ON PHOTOSPHERIC FLUORESCENCE AND THE NATURE OF THE 17.62 A FEATURE IN SOLAR X-RAY SPECTRA

摘要

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 La photo- spheric fluorescent line strength caused by irradiation from an overlying corona. These calculations demonstrate that the photospheric Fe La characteristic line is much too weak to account for the observed 17.62 A lineflux. Instead, we identify this line with the configuration interaction ZS22p43p 2P/3/2-ZsZp6 2S1/2 transition in Fe XVIII seen in Electron Beam Ion Trap spectra and predicted in earlier theoretical work on the Fe xvin 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.