Si X emission lines in spectra obtained with the Solar EUV Rocket Telescope and Spectrograph (SERTS)

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New R-matrix calculations of electron impact excitation rates for transitions among the 2s22p, 2s2p2 and 2p3 levels of Si x are presented. These data are subsequently used, in conjunction with recent estimates for proton excitation rates, to derive theoretical electron density sensitive emission-line ratios involving transitions in the ∼253–356 A wavelength range. A comparision of these with observations of a solar active region and subflare, obtained during the 1989 flight of the Solar EUV Rocket Telescope and Spectrograph (SERTS), reveals that the electron densities determined from most of the Si x line ratios are consistent with one another for both solar features. In addition, the derived densities are also in good agreement with the values of Ne estimated from diagnostic lines in other species formed at similar electron temperatures to Si x, such as Fe xii and Fe xiii. These results provide observational support for the general accuracy of the adopted atomic data, and hence line ratio calculations, employed in the present analysis. However, we find that the Si x 256.32-A line is blended with the He ii transition at the same wavelength, while the feature at 292.25 A is not due to Si x, but currently remains unidentified. The intensity of the 253.81-A line in the SERTS active region spectrum is about a factor of 3 larger than expected from theory, but the reason for this is unclear, and requires additional observations to explain the discrepancy.