1. Introduction

The Io plasma torus (IPT) in the inner Jovian magnetosphere contains charged particles confined by the action of the strong magnetic field and of the fast rotation of Jupiter, and ultimately supplied by the innermost Galilean satellite Io. The spatial distribution has a toroidal shape with inner and outer radii of approximately 5 and 12 ${\rm R_J}$(Jovian radii), respectively, and vertical thickness of 3${\rm R_J}$. Since different spacecraft have taken in situ measurements on at least 5 traversals of the torus, the IPT constitutes a valuable natural laboratory for comparing in situ measurements with remote plasma sensing techniques, for testing our understanding of the basic physics at work in the IPT and to provide a reliable model of its large-scale structure. Such a model is also a key requirement for understanding Jupiter as one of brightest radio sources in the sky.

The need for a new plasma torus model, especially its latitudinal structure, was driven by the Ulysses radio spectra acquired in 1992. In contrast to the Voyager 1 or Galileo spacecraft, Ulysses passed through the IPT on a north to south trajectory (see Figure 1)

Figure 1: The trajectories of four spacecraft that have flown through the Io torus: Voyager 1 (green), Voyager 2 (pink), Ulysses (red) and Galileo's initial orbit (blue). The contours show electron density from the model of Bagenal [1994] (isotropic case with $O_6$ magnetic field model and no current sheet)

and nearly tangentially to a magnetic shell ( $L \sim 8{\rm R_J}$), which allowed, for the first time, the determination of the electron density and temperature along the magnetic field. The principal and most unexpected result was that the electron temperature increased substantially with magnetic latitude (doubling over 7$^\circ$of latitude), and was anticorrelated with the electron density, obeying a polytropic law with an index $\sim 0.48$ [ Moncuquet et al., 1995, Meyer-Vernet, Moncuquet and Hoang, 1995]. This substantial variation of electron temperature was incompatible with previous IPT models [ Divine and Garrett, 1983, Bagenal, 1994]. In addition the observed latitudinal variation in electron temperature raises the question of whether the temperature of the ions varies similarly.

In this paper we find that even if we adopt multiple (Maxwellian) velocity distributions (core plus halo), the latitudinal variation in temperature and the equatorial confinement of the electrons is under estimated by the model when compared with the Ulysses observations. Introducing a temperature anisotropy at the equator helped confine the plasma to the equator, but yielded a temperature which decreases with latitude, contrary to Ulysses' observations. For these reasons we developed a new model.