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 Angstrom, in solar X-ray spectra is reexamined. Using a Monte Carlo technique, we compute a realistic theoretical upper limit to the observed Fe L alpha photospheric fluorescent line strength caused by irradiation from an overlying corona. These calculations demonstrate that the photospheric Fe L alpha characteristic line is much too weak to account for the observed 17.62 Angstrom, line flux. Instead, we identify this line with the configuration interaction 2s(2)2p(4)3p P-2(3/2)-2s2p(6) S-2(1/2) 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 Multi-mirror Mission.