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 trajector y at a Jovicentric distance of about 8 R(J). The quasi-thermal noise m easured 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 te mperature from the spin modulation of Bernstein waves. These results a re 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 crossin g is twice as large as that expected from the Divine-Garrett Voyager-b ased model at the same radial distance. The density scale height is lo wer than that found by Voyager 1 ; it is consistent with an ion temper ature of about 5 x 10(5) K, assuming an effective mass of about 20 pro ton masses. The fitting of the pressure distribution, symmetric with r espect 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 m agnitude as found by Voyager 1.