ASYMMETRIES IN THE IO PLASMA TORUS

Using Ulysses radio wave data taken during the 1992 Jupiter encounter, we conclude that there are significant large and small spatial scale azimuthal asymmetries at high latitudes in the Io plasma torus. During a period of time near perijove when the spacecraft motion was predomi nantly in the azimuthal direction and was relatively fixed in both lat itude and radial distance, inferred electron densities depart signific antly from the common assumption of longitudinal symmetry. Specificall y, electron plasma concentrations near 0 degrees system III longitude (and 0400 LT) are greater than those near 180 degrees (and 0000 LT). S uperposed on this large-scale variation are regularly spaced density d epletions, 30-50% in magnitude, and having a spatial periodicity of ab out 17 degrees. Some of these depletions may drive various known radio and plasma wave sources by means of large B parallel electric potenti als. The observations are compared with recent models and with the in situ Voyager observations.