Energetic electron beams in the duskside Jovian magnetosphere: Ulysses EPAC and HI-SCALE measurements

Abstract

The Ulysses flyby of Jupiter in February 1992 provided the first opportunity to investigate the high southern latitudes of the dusk side magnetosphere. We combine measurements taken by the Ulysses energetic particle 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 30°–36° and occur at distances between 14 and 80 Jovian radii ( R j) from the planet. These beams have electron distributions with small pitch angles and show a 10h 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 distribution (PAD) of the electron beams we also identify two different regions along the Ulysses trajectory similar to ion fluxes and anisotropies discussed by Cowley et al. (1996). These regions can be characterised by broad electron PADs at higher magnetic latitudes near the planet and very pronounced beams at higher magnetic latitudes far from the planet in the buter duskside magnetosphere. For the bi-directionality of the electron PAD we discuss whether the duskside magnetosphere is magnetically open or closed favouring a mainly closed magnetosphere.