The latitudinal structure of the nightside outer magnetosphere of Saturn as revealed by velocity moments of thermal ions

Abstract

In this study we investigate the latitudinal behavior of the azimuthal plasma velocities in the outer magnetosphere of Saturn using the numerical ion moments derived from the measurements of the Cassini Plasma Spectrometer. One of the new results presented is that although these moments display some scatter, a significant positive correlation is found to exist between the azimuthal velocity and the plasma density, such that on average, the higher the density the higher the rotation speed. We also found that both the azimuthal velocity and the density anticorrelate with the magnitude of the radial component of the magnetic field and drop rapidly with increasing distance from the magnetic equator. The azimuthal velocities show periodic behavior with a period near the planetary rotation period, which can also be explained by the strong dependence on magnetic latitude, taking into account the flapping of the magnetodisk. It is thus found that the dense plasma near the magnetic equator rotates around the planet at high speed, while the dilute plasma at higher latitudes in the northern and southern hemispheres rotates significantly slower. The latitudinal gradient observed in the azimuthal speed is suggested to be a direct consequence of the sub-corotation of the plasma in the outer magnetosphere, with highest speeds occurring on field lines at lowest latitudes mapping to the rapidly rotating inner regions of the plasma sheet, and the speed falling as one approaches the lobe, where the field lines are connected to strongly sub-corotating plasma.