Fe XVII soft X-ray spectral emission lines are examined using data from the
Flat Crystal Spectrometer (FCS) on the Solar Maximum Mission satellite. Re
sults are compared with theoretical calculations and with other recent obse
rvational results. Disparate findings from different studies on the inferre
d 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 abundan
ces 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 q
uiescent active regions on the solar disk; the missing fraction increases a
t 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 li
ne center of the 15.0 Angstrom line is tau(0) similar to 2 on the disk, bas
ed on a simple escape probability treatment for a slab geometry. This sugge
sts 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 negligib
le for quiescent active region conditions. This is consistent with the lack
of systematic center-to-limb dependence found previously for FCS lines oth
er 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 locatio
n 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 li
mb compared to disk center.