Cassini plasma spectrometer thermal ion measurements in Saturn's inner magnetosphere

Abstract

Plasma data from the Cassini Plasma Spectrometer experiment are analyzed using a robust forward modeling technique for dayside equatorial orbits within the range 5.5 to 11 Saturn radii (1 R_S = 60,268 km). It is assumed the measured ion data may be represented by two anisotropic Maxwellian distributed species, H⁺ and a water group ion, W⁺. Saturn's magnetospheric plasma is shown to subcorotate by 15-30% below rigid corotation within this region, with a minimum in fractional lag between 7 and 9 R_S. There is a suggestion of a small radial outflow, but the selection of data for this study precluded the inclusion of interchange injection events. Ion densities are in excellent agreement with the Cassini plasma wave instrument, giving confidence in the forward modeling technique. Plasma moments including density, temperatures, and velocities are presented, along with empirical models for density and azimuthal velocity. Water group temperature anisotropies T_⊥/T_∥ have values between 3 and 8 near 5.5 R_S, becoming less anisotropic as distance increases, but are still not isotropic by 10 R_S. The implications of these results for mass loading in the Saturnian magnetosphere are discussed, with the conclusion that an important fraction of the plasma source is located inside of the 5.5 R_S boundary of this study.