FE-XVIII EMISSION-LINES IN SOLAR-X-RAY SPECTRA

We have calculated intensity ratios for emission lines of Fe XVIII in the 13-94 Angstrom wavelength range at electron temperatures character istic of the solar corona, T-e = 2-10 x 10(6) K. Our model ion include s data for transitions among the 2s(2)2p(5), 2s2p(6), 2s(2)2p(4)3l, an d 2s2p(5)3l (l = s, p, and d) states. Test calculations which omit the 2s2p(5)3l levels show that cascades from these are important. We comp are our results with observed ratios determined from four solar X-ray instruments, a rocket borne spectrograph, and spectrometers on the P78 -1, OV1-17 and Solar Maximum Mission (SMM) satellites. In addition, we have generated synthetic spectra which we compare directly with flare observations from SMM. Agreement between theory and observation is ge nerally quite good, with differences that are mostly less than 30%, pr oviding limited support for the accuracy of the atomic physics data us ed in our calculations. However, large discrepancies are found for rat ios involving the 2s(2)2p(52)P(3/2)-2s2p(62)S line at 93.84 Angstrom, which currently remain unexplained. Our analysis indicates that the Fe XVIII feature at 15.83 Angstrom is the 2s(2)2p(52)P(3/2)-2s(2)2p(4)(P -3)3s(4)P(3/2) transition, rather than 2s(2)2p(52)P(3/2)-2s(2)2p(4)(P- 3)3s(2)P(3/2), as suggested by some authors.