We study UV spectra obtained with the SO82-B slit spectrograph on board SKYLAB to estimate the fine structure splitting of the Cl-like 3p⁴3d⁴D_5/2 and 3p⁴3d⁴D_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 3p⁴3d⁴D_7/2 to 3p^{5 2}P^o_3/2 for modest magnetic field strengths characteristic of the active solar corona.

Raw S082B image with wavelength scale calibrated as described in the text. The image shown is an off-limb exposure, catalog number 3B158 007, a 19 minute 59 second exposure obtained on January 9th 1974 beginning at 18:41 UT. The “intensity” scale shown on the right is . The images of the spectrograph slit are seen along the abscissa. But because the slit was not in a solar image plane, there is no solar information in the slit direction.

We estimate the splitting using the Ritz combination formula applied to two lines in the UV region of the spectrum close to 1603.2 Å, which decay from the level 3p⁴(¹D)3d²G_7/2 to these two lower levels. The MIT and accompanying spin-forbidden transition lie near 257 Å. By careful inspection of a deep exposure obtained with the S082B instrument we derive a splitting of −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.