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

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 subcorotating plasma.

Closed field line vortices in planetary magnetospheres

“…The plasma density changes smoothly in the region in question, in accordance with an initially exponential falloff. There are no detectable boundaries where the magnetospheric plasma abruptly disappears (see Nemeth et al 2015). Thus the behavior of the thermal plasma supports the notion that these are indeed closed-field vortices.…”

“…Such a flow pattern should be observable in the plasma measurements. Nemeth et al (2015) already identified such a flow pattern in the ion measurements of the Cassini spacecraft, but attributed the decreasing azimuthal velocity to sub-corotation intensifying for larger L values, and did not offer an explanation for the observed retrograde motion. In the next section, we revisit these observations in more detail, extending them to latitudinal as well as azimuthal flow patterns and showing how the experimental data support the existence of giant closed field line vortices in the tail lobes.…”

“…In Fig. 6 we show the azimuthal magnetic field as a function of the radial magnetic field component, the latter representing the distance from the magnetic equator (since it is a monotonic function of said distance, and we can eliminate the effects of magnetodisk flapping this way (see Nemeth et al 2015Nemeth et al , 2016. We can see a sinusoidal field behavior, which agrees with the closed field line vortex model -linear increase, decrease, zero, and a directional change as one moves away from the magnetic equator.…”

Mapping Saturn's Nightside Plasma Sheet Using Cassini's Proximal Orbits

“…The Saturnian plasma sheet, in particular, was extensively studied during the last decade. Its three‐dimensional structure and its dynamical (periodic and episodic) variability was revealed through multi‐instrumental data analyses (e.g., Arridge et al, ; Carbary et al, ; Cowley & Provan, ; Kellett et al, ; Krupp et al, ; Nemeth et al, ; Sergis et al, , ; Thomsen et al, ; Wilson et al, , ) and modeled by several sophisticated approaches (e.g., Achilleos, Guio, & Arridge, ; Arridge et al, ; Cowley & Provan, ; Jia & Kivelson, ).…”