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J.L.R. Saba (Lockheed Martin), S.O. Kastner, A.K. Bhatia (NASA/GSFC)
Motivated by the attempt to understand observations of Fe XVII X-ray line emission from solar active regions (see review by Saba et al. 1999, ApJ 510, 1064), Bhatia & Kastner (1999, ApJ in press) have recently calculated level populations, and X-ray and EUV line fluxes, for a 37-level model of the Fe XVII ion, as a function of column density. The model system was simulated by multiplying transition probabilities by population-dependent escape factors for two geometries: plane-parallel, traditionally adopted for tractability, and cylindrical, which better resembles coronal loop structures.
To date most analyses of active region line emission have assumed that the primary effect of low to moderate opacity (tau ~1 - 10) will be to scatter resonance line photons out of the line of sight, depleting the line intensity compared to the optically thin case. The new results show that the effects of opacity can run counter to intuition; in particular, the ratio of an optically thick resonance line intensity to that of a line less affected by opacity, can initially increase above the optically thin ratio with increasing column density. The counter-intuitive initial relative enhancement of the optically thick line occurs because at first the increase in level population can dominate over reduction by the emergent escape probability.
These results indicate that current analyses of Fe XVII lines in solar active regions and flares will need to be re-evaluated. It is important to establish whether the predicted behavior occurs in real sources. A study is under way to see if similar results apply more generally to other spectral lines commonly used in coronal observations. The onset of opacity effects on level populations at tau ~ 1, if present, could have a major impact on interpretation of coronal line-ratio diagnostic measurements.
This work was supported by NASA-RTOP Grant No. 344-12-53-14.
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