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Magnetospheric environments of outer planet rings: Influence of Saturn's axially symmetric magnetic field
Institution:1. Center for Forest Ecology, Department of Forest Resources, University of Minnesota, St. Paul, MN, United States;2. Institute of Arctic Biology, University of Alaska, Fairbanks, AK, United States;3. YukonU Research Centre, Yukon University, Whitehorse, YT, Canada;4. Department of Forest Sciences, University of Helsinki, Helsinki, Finland;1. Sandra and Edward Meyer Cancer Center, United States;2. Department of Medicine, United States;3. Department of Biochemistry, Weill Cornell Medicine, New York 10021, United States;4. Weill Cornell/Rockefeller/Sloan Kettering Tri-I MD-PhD Program, New York 10065, United States
Abstract:The Jovian and Uranian rings exist within severe energetic particle and plasma environments where magnetosphere-related losses of small ring particles and surface reflectance alteration by sputtering are likely to be important. In contrast, the main Saturnian rings exist within a zone where magnetospheric losses and surface alteration effects are negligible, primarily because of solid-body absorption of inwardly diffusing magnetospheric particles. It is shown here that solid-body absorption of radially diffusing ions is a much more efficient process in the inner Saturnian magnetosphere than in the inner Jovian and Uranian magnetospheres because of the near axial symmetry of the planetary magnetic field with respect to the rotational equatorial plane. This is especially true for continuous rings (as opposed to satellites) for which the approximate time scale against absorption is the particle bounce period in an axially symmetric field, whereas it is the particle drift period in an asymmetric field. Assuming comparable diffusion rates, inward transport of magnetospheric particles is much more strongly inhibited in the inner Saturnian magnetosphere than in the inner magnetospheres of Jupiter and Uranus. This remains true when only rings of comparable widths and optical depths are considered (e.g., the F ring at Saturn and the ϵ ring at Uranus). The most extreme possible consequence of this difference in solid-body absorption efficiency may have been the preferential development of a radially extensive, optically thick ring system at Saturn where magnetospheric losses are minimized in comparison to those at Jupiter and Uranus. A more definite consequence is that the Uranian rings are most probably directly exposed to nearly the same proton fluxes measured at Voyager 2's closest approach. Exposure of ring particle surfaces to radiation belt ion fluxes therefore remains as a viable explanation for the low albedos of the Uranian rings.
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