A comparison of measurements of solar extreme-ultraviolet spectral line intensities emitted by C, N, O, and S ions with theoretical calculations

Atomic data for ionized atoms are important for many astrophysical applicat ions. The launch of the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) extreme-ultraviolet (EUV) spectrometer aboard the Solar and Helios pheric Observatory (SOHO) allows us to test the accuracy of certain compute d relative excitation rate coefficients and transition probabilities for a number of important astrophysical ions. We use spectral line intensity rati os derived from SUMER spectra to compare these quantities with the best ava ilable theoretical calculations for transitions within the ions C II, N III , N IV, O III, O IV, O V, S III, S IV, and S V. The results of this work ar e important for many current and upcoming NASA astrophysics missions. In ad dition to the published atomic data, we calculate some new atomic data usin g the Hebrew University Lawrence Livermore Atomic Code (HULLAC). Our compar ison of measured intensity ratios with theoretical predictions reveals sign ificant discrepancies between the predicted and measured intensity ratios f or several ions, particularly for S III, S IV, and S V. S III and S IV prod uce strong line emission in the Io torus. We discuss the methods we used to ensure that our ratios are accurate, the possible effects of Lyman continu um absorption on our data, and the ramifications of ignoring dielectronic c apture resonances in certain transitions as a possible explanation for some of the discrepancies.