An explanation for the absence of sounder‐stimulated gyroharmonic resonances in the Io plasma torus by the Ulysses relaxation sounder

Abstract

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 spectra were quite different than expected, based on earlier published observations from topside sounders in the terrestrial ionosphere and relaxation sounders in the terrestrial magnetosphere, in that the URAP sounder spectra did not contain resonances at the harmonics of the electron gyrofrequency (which are commonly excited in the ionosphere and magnetosphere). Arguments are presented here to support the earlier suggestion that the absence of sounder‐excited gyroharmonic resonances in the Io plasma torus was due to the relatively long receiver dead time and to the inability to satisfy the required echo rendezvous conditions.