Abstract
Abstract : We have calculated intensity ratios for emission lines of Fe XVIII in the 19-94A wavelength range at electron temperatures characteristic of the solar corona, Te = 2-10 x 10 to the 6th power K. Our model ion includes data for transitions among the 2s22p5, 2s2p6, 2s22p43l, and 2s2p53l (l = s, p, and d) states. Test calculations that omit the 2s2p53l levels show that cascades from these are important. We compare 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 generally quite good, with differences that are mostly less than 30%, providing limited support for the accuracy of the atomic physics data used in our calculations. However, large discrepancies are found for ratios involving the 2s22p5 2P3/2-2s2p6 2S line at 93.84A, which currently remain unexplained. Our analysis indicates that the Fe XVIII feature at 15.83A is the 2s22p5 2P3/2-2s22p4(3P)3s4 P3/2 transition, rather than 2s22p5 2P3/2-2s22p4(3P)3s 2P3/2, as suggested by some authors.
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