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

Abstract

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 A, Fe XVII at 15.25 A, Fe XVII at 15.01 A, Ne IX at 13.45 A, and Mg XI at 9.17 A. 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 A, Fe XVII at 16.78 A, Ne IX at 13.70 A, and Mg XI at 9.31 A. In the latter two cases, the comparison line is the forbidden line of the He-like triplet. The spectra are from the Solar Maximum Mission Flat Crystal Spectrometer, which had a FWHM field of view of 15'' and could scan the soft X-ray resonance lines of prominent ions in the 1.5-20.0 A portion of the spectrum. Here 33 spectra are analyzed, all of which were obtained from nonflaring, quasi-stable active regions. For the quiescent regions selected, the data for the Fe XVII line at 15.01 A are clearly consistent with resonance scattering, with an increasing trend from Sun center to the limb. For the other lines tested, however, we find neither significant opacity effects nor center-to-limb variations.