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

New R-matrix calculations of electron impact excitation rates for transitio ns among the 2s(2)2p, 2s2p(2) and 2p(3) levels of SI X are presented. These data are subsequently used, in conjunction with recent estimates for proto n excitation rates, to derive theoretical electron density sensitive emissi on-line ratios involving transitions in the similar to 253-356 Angstrom wav elength range. A comparision of these with observations of a solar active r egion and subflare, obtained during the 1989 flight of the Solar EUV Rocket Telescope and Spectrograph (SERTS), reveals that the electron densities de termined 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 goo d agreement with the values of N-e estimated from diagnostic lines in other species formed at similar electron temperatures to SI X, such as Fe XII an d Fe XIII. These results provide observational support for the general accu racy of the adopted atomic data, and hence line ratio calculations, employe d in the present analysis. However, we find that the SI X 256.32-Angstrom l ine is blended with the He ii transition at the same wavelength, while the feature at 292.25 Angstrom is not due to SI X, but currently remains uniden tified. The intensity of the 253.81-Angstrom line in the SERTS active regio n spectrum is about a factor of 3 larger than expected from theory, but the reason for this is unclear, and requires additional observations to explai n the discrepancy.