Galileo observations of electron beams and thermal ions in Jupiter's magnetosphere and their relationship to the auroras

The thermal ion parameters within the energy range 7 V to 53 kV and the electron velocity distributions for the energy range 1 kV to 53 kV are presented for three Galileo spacecraft orbits around Jupiter. The orbital segments extended from perijove at radial distances about 9 R-J out to positions in the plasma sheet at 50 R-J. Inbound segments and outbound segments were positioned in the local morning and premidnight sectors, respectively, in the Jovian equatorial magnetosphere. Because the magnetic equatorial plane is tilted with respect to the spacecraft orbital plane, the plasma sheet is traversed about every 5 hours at System III longitudes about 110degrees and 290degrees alternatively. At radial distances extending to about 30 R-J, there is a strong dependence of ion temperatures on System III longitude. The ion temperatures are higher at 290degrees relative to those at 110degrees. At perijove of 9 R-J the ion temperatures were about 5x10(6) K and similar to those in the hot Io torus at 7 R-J. Ion temperatures increased over the radial distance range of 10 to 25 R-J to values of about 5x10(7) to 10(8) K. At 9 to 20 R-J the ion bulk flows were about 60% of rigid corotational values and were well ordered and directed in the corotational direction. At greater distances in the plasma sheet the bulk motions became highly variable with large reductions in the corotational direction but with large radial outflows. This behavior exhibits a significant dependence on the local-time position of the spacecraft with greater radial flows in the premidnight sector. Intense electron beams which were aligned parallel and antiparallel to the magnetic field were detected at radial distances of about 20 to 30 R-J. For Jupiter's plasma sheet at these radial distances it appears likely that a region of intense plasma heating at low altitudes is responsible for the electron beams. These electron beams are identified with the main auroral ring. The electron energies in the beams span the range of several keV to tens of keV. Their projection into the auroral ionosphere provides for arc widths in the range of 1000 km or less. The energy fluxes at the Jovian auroral ionosphere, after correction for the angular width of the field of view of the electrostatic analyzer, extend up to 100 ergs/cm(2)-s and are sufficient to excite maximum far-ultraviolet emissions in the megarayleigh range as remotely observed with cameras.