F. Herbert et Br. Sandel, RADIAL PROFILES OF ION DENSITY AND PARALLEL TEMPERATURE IN THE IO-PLASMA TORUS DURING THE VOYAGER-1 ENCOUNTER, J GEO R-S P, 100(A10), 1995, pp. 19513-19529
We have analyzed a moderately large set of Voyager 1 ultraviolet spect
rometer preencounter scans of the Io warm plasma torus in order to ded
uce its three-dimensional spatial structure. The density distributions
of the five ions O+, O++, S+, S++, and S+++ have been determined in o
rder to deduce their scale heights and thus the ion kinetic temperatur
e parallel to the magnetic field. We find that the parallel temperatur
e of the ions decreases with increasing Jovicentric distance. This var
iation is opposite to the profile of perpendicular ion temperature der
ived from in situ plasma measurements but consistent with an adiabatic
thermal profile. To reconcile these measurements, we propose that rap
id radial transport is adiabatically cooling the torus plasma and mixi
ng it with hot plasma from the middle magnetosphere. We also derive a
plasma ionization profile that increases with Jovicentric distance, co
nsistent 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 he
ated by a process that neglibly heats the ions; and (3) this electron
heating process is likely to be collisional thermalization with inward
ly 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 measureme
nts.