Ion Flows in Saturn's Nightside Magnetosphere

Details Ion Flows in Saturn's Nightside Magnetosphere Ion Flows in Saturn's Nightside Magnetosphere

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmsm53a1085m&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #SM53A-1085

Physics

2731 Magnetosphere: Outer, 2740 Magnetospheric Configuration And Dynamics, 2744 Magnetotail, 2756 Planetary Magnetospheres (5443, 5737, 6033), 2760 Plasma Convection (2463)

Scientific paper

Within Saturn's magnetosphere, plasma produced by the system of rings and moons continually loads the field lines. To avoid unlimited mass build-up, the release of plasma is predicted downtail either as a planetary wind or during substorm-like processes. The newly emptied flux tubes will then return to the dayside, depleted of plasma, to complete the circulation. The goal of the present study is to determine the equatorial flows in the night-side region to explore this scenario. Does the plasma appear to corotate? Are radial flows observed both outwards suggestive of tailward plasma release and inwards indicating the return of newly closed field-lines? If so, at what radial distances and local times are these observed? What plasma characteristics do these flows exhibit? Approximately 30 entries into the plasma sheet have been identified between October 2005 and April 2007 when Cassini was between 5 RS and 45 RS from Saturn, at local times between 5 to 20 LT, and close to the equatorial plane. This data set includes the preliminary events previously presented by Sittler et al., [2007a, b, c], where both outward and inward flows super-imposed on rotational motion were identified. For each plasmasheet encounter we employ data from the Cassini Plasma Spectrometer (CAPS) to determine preliminary ion flow directions and magnitude, as well as the ion and electron energy spectra and the ion composition. Our preliminary results find that the ion flows are generally in the corotation direction, but significantly lagging the corotation speed. There is also generally an outward (tailward) component to the derived velocities. Of the events surveyed, the IMS instrument was looking tailward (and therefore able to see inward flows) approximately 60 % of the time. However, inward flows (i.e. VR < 0) were only observed approximately 10 % of the time, demonstrating the rarity of such signatures in this region. Moreover, the ion composition persistently has a strong water-group component, rather than being relatively depleted of heavy ions as expected for recently emptied flux tubes returning to the inner magnetosphere. E. C. Sittler et al., Cassini Observations of Saturn's Dawn-Magnetotail Region: Preliminary Results, Magnetospheres of the Outer Planets (MOP), San Antonio, TX, June 25-29, 2007a. E. C. Sittler et al., Cassini Observations of Saturn's Dawn-Magnetotail Region: Preliminary Results, Europlanet, EPSC2007-A-00428, Potsdam, Germany, August 19-24, 2007b. E. C. Sittler et al., Cassini Observations of Saturn's Magnetotail Region: Preliminary results, Fall AGU, San Francisco, CA, December 10-14, 2007c.

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