Energetic Protons in the Magnetosphere of Neptune

Citation

Looper, Mark Dixon (1993) Energetic Protons in the Magnetosphere of Neptune. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/QEPK-4523. https://resolver.caltech.edu/CaltechTHESIS:12122012-085110897

Abstract

The Cosmic Ray Subsystem aboard Voyager 2 measured large fluxes of trapped energetic protons and electrons in the inner magnetosphere of Neptune during the 1989 flyby; the protons above 1.9 MeV observed by the Low Energy Telescopes are analyzed in this thesis. Proton events are extracted from pulse-height distributions dominated by low-energy electron pileup noise, and fluxes are calculated with corrections for discriminator deadtime. Theoretical models for satellite absorption of charged particles are adapted to the large gyroradii of energetic protons, and model magnetospheres are constructed that involve diffusion of particles in the presence of this absorption; parameters of these model magnetospheres are adjusted to reproduce the observations. The inward-diffusing proton flux is limited by absorption due to the moon 1989N1 (Proteus), with absorption at high magnetic latitudes (whence high Ls) proving to be most important. The proton radial diffusion coefficient is an order of magnitude less than that inferred elsewhere for the electrons; this prevents protons from diffusing inward past 1989N1 before they are absorbed, and in fact the proton flux returns to background levels within a limit well outside the minimum L-shell of 1989N1, while electrons can diffuse past this satellite so that their flux recovers before they are absorbed by the other moons and rings closer to the planet. The rate of proton radial diffusion, in comparison with that for electrons, is consistent with the diffusion being driven by electric fields from wind fluctuations in the ionosphere of Neptune. Radial diffusion alone, however, produces too much pitch-angle anisotropy as particles with mild anisotropy in the outer magnetosphere are transported inward, and pitch-angle diffusion must be invoked to reduce the excess anisotropy and reproduce the observations. The pitch-angle distributions at different Ls are consistent with the diffusion coefficient for this process being comparable in magnitude to that for radial diffusion inside L of about 6.8, though still much less than the strong-diffusion limit, and negligible outside that L.

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