OBSERVATIONS OF ELECTRON-PLASMA WAVES UPSTREAM OF THE JOVIAN BOW SHOCK

The Ulysses spacecraft encountered the planet Jupiter in February 1992 , on its journey towards high heliospheric latitude. During the approa ch to the planet, as well as on the outbound pass, while receding from the Jovian bow shock, the Plasma Frequency Receiver that is part of t he Unified Radio and Plasma Wave experiment (URAP) recorded bursts of plasma waves in the frequency range of a few kHz. These emissions, fir st observed by the PWS experiment onboard the Voyager spacecraft, have been identified as upstream electron plasma waves. In this paper, we present the first analysis of the characteristics of these emissions, which are very similar to those found in the Earth's electron foreshoc k, upstream of the Earth's bow shock. These bursty emissions, with a p eak frequency very close to the local electron plasma frequency F(pe), have a typical electric field amplitude in the range 0.01-0.1 mV m-1, with some bursts above 1 mV m-1. The frequency bandwidth over which s ignificant power can be found above the instrument background noise ra nges from below 0.2 F(pe) to about 2 F(pe). On the basis of our presen t knowledge of similar emissions observed at Earth, we suggest that th e broadband emissions are triggered by suprathermal (a few tens of eV) electrons, streaming back from Jupiter's bow shock.