Evidence for rotationally driven plasma transport in Saturn's magnetosphere

Abstract

[1] Radial convective transport of plasma in a rotationdominated magnetosphere implies alternating longitudinal sectors of cooler, denser plasma moving outward and hotter, more tenuous plasma moving inward. The Cassini Plasma Spectrometer (CAPS) has provided dramatic new evidence of this process operating in the magnetosphere of Saturn. The inward transport of hot plasma is accompanied by adiabatic gradient and curvature drift, producing a V-shaped dispersion signature on a linear energy-time plot. Of the many ($100) such signatures evident during the first two Cassini orbits, we analyze a subset (48) that are sufficiently isolated to allow determination of their ages, widths, and injection locations. Ages are typically <10.8 hr (Saturn's rotation period) but range up to several rotation periods. Widths are typically <1 R S (Saturn's radius) but range up to several R S . Injection locations are randomly distributed in local time and in Saturnian longitude. The apex of the V sometimes coincides with a localized density cavity in the cooler background plasma, and usually coincides with a localized diamagnetic depression of the magnetic field strength. These signatures are fully consistent with the convective motions that are expected to result from the centrifugal interchange instability.

Plasma and fields in the wake of Rhea: 3-D hybrid simulation and comparison with Cassini data

“…It would also be interesting to model how the system responds if there is a change in the upstream plasma configuration (e.g. during a plasma injection; see Hill et al, 2005). The latter scenario is theoretically very interesting: during a fresh injection event the flux of energetic ions (>50 keV) increases significantly, and these ions carry more than 70% of the total plasma pressure (Sergis et al, 2007).…”

Polar confinement of Saturn's magnetosphere revealed by in situ Cassini observations

“…The rapid rotation of the planet confines it to the equatorial plane and forms a magnetodisc. It then diffuses into the outer magnetosphere due to centrifugal instabilities as described by Kivelson and Southwood [2005] and observed by Burch et al [2005] and Hill et al [2005]. Only the ions and electrons with sufficient energy can escape and travel along the field lines to higher latitudes.…”

Interaction of Titan's ionosphere with Saturn's magnetosphere

“…Third, there are excursions in energy down to approximately 10 eV; these may be associated either with interchange between Saturn's dense, cold inner magnetosphere and the rarer, hot outer magnetosphere (e.g. Burch et al 2005;Hill et al 2005), or with ionospheric material from Titan. Fourth, there is a prominent interaction interval of approximately 14 min in this case, where Cassini is within Titan's ionosphere and exosphere region, covering the interval approximately 15.18-15.32 UT.…”