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Session 1 - Chromosphere, Corona, Flares.
Display session, Friday, June 27
Ballroom B, Chair: Charles Kankelborg

[1.45] Clarifying the Picture of Fe XVII Opacity in the Solar Corona

J. L. R. Saba, K. T. Strong (Lockheed-Martin), J. T. Schmelz (Univ. of Memphis)

Several recent studies of opacity in the bright resonance line of Fe XVII at 15.01 Å\ have yielded disparate results. Discrepancies include the magnitude of the inferred resonance scattering effects, the amount of center-to-limb variation, and even the sign of the center-to-limb change.

The larger optical depths inferred by Waljeski et al. (1994 ApJ 429, 909) compared to those found by Schmelz, Saba, amp; Strong (1992 ApJ 398, L115) and by Saba et al. (1997, in prep.) in active region data from the Solar Maximum Mission Flat Crystal Spectrometer (FCS) could be explained by allowing for an estimated 20% relative uncertainty in the calculated emissivities of pairs of Fe XVII lines (A.K. Bhatia, private communication). The lower values of optical depth are supported by a demonstrated lack of measurable opacity in other potentially affected FCS lines (Schmelz et al. 1997 ApJ 477, 509).

On the other hand, it is impossible to reconcile the report by Phillips et al. (1997 ApJ 469, L57) of a large decrease in opacity from disk center to the limb, with the more usual behavior found by Schmelz et al. (1997) and Saba et al. (1997), i.e., a slight increase in opacity for regions at or near the limb due to the increased line of sight. A reality check can be made with Yohkoh SXT intensities and with other soft X-ray spectroscopic data sets.

We will also discuss the use and abuse of resonance scattering as a coronal plasma diagnostic tool. Resonance scattering is likely to be relevant for SOHO observations, for some bright EUV resonance lines with high elemental abundance, large ionization fraction, and moderate oscillator strength, such as Fe XV at 284 ÅP>This work was supported by NASA contract NASW-4814 and the Lockheed-Martin Independent Research Program.


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