Charged particle anisotropies in Saturn's magnetosphere

We report observations of anisotropies and pitch angle distributions for 0.5–1.8 MeV protons, 7–17 MeV electrons and ⪞3.4 MeV electrons in Saturn's magnetosphere made with the University of Chicago experiments on Pioneer 11. In the outer magnetosphere (L >6) there is clear evidence for corotation of the proton flux, and the proton pitch angle distribution shows maximum flux perpendicular to the magnetic field (‘pancake’ distribution). Observed changes in the amplitude and shape of the pitch angle distributions suggest the existence of significant temporal variations in the outer magnetosphere. From L=6 to L=4, the proton intensity decreased by more than two orders of magnitude, and the pitch angle distribution shifted to a ‘dumbbell’ form (maximum flux parallel to magnetic field). The shift in pitch angle distribution most likely results from preferential absorption of large pitch angle particles by the tenuous E‐ ring found in the equatorial plane out to at least R=5 Rs. For L <4, the proton intensity increased inwards, implying an inner edge for the E ring at R ≈4 Rs. Except for regions where the flux was reduced by satellite absorption, the pitch angle distributions remained dumbbell. In absorption regions, pancake distributions were found. The observations are consistent with the suggestion by McKibben and Simpson (this issue) that inward diffusion and acceleration at Saturn may proceed primarily via large, infrequent disturbances. Electron anisotropies were measurable only for L ⪝4.5, and the pitch angle distributions were found to be pancake for the entire region L <4.5, suggesting that the absorbing particles in the E ring have radii that lie between the range of ∼1 MeV proton and ∼10 MeV electrons, or of the order of millimeters.