Corotation of Saturn's magnetosphere: Evidence from energetic proton anisotropies

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Abstract
The theory and technique of Northrop and Thomsen [1980] are applied to observations of energy spectra and directional anisotropies of 0.61‐ to 3.41‐MeV protons in Saturn's magnetosphere. The observations were made by the Goddard Space Flight Center/University of New Hampshire and University of Iowa instruments aboard Pioneer 11 during the Pioneer encounter with Saturn in August–September 1979. Fourier fits to 15‐min intervals of data are combined with spectral indices to yield information about the E × B convection velocity and temporal changes in the particle population. There is a fundamental inability to distinguish unambiguously between the two, but if one can be assumed, the other then follows from these calculations. It is found that although these data do not by themselves allow an unambiguous determination of the extent of corotation in Saturn's outer magnetosphere, they are consistent with exact corotation at the nominal rotation period in the presence of significant but not unreasonable temporal variations in the energetic proton population.