Energetic Protons in the Magnetosphere of Neptune

Voyager;Neptune;radiation;magnetosphere

Looper, Mark Dixon ; Stone, Edward C.

University:California Institute of Technology

Department:Physics, Mathematics and Astronomy

关键词: Voyager;    Neptune;    radiation;    magnetosphere;   

Others  :  https://thesis.library.caltech.edu/7329/1/Looper_md_1993.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

The Cosmic Ray Subsystem aboard Voyager 2 measured large fluxes of trapped energeticprotons and electrons in the inner magnetosphere of Neptune during the 1989 flyby; theprotons 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 electronpileup noise, and fluxes are calculated with corrections for discriminator deadtime. Theoreticalmodels for satellite absorption of charged particles are adapted to the large gyroradii ofenergetic protons, and model magnetospheres are constructed that involve diffusion of particlesin the presence of this absorption; parameters of these model magnetospheres areadjusted to reproduce the observations. The inward-diffusing proton flux is limited byabsorption 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 anorder of magnitude less than that inferred elsewhere for the electrons; this prevents protonsfrom diffusing inward past 1989N1 before they are absorbed, and in fact the proton fluxreturns 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 areabsorbed by the other moons and rings closer to the planet. The rate of proton radialdiffusion, in comparison with that for electrons, is consistent with the diffusion being drivenby electric fields from wind fluctuations in the ionosphere of Neptune. Radial diffusionalone, however, produces too much pitch-angle anisotropy as particles with mild anisotropyin the outer magnetosphere are transported inward, and pitch-angle diffusion must be invokedto reduce the excess anisotropy and reproduce the observations. The pitch-angle distributionsat different Ls are consistent with the diffusion coefficient for this process being comparablein magnitude to that for radial diffusion inside L of about 6.8, though still much less than thestrong-diffusion limit, and negligible outside that L.