The structure of Jupiter's Io plasma torus inferred from Ulysses radio occultation observations

Abstract

The radio downlinks from Ulysses were monitored during the passage of the spacecraft through the Io plasma torus shortly after closest approach to Jupiter. The electron content of the Io torus was derived from the dispersive phase shift of the S-band (2.3 GHz) signal with respect to the X-band (8.4 GHz) signal. Corrections were applied for non-Jovian contributions to the change in electron content arising from the interplanetary medium and the terrestrial ionosphere. Ulysses' unusual North-to-South encounter trajectory was well suited for investigating the global structure of the Jovian plasma environment near Io's orbit, especially its latitudinal extent and azimuthal variations. The Io torus was found to have the same peak electron density (to within about ±20%) during the Ulysses flyby at Jupiter, as predicted by the Voyager-based models. The latitudinal scale height, however, was significantly smaller, indicating a reduction in the ion plasma temperature by a factor of 0.5 ± 0.2 with respect to the value inferred from Voyager measurements. The signal ray path sliced through bounded sectors of the Io plasma torus in front of and behind Jupiter, respectively, at different times and at different speeds. Model parameters derived from the best-fit electron content profiles for the sector restricted to Jovian System III longitudes 35° < λ III < 110° (front) differ significantly from those for the sector at 230° <λ III < 370° (rear), thereby implying longitudinal asymmetry of the Io torus.