Latitudinal structure of outer Io plasma torus

Abstract

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 et al., 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 [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 quasiadiabatic radial temperature decrease at the torus equator.