The Transparency of Solar Coronal Active Regions

Resonance scattering has often been invoked to explain the disagreementbetween the observed and predicted line ratios of Fe XVII 15.01 A to Fe XVII15.26 A (the ``3C/3D'' ratio). In this process photons of 15.01, with its muchhigher oscillator strength, are preferentially scattered out of the line ofsight, thus reducing the observed line ratio. Recent laboratory measurements,however, have found significant inner-shell Fe XVI lines at 15.21 and 15.26Angstroms, suggesting that the observed 3C/3D ratio results from blending.Given our new understanding of the fundamental spectroscopy, we havere-examined the original solar spectra, identifying the Fe XVI 15.21 line andmeasuring 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 experimentalratio; hence, we find no need to invoke resonance scattering. Low opacity in FeXVII 15.01 also implies low opacity for Fe XV 284.2, ruling out resonancescattering as the cause of the fuzziness of TRACE and SOHO EIT 284-Angstromimages. The images must, instead, be unresolved due to the large number ofstructures at this temperature. Insignificant resonance scattering implies thatfuture instruments with higher spatial resolution could resolve the activeregion plasma into its component loop structures.Comment: accepted to Ap J Letter