Plasma Observations Near Jupiter: Initial Results from Voyager 2

Abstract

The first of at least nine bow shock crossings observed on the inbound pass of Voyager 2 occurred at 98.8 Jupiter radii (R_J) with final entry into the magnetosphere at 62 R_J. On both the inbound and outbound passes the plasma showed a tendency to move in the direction of corotation, as was observed on the inbound pass of Voyager 1. Positive ion densities and electron intensities observed by Voyager 2 are comparable within a factor of 2 to those seen by Voyager 1 at the same radial distance from Jupiter; the composition of the magnetospheric plasma is again dominated by heavy ions with a ratio of mass density relative to hydrogen of about 100/1. A series of dropouts of plasma intensity near Ganymede may be related to a complex interaction between Ganymede and the magnetospheric plasma. From the planetary spin modulation of the intensity of plasma electrons it is inferred that the plasma sheet is centered at the dipole magnetic equator out to a distance of 40 to 50 R_J and deviates from it toward the rotational equator at larger distances. The longitudinal excursion of the plasma sheet lags behind the rotating dipole by a phase angle that increases with increasing radial distance.