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Floyd Herbert, Bill Sandel (LPL)
About 47 hours of Voyager 1 ultraviolet spectrometer (UVS) observations of the Io plasma torus are analyzed to determine the distribution of ion and electron density and electron temperature (Te) as functions of (1) radius, (2) system III longitude (\lambdaIII), (3) Jovian local solar time, and (4) azimuth relative to Io. The uncertainty of these angular determinations is about ±5\circ.
We find a radial Te profile substantially the same as determined by in situ observations by the Voyager 1 plasma science investigation. We find that the ne profile has a very broad peak extending from \lambdaIII=90 to 180\circ, which includes the portion of the torus most closely approached by its mass source Io. The Te profile peaks around \lambdaIII=300\circ. The Jovian surface magnetic field reaches its lowest magnitude on the Io footprints near these longitudes, suggesting that torus electron heating might be associated with Birkeland currents connecting the torus with the Jovian ionosphere. Regarding local solar time, we find that the Te profile is highest near noon, while the ne profile peaks around dusk. The latter result agrees with the fact that torus plasma trajectories converge there, compressing the plasma. The Te maximum at the region of most rapid compression rate suggests a short cooling time. As for correlations with Io's position, we find the highest Io-frame Te near Io, while ne is maximized between 90 and 180\circ downstream from Io. The location of the Te maximum also suggests local heating associated with Birkeland currents, while the downstream ne enhancement indicates mass loading.
The author(s) of this abstract have provided an email address for comments about the abstract: herbert@argus.LPL.arizona.edu