2009A&A...499..943L

We present the results of an intermediate coupling frame-transformation R-matrix calculation for the electron-impact excitation of Si⁹⁺. The target and close-coupling expansions are both taken to be the 125 fine-structure levels (58 LS terms) belonging to the configurations 2s^x2p^y (x+y=3) and 2s^α2p^β3l (α+β=2, l=s, p, and d). Due to the additional resonances included in our calculation, we find significant differences at low temperatures with the widely used n=2-2 excitation rates, also obtained with the R-matrix method, as well as with the n=2-3 excitation rates calculated by using the distorted wave (DW) approximation. We present a list of prominent transition lines and comparisons with SERTS and Hinode/EIS EUV spectra of the solar corona, SUMER observations for the quiet sun, as well as Chandra LETG and Rocket soft X-ray spectra of the Procyon corona and solar flares, respectively. Line emissivities of some transitions are enhanced up to 40% when compared with those obtained from using the previous atomic data at the same electron density (1.6x10⁹cm^–3) and temperature (1.3x10⁶K). The comparison with Chandra LETG observation of Procyon reveals that the 3s-2p line flux was significantly underestimated (by a factor of 4-5) in previous analyses. Some EUV and soft X-ray emission line ratios are n_e-sensitive and T_e-insensitive. Estimated electron densities from them shift downwards due to the new resonant-enhanced excitation data used in the present modelling.