Solar Si  XI Line Ratios Observed by the Normal Incidence Spectrometer on SOHO CDS

New measurements of line intensity ratios in the Be-like ion Si xi are presented for observations of the quiet Sun, active regions, coronal holes and above-limb regions obtained using the Coronal Diagnostic Spectrometer on the Solar and Heliospheric Observatory. A model ion, constructed using the best available atomic data, is used to predict the line intensity ratios for a wide range of electron temperatures and densities. Comparisons of the theoretical ratios with the new intensity ratios as well as with those from previous solar observations and laboratory measurements are given. The usefulness of the ratios for electron temperature and density diagnostics, as well as for spectrometer calibration, is discussed.