Investigating the Effect of Opacity in Soft X‐Ray Spectral Lines Emitted by Solar Coronal Active Regions

Current literature suggests that several lines in the soft X-ray portion of the coronal spectrum may not be optically thin. Here, we wish to check this possibility for five of the brightest resonance lines in this part of the spectrum—O VIII at 18.97 Å, Fe XVII at 15.25 Å, Fe XVII at 15.01 Å, Ne IX at 13.45 Å, and Mg XI at 9.17 Å. A comparison is made between each of these resonance lines and an optically thin "reference" line produced by the same element in the same ionization state—O VIII at 16.01 Å, Fe XVII at 16.78 Å, Ne IX at 13.70 Å, and Mg XI at 9.31 Å. In the latter two cases, the comparison line is the forbidden line of the He-like triplet.