EMISSION LINES OF Ni XVIII IN THE SOLAR EUV SPECTRUM

Using electron excitation rates calculated with the R-matrix code, theoretical Nixviii electron-temperature-sensitive emission line ratios are presented for R ₁ = I(220.41 Å)/I(320.56 Å) , R ₂ = I(233.79 Å)/I(320.56 Å) , and R ₃ = I(220.41 Å)/I(292.00 Å) . A comparison of these with observational data for two solar flares, obtained by the Naval Research Laboratory's S082A slitless spectrograph on board Skylab, reveals good agreement between theory and observation for R ₁ and R ₂ in two spectra, which provides limited support for the accuracy of the atomic data adopted in the analysis. However, several of the measured ratios are much larger than theory predicts, which is probably due mainly to saturation of the strong 292.00 and 320.56 Å lines on the photographic film used to record the S082A data. A comparison of our line ratio calculations with active region observations made by the Solar EUV Rocket Telescope and Spectrograph (SERTS) indicate that a feature at 236.335 Å, identified as the Nixviii 3p ² P _3/2 - 3d ² D _3/2 transition in the SERTS data, is actually the Arxiii 2s ²2p ^{2 3} P ₀ - 2s2p ^{3 3} D ₁ line. The potential usefulness of the Nixviii line ratios as electron temperature diagnostics for the solar corona is briefy discussed.