Relativistic electron flux anisotropies in the duskside Jovian magnetosphere: a test for source location and escape mechanism

Abstract

Results of anisotropy analyses of >∼ 3 MeV electron flux data obtained by the high-energy telescope of the COSPIN experiment on Ulysses are presented for the period from day 40 to day 45 of 1992, when the spacecraft was outbound from Jupiter in the duskside high-latitude magnetosphere and magnetosheath. At small L-shells or minimum magnetic latitudes near the planet, the electron pitch-angle distributions are peaked at α = 90° and have net inward flows to the South Pole of the magnetic dipole. A majority of the high-latitude magnetosphere regions are dominated by electrons with field-aligned bidirectional distributions. The electron flows are essentially directed outward from the southern polar region, but inward electron flows which come from the northern polar region have also been detected, particularly near the magnetospheric boundary. The pattern of electron angular distributions in the magnetosphere is compatible with Nishida's model for recirculation of magnetospheric energetic particles by diffusion at low altitudes. Mechanisms for electron acceleration in the magnetosphere are discussed on the basis of our new observations. In the magnetosheath, alternating field-aligned electron flows are present. The observations suggest that electrons escape from the magnetosphere to the interplanetary space through diffusion across closed magnetic field lines on the boundary. It is found that at least a significant fraction of the interplanetary Jovian electrons (>∼ 10 26s −1) may escape the magnetosphere in this way.