  Resonance scattering has often been invoked to explain the disagreement
between the observed and predicted line ratios of Fe XVII 15.01 A to Fe XVII
15.26 A (the ``3C/3D'' ratio). In this process photons of 15.01, with its much
higher oscillator strength, are preferentially scattered out of the line of
sight, thus reducing the observed line ratio. Recent laboratory measurements,
however, have found significant inner-shell Fe XVI lines at 15.21 and 15.26
Angstroms, suggesting that the observed 3C/3D ratio results from blending.
Given our new understanding of the fundamental spectroscopy, we have
re-examined the original solar spectra, identifying the Fe XVI 15.21 line and
measuring its flux to account for the contribution of Fe XVI to the 15.26 flux.
Deblending brings the 3C/3D ratio into good agreement with the experimental
ratio; hence, we find no need to invoke resonance scattering. Low opacity in Fe
XVII 15.01 also implies low opacity for Fe XV 284.2, ruling out resonance
scattering as the cause of the fuzziness of TRACE and SOHO EIT 284-Angstrom
images. The images must, instead, be unresolved due to the large number of
structures at this temperature. Insignificant resonance scattering implies that
future instruments with higher spatial resolution could resolve the active
region plasma into its component loop structures.

Astrophysics