HOPKINS ULTRAVIOLET TELESCOPE DETERMINATION OF THE IO TORUS ELECTRON-TEMPERATURE

Sulfur ion emissions from the Io plasma torus observed by the Hopkins Ultraviolet Telescope (HUT) in 1990 December have been analyzed to det ermine the effective temperature of the exciting electrons. Spectra we re obtained with a long slit that extended from 3.1 to 8.7 Jupiter rad ii (R(j)) on both dawn and dusk tours ansae. The average temperature o f electrons exciting S2+ emissions from the dawn ansa is (4800 +/- 240 0) K lower than on the dusk ansa, a dawn-dusk asymmetry comparable in both sign and magnitude to that measured by the Voyager Ultraviolet Sp ectrograph (UVS) experiment. Emissions from S2+ ions are generated in a source region with electron temperatures in the range 32,000-56,000 K; S3+ ion emissions are excited by electrons that average 20,000-40,0 00 K hotter. This distinct difference suggests that the S3+ emission s ource region is spatially separate from the S2+ source region. Estimat ed relative aperture filling factors suggest that the S3+ emissions or iginate from a region more extended out of the centrifugal plane than the S emissions.