of approximately 7 cm-1, which is relevant for understanding magnetic effects in the solar corona.

Contribution

The paper provides the first spectroscopic estimate of the fine structure splitting of Fe X 3p4 3d 4D levels using UV spectral lines, highlighting its importance for solar magnetic field studies.

Findings

Estimated splitting of ~7 cm-1 with uncertainty of 3 cm-1.

Identified the impact of non-thermal broadening on splitting measurements.

Provided optimal line width and splitting values from photographic data.

Abstract

We study UV spectra obtained with the SO82-B slit spectrograph on board SKYLAB to estimate the fine structure splitting of the Cl-like 3p4 3d 4D J=5/2 and 3p4 3d 4D J=7/2 levels of Fe X. The splitting is of interest because the Zeeman effect mixes these levels, producing a "magnetically induced transition" (MIT) from 3p4 3d 4D J=7/2 to 3p5 2Po J=3/2 for modest magnetic field strengths characteristic of the active solar corona. We estimate the splitting using the Ritz combination formula applied to two lines in the UV region of the spectrum close to 1603.2 Angstrom, which decay from the level 3p4(1D)3d 2G J=7/2 to these two lower levels. The MIT and accompanying spin-forbidden transition lie near 257 Angstrom. By careful inspection of a deep exposure obtained with the S082B instrument we derive a splitting of <~ 7 +/- 3 cm-1. The upper limit arises because of a degeneracy between the effects of non-thermal line broadening and fine-structure splitting for small values of the latter parameter. Although the data were recorded on photographic film, we solved for optimal values of line width and splitting of 8.3 +/- 0.9 and 3.6 +/- 2.7 cm-1.