S XI emission lines in active region spectra obtained with the Solar EUV Rocket Telescope and Spectrograph (SERTS)

Theoretical electron density sensitive emission line ratios involving a tot al of eleven 2s(2)2p(2)-2s2p(3) transitions in S XI between 187 and 292 Ang strom are presented. A comparison of these with solar active region observa tions obtained during rocket flights by the Solar EUV Rocket Telescope and Spectrograph (SERTS) reveals generally good agreement between theory and ex periment. However, the 186.87 Angstrom line is masked by fairly strong Fe X II emission at the same wavelength, while 239.83 Angstrom is blended with a n unknown feature, and 285.58 Angstrom is blended with possibly N IV 285.56 Angstrom. In addition, the 191.23 Angstrom line appears to be more serious ly blended with an Fe XIII feature than previously believed. The presence o f several new S XI lines is confirmed in the SERTS spectra, at wavelengths of 188.66, 247.14 and 291.59 Angstrom, in excellent agreement with laborato ry measurements. In particular, the detection of the 2s(2)2p(2) P-3(1) -2s2 p(3) P-3(0,1) transitions at 242.91 Angstrom is the first time (to our know ledge) that this feature has been identified in the solar spectrum. The pot ential usefulness of the S XI line ratios as electron density diagnostics f or the solar transition region and corona is briefly discussed.