Resonance Scattering of Fe xvii X‐Ray and Extreme‐Ultraviolet Lines

Abstract

Over the years, a number of calculations have been carried out to derive intensities of various X-ray and extreme-ultraviolet (EUV) lines in Fe XVII for comparing with observed spectra. The predicted intensities have not agreed with solar observations, particularly for the line at 15.02 Å; resonance scattering has been suggested as the source for much of the disagreement. The atomic data calculated earlier by Bhatia & Doschek used seven configurations having n = 3 orbitals, and the scattering calculations were carried out only for incident energies above the threshold of the highest fine-structure level. These calculations have now been extended to thirteen configurations having n = 4 orbitals, and the scattering calculations are carried out below as well as above the threshold of the highest fine-structure level. These improved calculations of Fe XVII change the intensity ratios compared to those obtained earlier, bringing the optically thin F(15.02)/F(16.78) ratio and several other ratios closer to the observed values. However, some disagreement with the solar observations still persists, even though the agreement of the currently calculated optically thin F(15.02)/F(15.26) ratio with the experimental results of Brown and coworkers, and Laming and coworkers has improved. Some of the remaining discrepancy is still thought to be the effect of opacity, which is consistent with expected physical conditions for solar sources. EUV intensity ratios are also calculated and compared with observations. Level populations and intensity ratios are calculated as a function of column density of Fe XVII in the slab and cylindrical geometries. As found previously, the predicted intensities for the resonance lines at 15.02 and 15.26 Å exhibit initial increases in flux relative to the forbidden line at 17.10 Å and the resonance line at 16.78 Å as optical thickness increases. The same behavior is predicted for the lines at 12.262 and 12.122 Å. Predicted intensities for some of the allowed EUV lines are also affected by opacity.