On the fine structure splitting of the 3p⁴3d ⁴D_5/2 and 3p⁴3d ⁴D_7/2 levels of Fe x

We study UV spectra obtained with the SO82-B slit spectrograph on board SKYLAB to estimate the fine structure (FS) splitting of the Cl-like $3p^{4}3d{}^4{D}_{5/2}$ and $3p^{4}3d{}^4{D}_{7/2}$ levels of Fe x. The splitting is of interest because the Zeeman effect mixes these levels, producing a "magnetically induced transition" (MIT) from $3p^{4}3d{}^4{D}_{7/2}$ to $3p^5{}^2{P}_{3/2}^o$ 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 Å, which decay from the level $3p^4{(}^{1}D)3d{}^2{G}_{7/2}$ 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 $\lesssim 7\pm 3$ cm⁻¹. The upper limit arises because of a degeneracy between the effects of non-thermal line broadening and FS 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.