Impact of the Ulysses velocity on the diagnosis of the electron density by the unified radio and Plasma wave sounder in the outskirts of the Io torus

The resonance spectra collected on February 8, 1992, in the outskirts of the Io plasma torus by the Unified Radio and Plasma wave (URAP) relaxation sounder on board the Ulysses spacecraft present significant differences from the active spectra gathered by earthbound spacecraft in similar plasma conditions. The most striking anomaly is the lack of resonances at the harmonics of the electron gyrofrequency, one of the most common signatures found in usual sounder spectra. These differences are interpreted, when computing the dispersion relation of the Bernstein modes triggered by the sounder, by taking into account the large frequency shift induced on the resonances by the high speed of the Ulysses spacecraft in the plasma. The frequency of the shifted resonances depends on the ratio V_sat/p /Vth of the spacecraft speed relative to the plasma and the thermal speed of the electrons. The observed resonances are all found to be in excellent agreement with the computed frequencies obtained in this way. Some predicted resonances are not observed on the Ulysses spectra, but it is suggested that they have too long a wavelength to be easily detected by the URAP antenna. As a by-product of the electron density diagnosis allowed by this analysis a rough estimate of the electron temperature can be deduced.