Corotation anisotropies in Saturn's magnetosphere

Abstract

Ion data from the Voyager 1 and 2 low‐energy charged particle (LECP) experiment are fit to a second‐order harmonic expansion to determine anisotropies within the Saturnian magnetosphere. Anisotropies from the low‐energy channels (28–130 keV) are often consistent with those expected from corotation (generally with a small radial component on the dayside) but are at times distinctly different from corotation. On the dayside, near the noon meridian, first‐order anisotropies often depart significantly from the corotation direction. On the nightside, amplitudes are consistent with full corotation but begin to drop below corotation values at a dipole L of ∼27 Rs (corresponding to a velocity of ∼265 km/s). Second‐order anisotropies are significant, even dominant at times, on the dayside where a relatively broad range of pitch angles is sampled by the instrument. In all cases there is a departure of ∼180° from the first‐order anisotropies expected due to corotation beginning outside the Rhea L shell and continuing through at least the orbit of Dione. This is the region where the cold and hot plasma tori have been observed. We have computed the anisotropies expected from rigid corotation of the observed flux distributions and conclude that parts of the Saturnian magnetosphere are not rigidly corotating. Calculations based on the combined effects of corotation plus solar wind generated convective electric fields appear to provide insights into the anisotropies along the dawn meridian. However, on the dayside near noon, turbulence and/or time variations seem necessary to explain the departure from simple convection models.