Energetic particle, plasma and magnetic field signatures of a poloidal pulsation in Jupiter's magnetosphere

Abstract

During its February 1992 inbound journey into Jupiter's magnetosphere, Ulysses made several near-encounters with (and actual traversals of) the Jovian magnetodisk. In at least one such event (investigated here) the anticipated variations in energetic particle fluxes, plasma density, magnetic field intensity, and magnetic field direction at the planetary rotation period were further modulated (strongly and quasi-periodically) at a higher frequency, as if the magnetodisk and magnetic-equatorial surface were “flapping” relative to the position of the spacecraft. This additional modulation occurred around 14: 00–19: 00 U.T. on 7 February (Day 38) 1992, with Ulysses at a nominal (dipolar) magnetic latitude ≈ 11°–22° and at a zenocentric distance r ≈ 18–22 R j. The quasi-period 2π/Ω revealed by autocorrelation analysis of the energetic-proton data (and confirmed by visual examination of plotted data from the various instruments) was about 6.8 ks (≈1.9 h). Oscillations in energetic particle fluxes and plasma density were typically 180° out-of-phase with those in magnetic field intensity. The 6.8 ks period is consistent with oscillation of the magnetic shell of radius r ≈ 21 R j at the second poloidal harmonic, which is the mode that would correspond to a “flapping” of the magnetodisk and (more generally) of the magnetic-equatorial surface.