Energetic particles in the predawn magnetotail of Jupiter

Abstract

A detailed account is given of the energetic electron and proton populations as observed with Voyager 1 and 2 during their passes through the dawn magnetotail of Jupiter. In general, the results of the Pioneer 10 pass at 90° to the Jupiter‐sun line have been confirmed and extended. The region between 20 and 150 RJ is dominated by a thin plasma sheet, and open field lines were observed at 42 RJ at a magnetic latitude of only 15°. Trapped energetic electron and proton fluxes reach their maximum in the plasma sheet and decrease rapidly even a few degrees away from it. The spectra of trapped protons can be represented by an exponential in rigidity and have a characteristic energy of ∼50 keV. Proton anisotropies in the plasma sheet were consistent with corotation even at 100 RJ, but the preliminary analysis is not yet conclusive. A major proton acceleration event and several cases of field‐aligned proton streaming were observed. Comparable proton fluxes were observed in the plasma sheet by Voyager 1 and 2. The flux of >0.4‐MeV protons decreases by 3 orders of magnitude between 20 and 90 RJ and then remains relatively constant from there to the boundary layer near the magnetopause. Between 20 and 30 RJ in the antisolar direction, the trapping region has a latitudinal extent which is comparable to that observed during the inbound pass at −30° solar aspect. The plasma sheet positions in the magnetotail can be represented by a distorted disk which rotates about the Jovian spin axis. Fine structure in the data indicates longitudinal asymmetries with respect to the dipole orientation. Electron spectra in the outer magnetosphere, the magnetosheath, and interplanetary space are modulated by the Jovian longitude relative to the subsolar point; this confirms the Pioneer 10 and 11 results.