ENERGETIC ELECTRON-BEAMS IN THE DUSKSIDE JOVIAN MAGNETOSPHERE - ULYSSES EPAC AND HI-SCALE MEASUREMENTS

The Ulysses flyby of Jupiter in February 1992 provided the first oppor tunity to investigate the high southern latitudes of the dusk side mag netosphere. We combine measurements taken by the Ulysses energetic par ticle instruments EPAC and HI-SCALE and identify bi-directional, field -aligned electron beams in an energy range from 100-380 keV. The beams are detected in southern Jovigraphic latitudes of 31 degrees-36 degre es and occur at distances between 14 and 80 Jovian radii (R-j) from th e planet. These beams have electron distributions with small pitch ang les and show a 10 h period. The periodic appearance of these electron beams is related to the position of the planetary magnetic dipole axis relative to the position of Ulysses. From the pitch angle distributio n (PAD) of the electron beams we also identify two different regions a long the Ulysses trajectory similar to ion fluxes and aniostropies dis cussed by Cowley et al. (1996). These regions can be characterised by broad electron PADs at higher magnetic latitudes near the planet and v ery pronounced beams at higher magnetic latitudes far from the planet in the outer duskside magnetosphere. For the bi-directionality of the electron PAD we discuss whether the duskside magnetosphere is magnetic ally open or closed favouring a mainly closed magnetosphere. (C) 1997 Elsevier Science Ltd. All rights reserved .