Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsm32b..02t&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #SM32B-02
Physics
[6275] Planetary Sciences: Solar System Objects / Saturn
Scientific paper
The Cassini Plasma Spectrometer (CAPS) detected freshly-produced water-group ions (O+, OH+, H2O+, H3O+) and heavier water dimer ions (HxO2)+ very close to Enceladus where the plasma begins to emerge from the south polar plume (1). The data were obtained during two close (52 and 25 km) flybys of Enceladus in 2008 (E3 and E5) and are consistent with measurements from the Cassini Ion Neutral Mass Spectrometer (INMS). The ions are observed in CAPS detectors looking in the Cassini ram direction close to the ram kinetic energy, indicative of a nearly stagnant plasma flow in the plume. North of Enceladus the plasma slowing commences about 4 to 6 Enceladus radii away, while south of Enceladus signatures of the plasma interaction with the plume are detected 22 Enceladus radii away. Here we review and contrast these observations including the E7 flyby (anticipated Nov. 2, 2009). E7 is planned for a closest approach ~103 km south of Enceladus and CAPS should detect ions at rest with respect to Enceladus and over a broad range of gyrophase angles. Plasma fluid parameters both upstream and downstream of these encounters are extracted from the CAPS data. In addition, we compare the CAPS ion measurements with both fluid and 3D hybrid simulations. The MHD simulations (BATSRUS) are tuned to agree with Cassini Magnetometer (MAG) observations during the encounters then compared with CAPS observations. For example, for the E3 encounter the CAPS/BATSRUS comparison is striking, with features reproduced such as: the overall spatial scale of the interaction, the slowing of the ion flow within the dust plume to less than 5 km/s with respect to Enceladus, the temperature, flow and density signature of the geometric wake, and the flow perturbation along the magnetic field due to wake expansion. For E5, BATSRUS tuned against MAG suggests a 15 km/s bulk plasma flow toward Saturn during the encounter. We search for signatures of this flow in the CAPS ion data. 1.) Tokar,R.L. et al. Geophys. Res. Lett., 36, L13203, doi:10.1029/2009GL038923, 2009.
Coates Andrew J.
Crary Frank J.
Goldstein Raymond
Jia Yu
Johnson Robert E.
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