HR: 0830h
AN: SM31C-1127    [PDF]
TI: Saturn's Polar Ionospheric Flows and Their Relation to the Main Auroral Oval
AU: Cowley, S W
EM: stan.cowley@ion.le.ac.uk
AF: Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH United Kingdom
AU: * Bunce, E J
EM: emma.bunce@ion.le.ac.uk
AF: Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH United Kingdom
AU: Prange, R
EM: renee.prange@obspm.fr
AF: LESIA, Observatoire de Paris, Meudon, 92195 France
AB: We consider flows and currents in Saturn's polar ionosphere implied by the large-scale magnetospheric flow driven by planetary rotation and the solar wind interaction. With increasing radial distance in the equatorial plane, the components are the region where planetary plasma sub-corotates on closed field lines, a surrounding region where plasma is lost down the dusk tail by the stretching out of closed field lines followed by plasmoid pinch-off, as first described for Jupiter by Vasyliunas, and an outer region driven by the solar wind interaction, specifically reconnection, first discussed for Earth by Dungey. Observations show the sub-corotating flow on closed field lines in the dayside magnetosphere falls to around half of rigid corotation in the outer magnetosphere, while here we provide arguments that the flow should drop to much smaller values on open field lines in the polar cap. The implied ionospheric current system requires a four-ring pattern of field-aligned currents, with distributed downward currents on open field lines in the polar cap, a narrow ring of upward current near the boundary of open-closed field lines, and regions of distributed downward and upward current on closed field lines at lower latitudes associated with the transfer of angular momentum from the atmosphere to the sub-corotating magnetospheric plasma. Recent work has shown that the upward current associated with sub-corotation is not sufficiently intense to produce significant auroral acceleration and emission. Here we suggest that the observed auroral oval at Saturn instead corresponds to the ring of upward current bounding the region of open and closed field lines. Estimates indicate that auroras of brightness from a few kR to a few tens of kR can be produced by precipitating accelerated magnetospheric electrons of a few keV to a few tens of keV energy if the current flows in a region which is sufficiently narrow, of order or less than $\sim$1000 km ($\sim$$1\deg$ latitude) wide. We also suggest that the auroras should be significantly brighter on the dawn side of the oval than at dusk, and additionally displaced somewhat towards dawn by the down-tail outflow at dusk.
DE: 2409 Current systems (2708)
DE: 2704 Auroral phenomena (2407)
DE: 2756 Planetary magnetospheres (5443, 5737, 6030)
DE: 2784 Solar wind/magnetosphere interactions
SC: SPA - Magnetospheric Physics [SM]
MN: 2003 Fall Meeting