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 applications. The launch of the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) extreme-ultraviolet (EUV) spectrometer aboard the Solar and Heliospheric Observatory (SOHO) allows us to test the accuracy of certain computed relative excitation rate coefficients and transition probabilities for a number of important astrophysical ions. We use spectral line intensity ratios derived from SUMER spectra to compare these quantities with the best available 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 are important for many current and upcoming NASA astrophysics missions. In addition to the published atomic data, we calculate some new atomic data using the Hebrew University Lawrence Livermore Atomic Code (HULLAC). Our comparison of measured intensity ratios with theoretical predictions reveals significant discrepancies between the predicted and measured intensity ratios for several ions, particularly for S III, S IV, and S V. S III and S IV produce 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 continuum absorption on our data, and the ramifications of ignoring dielectronic capture resonances in certain transitions as a possible explanation for some of the discrepancies.