MEASUREMENTS OF HOT PLASMAS IN THE MAGNETOSPHERE OF JUPITER

This paper presents an overview of a number of the principal findings regarding the hot plasmas (E greater than or similar to 50 keV) in Jup iter's magnetosphere by the HI-SCALE instrument during the encounter o f the Ulysses spacecraft with the planet in February 1992. The hot pla sma ion fluxes measured by HI-SCALE in the dayside magnetosphere are s imilar to those measured in the same energy range in this region by th e Voyager spacecraft in 1979. Within the dayside plasma sheet, the hot -ion energy densities are comparable with, or larger than, the magneti c field energy densities; these hot ions are found to corotate at abou t one-half the planetary corotational speed. For ions of energies grea ter than or similar to 500 keV/nucleon, the protons contributed from 5 0-60% to as much as 80% of the energy content of these plasmas. Strong , magnetic-field-aligned streaming was found for both the ions and ele ctrons in the high-latitude duskside magnetosphere. The ion and electr on pitch-angle distributions could be characterized by cos25 a through out many of the high anisotropy intervals of the outbound pass. There is some evidence in the ion pitch-angle distributions for a corotation al component in the hot plasmas at high Jovian latitudes. While there are limitations owing to the finite geometries of the detector telesco pe systems on the determination of the angular spreads of the ion and electron beams. the measurements show that there are intervals when th e particle distributions are not bidirectional. At such times, locally the hot plasmas could be carrying currents of approximately 10(-4) mu Am-2. The temporal variations in the streaming electron fluxes are sub stantially larger than the variations measured for the fluxes that are more locally mirroring. The temporal variations contain periodicities that may correspond to hydromagnetic wave frequencies in the magnetos phere as well as to larger scale motions of magnetospheric plasmas. On nearly half of the days for about a 130 day interval around the time of the Ulysses encounter with the planet, particles of Jovian origin w ere measured in the interplanetary medium. An event discussed herein s hows evidence of an energy dependence of the particle release process from the planetary magnetosphere into the interplanetary medium.