Summary (version française ici)

The inner Jovian magnetosphere known as the ``Io plasma torus'' (IPT) contains charged particles confined by the action of the strong magnetic field and fast rotation of Jupiter and ultimately supplied by mass loss from the innermost Galilean satellite Io. Its spatial distribution is a toroidal plasma cloud with inner and outer radii of approximately 5 and 10 Jovian radii, respectively, and vertical thickness of   3 Jovian radii. It is the most beautiful example of plasma confinement in a planetary magnetosphere. Because, previously, five different spacecrafts have performed in situ measurements and, currently, the Galileo spacecraft is making repeated measurements of magnetospheric properties, the IPT constitutes a precious natural laboratory for testing and validating in situ as well as remote plasma sensing techniques. A reliable model of the IPT structure is also a key requirement for understanding Jupiter as one of brightest radio sources in the sky.

My thesis is divided in two parts: In the first part, I present two novel methods of using radio spectra to determine the electron density and temperature in the IPT. These methods exploit characteristics of the URAP experiment onboard the Ulysses spacecraft (the radio receiver's great sensitivity, a long wire antenna, and a spinning spacecraft) and the plasma environment which is a quasi-collisionless, magnetized, rotating plasma that supports oscillation of Bernstein waves. These methods can be summarized as " spectroscopy of quasi- thermal noise measured by an antenna imbedded in a magnetized plasma " [Moncuquet, Meyer-Vernet and Hoang, J. Geophys. Res.,100, 21697, 1995; Moncuquet et al.,J. Geophys. Res.,102, 2373, 1997]. In contrast to the Voyager 1 or Galileo spacecraft, Ulysses passed through the IPT basically on a north to south trajectory and nearly tangentially to a magnetic shell, which allowed, for the first time, the determination of the electron density and temperature along the magnetic field lines. The principal and most unexpected result is that the electron temperature increases substantially with magnetic latitude (it doubles within of latitude range), is anticorrelated with the electron density, and obeys a polytropic law with an index . The substantial variation of electron temperature found along lines is incompatible with the hypothesis of constant temperatures along magnetic field lines for each plasma specie assumed in all previous models of the IPT. Thus these models may be invalid if the ion temperatures vary as much with magnetic latitude as the electron temperature.

The second part is devoted to a new model of the latitudinal structure of the IPT, which is able to explain the unexpected results from Ulysses and to reconcile several in situ data sets. To explain the observed temperature inversion and the polytropic law, we adopt the "velocity filtering" mechanism, first proposed by J.D. Scudder [Astrophys. J., 398, 299, 1992] to explain stellar coronal temperature profiles. This mechanism acts as a high pass filter for particle energies if the particles are confined in an attractive monotonic potential well and have a non-maxwellian velocity distribution. These conditions are met in the IPT, where the attractive potential is due to centrifugal force that confines plasma ions (since the plasma is corotating with Jupiter) and hence electrons by an ambipolar electric field to preserve neutrality and the electron velocity distribution has a suprathermal tail. The suprathermal electron population has a velocity distribution that decreases with increasing energy as a power law, as is frequently the case in space plasmas, and the velocity distribution can be conveniently modeled mathematically with a " kappa " distribution [Meyer-Vernet, Moncuquet and Hoang, Icarus, 116, 202, 1995]. Adopting a kappa distribution for the electrons and all ion species detected in the torus and including anisotropy effects with respect to the magnetic field, I construct a kinetic model based on the so-called ``anisotropic bi-kappa'' distributions to calculate the latitudinal structure. Following F. Bagenal [J. Geophys. Res., 99, 11043, 1994], I adopt the nearly equatorial data set from Voyager 1 to empirically represent the radial structure. My 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.