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

New R-matrix calculations of electron impact excitation rates for transitions among the 2s²2p, 2s2p² and 2p³ levels of Six 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 ~253-356Å 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 Six 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 N_e estimated from diagnostic lines in other species formed at similar electron temperatures to Six, such as Fexii and Fexiii. 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 Six 256.32-Å line is blended with the Heii transition at the same wavelength, while the feature at 292.25Å is not due to Six, but currently remains unidentified. The intensity of the 253.81-Å 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.