Radial diffusion and losses of energetic protons in the 5 to 12 RS region of Saturn's magnetosphere

Abstract

Phase space density profiles for protons with first invariants µ = 600 and 3000 MeV/G (integral invariants K = 0.30 and 0.003 G½ RS) previously derived from measurements by the University of Iowa detector G on Pioneer 11 during the 1979 Saturn encounter are analyzed using solutions of the time‐averaged radial diffusion equation in a dipolar planetary magnetic field. A series of loss models ranging from satellite absorption only to satellite plus ring E absorption and added distributed losses is assumed and the corresponding form of the time‐averaged radial diffusion coefficient D(L) (taken to be of the form D(L) = DoLn, where n is an integer) is determined in each case by a minimum‐variance fit to the data‐derived phase space density profiles. The inferred forms for D(L) are characterized by a low‐order L‐dependence (∼L³–L6) and a relatively high amplitude (Do ≃ 10−9–10−8 RS² s−1) in approximate agreement with earlier studies. Loss models that include only satellite absorption result in pronounced Dione macrosignatures in the model phase space density profiles that are not present in the data profiles, indicating a need for additional distributed losses. Absorption by ring E would be sufficient to explain the absence of a Dione macrosignature only if both the presently accepted maximum normal optical depth and mean ring particle size are increased by at least 1 order of magnitude. Charge exchange losses, estimated from observational upper limits on the concentrations of relevant neutral species, also appear to be only marginally important. Pitch angle scattering precipitation losses occurring within the orbit of Rhea at a rate of at least 0.01 times that of the strong pitch angle diffusion limit could in principle provide the necessary distributed losses.