RADIAL PROFILES OF ION DENSITY AND PARALLEL TEMPERATURE IN THE IO-PLASMA TORUS DURING THE VOYAGER-1 ENCOUNTER

We have analyzed a moderately large set of Voyager 1 ultraviolet spectrometer preencounter scans of the Io warm plasma torus in order to deduce its three-dimensional spatial structure. The density distributions of the five ions O+, O++, S+, S++, and S+++ have been determined in order to deduce their scale heights and thus the ion kinetic temperature parallel to the magnetic field. We find that the parallel temperature of the ions decreases with increasing Jovicentric distance. This variation is opposite to the profile of perpendicular ion temperature derived from in situ plasma measurements but consistent with an adiabatic thermal profile. To reconcile these measurements, we propose that rapid radial transport is adiabatically cooling the torus plasma and mixing it with hot plasma from the middle magnetosphere. We also derive a plasma ionization profile that increases with Jovicentric distance, consistent with in situ measurements of the electron temperature, which also rises with distance. From these observations we conclude that (1) torus plasma transport and the associated adiabatic cooling is faster than ion heating in the outer torus; (2) outer torus electrons are heated by a process that neglibly heats the ions; and (3) this electron heating process is likely to be collisional thermalization with inwardly transported hot plasma from the middle magnetosphere, as postulated by other authors. We also propose a picture of torus dynamics that is consistent both with these conclusions and with the in situ measurements.