Electron-impact excitation of Be-like Mg

Abstract

We present an R-matrix calculation of electron-impact excitation of Be-like Mg. The calculation is similar to that one presented for Be-like Fe by Chidichimo et al. (2005, A&A, 430, 331), and was done with the intermediate-coupling frame transformation method and including a total of 98 fine-structure levels, up to n=4. We find significant differences with the widely used n=2 excitation rates of Keenan et al. (1986, Phys. Scr., 34, 216), calculated by interpolating R-matrix calculations along the Be-like sequence. We present a list of the most important transitions and a few comparisons with SOHO SUMER and CDS/GIS spectra of the solar corona. We show that previous long-standing discrepancies between observed and predicted line intensities are now resolved. We also show how temperatures of the solar corona were significantly underestimated. For example, a coronal hole inter-plume temperature of 850 000 K found by Wilhelm et al. (1998, ApJ, 500, 1023) is now revised to 1 160 000 K.