Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsm13f..04b&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #SM13F-04
Physics
[2736] Magnetospheric Physics / Magnetosphere/Ionosphere Interactions, [2740] Magnetospheric Physics / Magnetospheric Configuration And Dynamics, [2756] Magnetospheric Physics / Planetary Magnetospheres, [2778] Magnetospheric Physics / Ring Current
Scientific paper
A global picture of Saturn's magnetospheric periodicities is emerging from several observations and modeling efforts. In this presentation we demonstrate that these observations likely contain sufficient information to explain the mysterious periodicities at Saturn, without the need of any prescribed (and often, unobservable) longitudinal anomalies. In this picture plasmoids are released quasi-periodically down the tail, leading to fast planet-ward flows and particle energization ("injections") that enhance the plasma pressure in the night side magnetosphere in the 8-20 Rs region as clearly observed in energetic neutral atom (ENA) observations by the Ion Neutral Camera (INCA) on board Cassini. Both the fast flows and the enhanced pressure drive a 3D current system that closes through the ionosphere, whose upward field-aligned component can be linked to bursts of Saturn Kilometric Radition (SKR). The current system driven by the energetic particle pressure - the partial ring current (PRC) - also distorts the magnetic field significantly leading to its periodic oscillations as the enhanced particle pressure island drifts around Saturn with a period between 10-11 h. The missing link is how the plasmoid release can be periodic. We present global INCA observations showing that pre-existing energetic particle pressure distributions from a previous injection seem to trigger the next injection. This is likely to happen due to the inflation of the magnetic field and modification of the properties of the night side current sheet, leading to an unstable current sheet. The presence of a PRC rotating around Saturn also modifies the electric field in the magnetosphere due to its closure through the ionosphere. Such a modification is called a shielding electric field, and is commonly observed at Earth associated with a radially outward density enhancement of the cold, dense plasmasphere below the PRC. This can further contribute to triggering the plasmoid release. In regards to the "dual" SKR and field periodicities that appear to be different in the northern and southern hemispheres, we investigate the possibilities that this could be a combined effect of injections in to two different radial ranges and inter-hemispheric field-aligned currents (FAC) set up by the seasonal conductance differences of the two hemispheres. Injections have been observed in the two radial ranges of roughly 9-12 Rs and >15Rs where the drift periods differ. We will attempt to quantify if this holds statistically and visualize what the combination of a rotating PRC and inter hemispheric currents look like.
C:son Brandt Pontus
Carbary James F.
Ebihara Yasuhiro
Mitchell Donald G.
Tsyganenko Nikolai A.
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