Other
Scientific paper
Sep 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009dps....41.6407l&link_type=abstract
American Astronomical Society, DPS meeting #41, #64.07
Other
Scientific paper
A cloud of water-group (O, OH, H2O) neutrals extends outward from Saturn's main rings with a maximum density near the orbital location of the Enceladus. During close flybys of this moon, the Cassini spacecraft found that a plume of dust and water vapor originates from fractures at Enceladus’ southern pole. This water vapor spreads away from Enceladus, creating the large-scale cloud. This mass transport can occur by collisions with other neutrals or through an interaction with the ambient plasma. Recent modeling suggests that neutral-neutral collisions are capable of distributing the plume material throughout the saturnian system on the order of a month and would produce an azimuthally-symmetric cloud. At Saturn, the dominant neutral-plasma interaction is charge exchange, where a neutral gives up an electron to an ion. The neutralized ion ("fast neutral") will then move on a ballistic trajectory while the neutral joins the magnetospheric plasma. This interaction also produces electromagnetic waves - called ion cyclotron waves - that are observed by the Cassini magnetometer. Using a conservation of energy argument, we use these waves to calculate the local ionization rate of the neutral cloud. By assuming that the relationship between that rate and the neutral density is constant at a given radial distance, we study the azimuthal structure in the neutral cloud. We find that this structure is strongly controlled by longitudinal separation from Enceladus. There is an order-of-magnitude enhancement in the density at the moon, relative to the background. The enhancement extends about forty degrees upstream and one hundred and twenty degrees downstream, falling off with distance. This control of the neutral cloud extends more than sixty moon radii outward from Enceladus’ orbital distance, which requires rapid transport that can only take place via fast neutrals by the neutral-plasma interactions.
Dougherty K. M. K. M.
Leisner Jared S.
Russell Christopher T.
Strangeway Robert J.
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