On Photospheric Fluorescence and the Nature of the 17.62 Å Feature in Solar X-Ray Spectra

The identification of the emission-line feature at 17.62 A in solar X-ray spectra is reexamined. Using a Monte Carlo technique, we compute a realistic theoretical upper limit to the observed Fe Lα photospheric fluorescent line strength caused by irradiation from an overlying corona. These calculations demonstrate that the photospheric Fe Lα characteristic line is much too weak to account for the observed 17.62 A line flux. Instead, we identify this line with the configuration interaction 2s22p43p 2P3/2-2s2p6 2S1/2 transition in Fe XVIII seen in Electron Beam Ion Trap spectra and predicted in earlier theoretical work on the Fe XVIII X-ray spectrum. This line should be easily resolved and detected in stellar coronae by the spectrographs on the upcoming Chandra X-ray Observatory and X-ray Multimirror Mission.

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