Energetic Oxygen and Sulfur Ions in the Jovian Magnetosphere and Their Contribution to the Auroral Excitation

Creators: Gehrels, N.; Stone, E. C.

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 (R_J). The phase space density of the oxygen ions calculated from the spectra has a positive radial gradient between 6 and 17 R_J, indicating an inward diffusive flow. The upper limit for the diffusion coefficient D at 9 R_J 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 ∼10^(28) ions/s. If D(9 R_J) is ∼4×10^(−6) s^(−1), then ∼2×10^(24) oxygen and sulfur ions with > 70 MeV/nuc-G are lost per second as they diffuse inward from 12 to 8 R_J. Assuming these ions are scattered into the loss cone, they deliver ∼4×10^(12) W to the Jovian atmosphere. Extrapolations to lower magnetic moments suggest that the 10^(13)–10^(14) W required to produce the observed ultraviolet auroral emissions could result from the precipitation of ∼10^(26) oxygen and sulfur ions/s with magnetic moments ≳10 to 30 MeV/nuc-G (≳35 to 100 keV/nuc at 10 R_J). The ions with ≳70 MeV/nuc-G deposit their energy between ∼67° and ∼72° magnetic latitude at an average depth of ∼10^(19) cm^(−2) of H_2 (∼500-km altitude), which is above the homopause. If the extrapolated spectrum extends down to ∼10 MeV/nuc-G, then 10 times more energy (∼10^(13) W) is carried inward across 10 R_J 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.

Additional Information

© 1983 by the American Geophysical Union. Manuscript Accepted: 11 Apr 1983. Manuscript Received: 30 Nov 1982. We greatly appreciate the efforts of R. E. Vogt, both in his capacity as CRS Principal Investigator and as a colleague who has provided useful discussions. We are also indebted to our colleague J. H. Trainor for his contribution to this analysis. We are grateful to the California Institute of Technology and Goddard groups who have supported the investigation, with special thanks to W. E. Althouse, R. Burrell, W. R. Cook, and D. E. Stilwell. We also thank D. L. Chenette for helpful discussions N. F. Ness and his colleagues on the Voyager magnetometer team provided the magnetic field data used in this analysis, which we gratefully acknowledge. We thank Y. L. Yung and G. R. Gladstone for useful discussions concerning the Jovian atmosphere and its auroral emissions. This work was supported in part by NASA under grants NAGW-200 and NGR 05-002-160 and contract NAS7-918 (formerly NAS7-100). The Editor thanks D. Hamilton and C. K. Goertz for their assistance in evaluating this paper.

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