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On determining magnetospheric diffusion coefficients from the observed effects of Jupiter's satellite Io


  • M. F. Thomsen,

  • C. K. Goertz,

  • J. A. Van Allen


Several previously proposed techniques for determining the radial diffusion coefficient from the observed effects of the inner Jovian satellites on the energetic particle fluxes are discussed, and important shortcomings are pointed out. A new method is proposed which avoids the most important shortcoming by dealing with data from regions somewhat removed from the actual sweeping region. The new technique is applied to data obtained at the orbit of Io by the University of Iowa proton detector on Pioneer 11 and to a published electron phase space density profile constructed from data obtained (also at Io's orbit) by the University of California at San Diego instrument on Pioneer 10. If satellite sweeping with an effective satellite radius equal to the geometric radius is assumed to be the only loss process operating, the resulting diffusion coefficient for protons with µ = 1.7 MeV/G at L = 6 is D ∼ 3 × 10−8 RJ² s−1, and that for electrons with µ = 1 × 104 MeV/G is D ∼ 4 × 10−7 RJ² s−1. A possible alternative to satellite sweepup as an explanation for the large proton losses across Io's L shell is proposed. This alternative consists of enhanced precipitation of protons near Io's L shell due to resonant interaction with ion cyclotron waves. The region of enhanced precipitation is localized to the region near Io's orbit because that is a region of increased plasma density. Some consequences of such a hypothesis are discussed. One important consequence is that the radial diffusion coefficient for the protons has an upper limit of about 2 × 10−7 RJ² s−1 at L = 6. This upper limit corresponds to the case in which the pitch angle scattering occurs at the strong diffusion limit. Finally, the effects which other potentially significant processes, such as injection or additional loss mechanisms, might have on a determination of the diffusion coefficient from observed satellite effects are discussed. It is pointed out that until such processes can be either dealt with or dismissed, the diffusion coefficient cannot be reliably deduced from observed satellite effects.

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