We study UV spectra obtained with the SO82-B slit spectrograph on board SKYLAB to estimate the fine structure splitting of the Cl-like { { 3 p ^ { 4 } 3 d } { { } ^ { 4 } } { D _ { \mathrm { 5 / 2 } } } } and { { 3 p ^ { 4 } 3 d } { { } ^ { 4 } } { D _ { \mathrm { 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 { { 3 p ^ { 4 } 3 d } { { } ^ { 4 } } { D _ { \mathrm { 7 / 2 } } } } to { { 3 p ^ { 5 } } { { } ^ { 2 } } { P ^ { { o } } _ { \mathrm { 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 Å , which decay from the level { { 3 p ^ { 4 } ( ^ { 1 } D ) 3 d } { { } ^ { 2 } } { G _ { \mathrm { 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 \mathrel { \hbox to 0.0 pt { \lower 3.7 pt \hbox { $ \sim$ } \hss } \raise 1.7 pt \hbox { $ < $ } } 7 \pm 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 \pm 0.9 and 3.6 \pm 2.7 cm ^ { -1 } .