Fe XVIII Emission-Line Intensities in the Sun

Recently calculated electron-impact excitation rates among the 2s2 2p5, 2s2p6, and 2s2 2p4nl levels of Fe XVIII are used to derive theoretical emission-line ratios applicable to solar X-ray spectra. Oscillator strengths for all levels with n of less than or equal to 3 are used to take cascade involving the levels fully into account. Overall, the agreement between the theory and spectra measured by crystal spectrometers aboard the OV 1-17, P78-1, and SMM satellites is good. The effects of cascade from levels for which n is at least 4 are small, at least for the strongest lines. The labor required to take into account these higher levels is not justified by the current discrepancies between theory and observation.