Abstract
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.
Highlights
The first in situ measurements in the magnetosphere of Saturn were obtained by the Pioneer 11 and the Voyager 1 and 2 spacecraft, in 1979, 1980, and 1981, respectively
The Saturn equinox was on 11 August 2009, so this set of orbits is of particular interest for studying the nightside magnetodisk of Saturn under near-equinoctial conditions. For each of these orbits we examined the radial component of the magnetic field (Br), together with the azimuthal velocity and the density of the proton species derived by Thomsen et al (2010)
Our results suggest that the Kronian magnetodisk has a dense, rapidly rotating central plasma sheet with the rotation rate low in the lobes, and gradual variation in between
Summary
The first in situ measurements in the magnetosphere of Saturn were obtained by the Pioneer 11 and the Voyager 1 and 2 spacecraft, in 1979, 1980, and 1981, respectively. The proton sheet is smoothly modulated by the flapping of the magnetodisk; in the nightside the heavy ions form a narrow sheet surrounding the magnetic equator, and when the nominal center of the plasma sheet is crossed (Br = 0), highdensity heavy ion peaks can be observed The periodicity of these heavy rich events was found to be close to the southern SKR period. Carbary and Mitchell (2014) used energetic neutral atom (ENA) images to determine the rotational speeds of the plasma by measuring the speed of features (“blobs”) appearing in the images; their results show good agreement with in situ plasma measurements These papers established the basic behavior of the azimuthal flow with respect to radial distance near the equatorial plane in the nightside magnetosphere of Saturn, falling from near-rigid corotation at a distance of ∼ 3 Saturn radii (RS), to ∼ 50% of rigid corotation or less at ∼ 20 RS and beyond. This long data acquisition period gives us excellent statistics, which is essential for the investigation of the structure of the magnetodisk
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