Session 2: Solar, Planetary and Space Physics

Title: Energetic ions observed by ULYSSES in the Jovian magnetosheath
Author(s):  G.C. Anagnostopoulos, I. Karanikola (Oral)
Contact: G. Anagnostopoulos, Demokritos University of Thrace, Greece

Ulysses encountered a quasi-parallel bow shock and a thinned fast moving magnetosheath inbound, and spent long times in the dusk-side high latitude magnetosheath outbound, where it crossed the bow shock three times, under a quasi-perpendicular geometry. On the outbound pass, Ulysses observed two different types of ion events in the sheath: (1) In the inner magnetosheath, the ion events were characterized by intensities comparable to and energy spectra harder than those in the adjacent outer magnetosphere, and they were accompanied by the presence of significant fluxes of high energy (~400 - ~2 MeV) ions and (~40 - ~300 keV) electrons. (2) In the outer magnetosheath, the ion events were in general not accompanied by the presence of high energy ions and electrons (or they were accompanied by low fluxes), their intensities were lower and the energy spectra softer than those in the outer magnetosphere. The ion anisotropies in the sheath were variable; however, intensity gradient vectors looking in the general direction of the magnetopause and an outward streaming toward the bow shock can be checked in most of the time. A quasi-periodic
~10 h variation was found in ion / electron intensities outbound. A quasi-periodic ~40 min (also ~20 min, ~10 min) modulation in ion / electron intensities and anisotropy data of magnetospheric origin was also evident on the inbound and most of the time of the outbound Ulysses pass of the Jovian magnetosheath. Inbound, higher anisotropy and significant quasi-periodic modulation in ion data were observed in the outer sheath, before a reversal from an anti-sunward to sunward plasma velocity, due to the expansion of the magnetosphere (Phillips et al., 1993). A counter-streaming of low and high energy ions was found between the times of magnetopause crossings determined by the magnetic field and plasma experiments, which is consistent with the existence of two populations in the dayside boundary layer, a magnetospheric and a solar component. Evidence of electron and low energy (<~300 keV) ion acceleration and / or trapping was found downstream from the quasi-perpendicular bow shock outbound. No effects of a diffusive type acceleration were found near the quasi-parallel  bow shock inbound, under a radial magnetic field. The whole set of data analyzed strongly suggest that energy dependent leakage of energetic particles from the high latitude magnetosphere in general controlled the >~60 keV ion observations made by Ulysses in the Jovian magnetosheath.