DISPERSION OF ELECTROSTATIC-WAVES IN THE IO PLASMA TORUS AND DERIVED ELECTRON-TEMPERATURE

We present a detailed analysis of the set of radio spectra acquired during the Ulysses spacecraft passage through the outer part of the Io plasma torus (at similar to 8 R(J)). Since Ulysses is the first spacecraft to explore the torus far from the equator and the onboard plasma analyzers were shut off, these wave data are the only ones providing an in situ plasma diagnostics outside the equatorial region. We present here two main results. First, by comparing the observed spin modulation of the signal measured between electron gyroharmonics to the theoretical modulation for electrostatic waves propagating roughly norm al to B, we deduce experimental dispersion curves for these waves. These curves are very similar to Bernstein dispersion curves in a quasi-thermal plasma. Second, by fitting these theoretical dispersion relations to the experimental ones, we are able to deduce the core electron temperature with about 20% uncertainty when the density is measured independently. Otherwise, we can get a rough evaluation of both the density and the temperature. The corresponding latitu dinal variation of these parameters is analysed in a related paper (N, Meyer-Vernet, M. Moncuquet, and S. Hoang, 1995).