Energetic oxygen and sulfur ions in the Jovian magnetosphere and their contribution to the auroral excitation

Abstract

Observations of 1 to 20 MeV/nuc oxygen, sodium, and sulfur ions in the Jovian magnetosphere are reported. Measurements made by the cosmic ray subsystem on Voyager 1 and 2 are used to calculate abundances and energy spectra in the region from 5 to 20 Jovian radii (RJ). The phase space density of the oxygen ions calculated from the spectra has a positive radial gradient between 6 and 17 RJ, indicating an inward diffusive flow. The upper limit for the diffusion coefficient D at 9 RJ is ∼10−5 s−1. This limit, combined with the analysis of Voyager plasma observations by Siscoe et al. (1981), implies an upper limit to the production rate of oxygen and sulfur ions from Io of ∼1028 ions/s. If D(9 RJ) is ∼4×10−6 s−1, then ∼2×1024 oxygen and sulfur ions with > 70 MeV/nuc‐G are lost per second as they diffuse inward from 12 to 8 RJ. Assuming these ions are scattered into the loss cone, they deliver ∼4×1012 W to the Jovian atmosphere. Extrapolations to lower magnetic moments suggest that the 1013–1014 W required to produce the observed ultraviolet auroral emissions could result from the precipitation of ∼1026 oxygen and sulfur ions/s with magnetic moments ≳10 to 30 MeV/nuc‐G (≳35 to 100 keV/nuc at 10 RJ). The ions with ≳70 MeV/nuc‐G deposit their energy between ∼67° and ∼72° magnetic latitude at an average depth of ∼1019 cm−2 of H2 (∼500‐km altitude), which is above the homopause. If the extrapolated spectrum extends down to ∼10 MeV/nuc‐G, then 10 times more energy (∼1013 W) is carried inward across 10 RJ by the energetic oxygen and sulfur ions than flows outward with the plasma, indicating the presence of an energy source in the middle or outer magnetosphere.