HR: 11:20h
AN: P52B-05 INVITED    [Abstracts]
TI: How important is Enceladus in Saturn's global magnetosphere?
AU: * Delamere, P
EM: delamere@lasp.colorado.edu
AF: Laboratory for Atmospheric and Space Physics, University of Colorado, CB 392 Duane Physics D133, Boulder, CO 80309, United States
AB: Our knowledge of Saturn's neutral gas-dominated inner magnetosphere is based on measurements made by Pioneer 11, Voyager 1, Voyager 2, Cassini, and on observations using the Hubble Space Telescope. As was recently revealed by Cassini, the source for Saturn's neutral torus is Enceladus, venting 10²⁷ to 10²⁸ H₂O s^-1 from its south pole. Ionization of the neutral torus produces a tenuous plasma torus with electron densities < 100 cm^-3. Outside 1.9 {R_s}, the plasma torus is subject to a flux tube interchange instability provided that the total number of ions per unit of magnetic flux decreases radially outward. Consequently the radial transport of plasma from Enceladus can influence magnetospheric dynamics just as Io's plasma source drives Jupiter's magnetosphere. A key parameter that determines radial transport rates is the plasma mass per second transported outward, coupled with Saturn's ionospheric Pedersen conductivity. To assess the influence of Enceladus on Saturn's global magnetosphere we have adapted our Io plasma torus chemistry and magnetosphere-ionosphere coupling models to Saturn following Richardson et al. [1986] and Jurac and Richardson [2005] for implementing the water group chemistry. Compared to Jupiter, our results suggest that ionization at Saturn is fundamentally limited by the slower corotational flow velocity at Enceladus (26 km/s), resulting in a lower ion pickup temperature. The net result of cooler ions at Enceladus is a cooler thermal electron population (~ 2 eV) that is insufficient to generate significant ionization. Instead, the Enceladus plasma torus is maintained by the hot electron population (i.e. 1000 eV, measured by the Cassini Plasma Spectrograph [ Young et al., 2005]), and we suggest that the hot electrons are derived from global plasma dynamics. In addition, our initial results suggest that longitudinal variations in the hot electron abundance can explain the factor-to-two variation in electron density reported by Gurnett et al. [2006]. Preliminary estimates of the plasma mass outflow rate will be presented to address the question, "How important is Enceladus in Saturn's global magnetosphere?"
DE: 2730 Magnetosphere: inner
DE: 2732 Magnetosphere interactions with satellites and rings
DE: 2756 Planetary magnetospheres (5443, 5737, 6033)
DE: 2772 Plasma waves and instabilities (2471)
DE: 6280 Saturnian satellites
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting