Fe XVII soft X-ray lines: Theory and data comparisons

Fe XVII soft X-ray spectral emission lines are examined using data from the Flat Crystal Spectrometer (FCS) on the Solar Maximum Mission satellite. Results are compared with theoretical calculations and with other recent observational results. Disparate findings from different studies on the inferred opacity of the bright resonance line at 15.01 Angstrom and on its center-to-limb behavior are reviewed. Present limitations on the use of resonance scattering to infer coronal plasma densities and absolute elemental abundances are discussed. An analysis is made of the temperature-insensitive ratio of the 15.01 Angstrom line of Fe XVII to the optically thin 16.78 Angstrom. line. This analysis shows that approximately half of the photons expected in the 15.01 Angstrom line are missing from the bright emission cores of quiescent active regions on the solar disk; the missing fraction increases at most by 50% near the solar limb. If the missing flux has been resonantly scattered out of the line of sight, then the equivalent optical depth at line center of the 15.0 Angstrom line is tau(0) similar to 2 on the disk, based on a simple escape probability treatment for a slab geometry. This suggests that the effects of resonance scattering for other FCS lines, with the possible exception of the O vm doublet at 18.97 Angstrom should be negligible for quiescent active region conditions. This is consistent with the lack of systematic center-to-limb dependence found previously for FCS lines other than Fe XVII at 15.01 Angstrom. Yohkoh Soft X-ray Telescope measurements of the expected lines of sight for active regions as a function of location on the solar disk, and resonance scattering results from other soft X-ray active region data sets all support a trend of increased opacity at the limb compared to disk center.