We present a model of the latitudinal structure
of the Io plasma torus (IPT), which is able to explain Ulysses results
and to reconcile several
in situ data sets. Basically,
the observed temperature inversion and the polytropic law are due to
``velocity filtration'' of particles having non-Maxwellian
velocity distributions. This mechanism acts as a high
pass filter for
particle energies if the particles are confined in an attractive
monotonic potential well. These conditions are met in the IPT, where the
attractive potential is due to the centrifugal force that confines plasma
ions since the plasma is corotating with Jupiter, whereas electrons
are confined by an ambipolar electric field preserving electric neutrality,
and the electron
velocity distribution is known to have a suprathermal tail. The suprathermal
electron population has a velocity distribution that decreases with
increasing energy as a power law, as is frequently observed in space
plasmas, and the velocity distribution can be conveniently modeled
with a ``kappa'' function [
Meyer-Vernet, Moncuquet
and Hoang, Icarus, 116, 202, 1995]. Adopting such a kappa
distribution for the electrons and for all ion species detected in the
torus and including temperature anisotropy, we construct a
collisionless kinetic model based on the so-called ``bi-kappa distributions''
to calculate the latitudinal structure.
Following
Bagenal [
J. Geophys. Res., 99, 11043, 1994], we
adopt the nearly equatorial data set from Voyager 1 to
represent empirically the radial structure.
The model reconciles the Voyager 1 and
2 and Ulysses observations, and demonstrates that these data sets
possess similar latitudinal and radial variations of the IPT
densities and temperatures. This model also generates a radial ion
temperature profile past
7.5 Jovian radii, which is compatible
with a quasi-adiabatic radial temperature decrease at the torus
equator.
, Fran Bagenal
and Nicole
Meyer-Vernet