Radial diffusion of low‐energy ions in Saturn's radiation belts: A Combined analysis of phase space density and satellite microsignature data

Abstract

Phase space densities for low‐energy (80 MeV/G) ions in Saturn's inner magnetosphere are analyzed using solutions of the time‐averaged radial diffusion equation for charged particle transport in a dipolar magnetic field. A series of distributed loss models ranging from satellite absorption only to satellite and maximum estimated Ring E absorption losses plus pitch angle scattering losses occurring at the strong diffusion limit in the inner magnetosphere are assumed. In each case the corresponding form of the magnetospheric radial diffusion coefficient (assumed to be expressible as D(L) = DoLn, where Do is a constant and n is an integer) which yields a minimum rms residual between model and data is determined. Independent constraints on the diffusion rate at specific L values derivable from satellite microsignatures in low‐energy ions and electrons are then considered. Estimates previously derived from the Dione microsignature in Voyager 1 low‐energy electrons are supplemented by additional analyses of the Tethys microsignature in Voyager 2 low‐energy ions and the Rhea microsignature in Voyager 1 low‐energy electrons. The resulting diffusion rate estimates of D(4.9) ∼4 ± 2×10−8 Rs² s−1 and D(8.7) > 6 ± 5×10−8 Rs² s−1 are consistent with those derived from the Dione microsignature if D(L) increases outward. A comparison with the phase space density modeling results shows that satellite and maximum Ring E absorption losses alone are insufficient to yield diffusion rates at Tethys and Dione that are in agreement with the microsignature estimates. Models containing weak pitch angle scattering losses of low‐energy ions in the inner magnetosphere occurring at a rate less than one‐tenth that of the strong diffusion limit produce diffusion rates that are compatible with microsignature estimates and additionally result in an improved fit to the radial variation of the experimental phase space densities. The preferred radial diffusion coefficient for these low‐energy ions is characterized by a relatively high amplitude and low‐order L dependence that is most consistent with Jovian‐type diffusion mechanisms including the centrifugal interchange instability and the ionospheric dynamo mechanism.