Mgix emission lines in an active region spectrum obtained with the Solar EUV Rocket Telescope and Spectrograph (SERTS)

Abstract

Theoretical electron density sensitive emission line ratios involving a total of eleven 2s22p2–2s2p3 transitions in S xi between 187 and 292 A are presented. A comparison of these with solar active region observations obtained during rocket flights by the Solar EUV Rocket Telescope and Spectrograph (SERTS) reveals generally good agreement between theory and experiment. However, the 186.87 A line is masked by fairly strong Fe xii emission at the same wavelength, while 239.83 A is blended with an unknown feature, and 285.58 A is blended with possibly N iv 285.56 A. In addition, the 191.23 A line appears to be more seriously blended with an Fe xiii feature than previously believed. The presence of several new S xi lines is confirmed in the SERTS spectra, at wavelengths of 188.66, 247.14 and 291.59 A, in excellent agreement with laboratory measurements. In particular, the detection of the 2s22p23P1 –2s2p33P0,1 transitions at 242.91 A is the first time (to our knowledge) that this feature has been identified in the solar spectrum. The potential usefulness of the S xi line ratios as electron density diagnostics for the solar transition region and corona is briefly discussed.