AN EXPLANATION FOR THE ABSENCE OF SOUNDER-STIMULATED GYROHARMONIC RESONANCES IN THE IO PLASMA TORUS BY THE ULYSSES RELAXATION SOUNDER

Plasma resonances were excited by the relaxation-sounder component of the unified radio and plasma wave instrument (URAP) during the Ulysses encounter with Jupiter's Io plasma torus. The observed resonant spect ra were quite different than expected, based on earlier published obse rvations from topside sounders in the terrestrial ionosphere and relax ation sounders in the terrestrial magnetosphere, in that the URAP soun der spectra did not contain resonances at the harmonics of the electro n gyrofrequency (which are commonly excited in the ionosphere and magn etosphere). Arguments are presented here to support the earlier sugges tion that the absence of sounder-excited gyroharmonic resonances in th e Io plasma torus was due to the relatively long receiver dead time an d to the inability to satisfy the required echo rendezvous conditions.