ELECTRON-DENSITY AND TEMPERATURE IN THE IO PLASMA TORUS FROM ULYSSES THERMAL NOISE MEASUREMENTS

During the Ulysses flyby of Jupiter, the spacecraft crossed the outer part of the lo plasma torus along a basically North-to-South trajectory at a Jovicentric distance of about 8 R(J). The quasi-thermal noise measured by the Unified Radio and Plasma Wave (URAP) experiment is used to deduce the electron density and temperature along the trajectory. The density is deduced from the upper hybrid frequency line and the temperature from the spin modulation of Bernstein waves. These results are used to build a simplified Gaussian model of the torus. The density profile is roughly symmetric with respect to the centrifugal equator, with a scale height of about 0.9 R(J). The density at equator crossing is twice as large as that expected from the Divine-Garrett Voyager-based model at the same radial distance. The density scale height is lower than that found by Voyager 1 ; it is consistent with an ion temperature of about 5 x 10(5) K, assuming an effective mass of about 20 proton masses. The fitting of the pressure distribution, symmetric with respect to the centrifugal equator, yields a cold electron temperature of about 1.4 x 10(5) K at the equator, which is of the same order of magnitude as found by Voyager 1.