Energetic Oxygen and Sulfur in the Jovian Magnetosphere

Abstract

This paper reports measurements made by the cosmic ray subsystem onboard Voyager 1 and 2 in the Jovian magnetosphere. Energy spectra of oxygen ions in the energy range 1–20 MeV/nuc between 5 and 20 R_J are presented and used to calculate phase space densities. There is a steep positive radial gradient in the phase space density of the energetic oxygen ions in this region, indicating an inward diffusive flow. Solutions of the diffusion equation assuming a diffusion coefficient D and loss lifetime τ of the forms D=D_0L^n and τ=τ_0L^m, where D_0, τ_0, n, and m are constants, and L is the McIlwain parameter, are fit to the radial phase space density profile of oxygen ions with magnetic moments of 680 MeV/nuc-G. The best fits are found to have n + m ≈ 6 and 3 <n <6. On the basis of the diffusion coefficient upper limit obtained from these fits, the upper limit on the rate at which oxygen ions with >400 MeV/nuc-G diffuse inward across 10 R_J is 5 × 10^(21±1) ions s^(−1). The observations suggest that oxygen and sulfur ions in the Io plasma torus diffuse radially outward, are nonadiabatically accelerated in some region outside 17 R_J and then diffuse inward and outward from the acceleration region.