Fe XVIII EMISSION LINES IN SOLAR X-RAY SPECTRA

We have calculated intensity ratios for emission lines of Fexviii in the 13–94 Å wavelength range at electron temperatures characteristic of the solar corona, Te = 2–10 x 106 K. Our model ion includes data for transitions among the 2s22p5 , 2s2p6, 2s22p43l, and 2s2p53l (l = s, p, and d) states. Test calculations which 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 2s22p52P3/2- 2s2p62S line at 93.84 Å, which currently remain unexplained. Our analysis indicates that the FeXVIII feature at 15.83 Å is the 2s22p52P3/2 - 2s22p4(3P)3s 4P3/2 transition, rather than 2s22p52P3/2 - 2s22p4(3P)3s 2P3/2, as suggested by some authors.