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Ann. Geophys., 26, 619-637, 2008
www.ann-geophys.net/26/619/2008/
© European Geosciences Union 2008
Plasma and fields in the wake of Rhea: 3-D hybrid simulation and comparison with Cassini data
E. Roussos1, J. Müller2, S. Simon2, A. Bößwetter2, U. Motschmann2, N. Krupp1, M. Fränz1, J. Woch1, K. K. Khurana3, and M. K. Dougherty4
1Max Planck Institut für Sonnensystemforschung, Max Planck Str. 2, 37191, Katlenburg-Lindau, Germany
2Institut für Theoretische Physik, TU Braunschweig, Germany
3Institute of Geophysics and Planetary Physics, University of California at Los Angeles, USA
4Blackett Laboratory, Imperial College London, UK
Abstract. Rhea's magnetospheric interaction is simulated using a three-dimensional,
hybrid plasma simulation code, where ions are treated as particles and
electrons as a massless, charge-neutralizing fluid. In consistency with
Cassini observations, Rhea is modeled as a plasma absorbing obstacle. This
leads to the formation of a plasma wake (cavity) behind the moon. We find
that this cavity expands with the ion sound speed along the magnetic field
lines, resulting in an extended depletion region north and south of the moon,
just a few Rhea radii (RRh) downstream. This is a direct consequence of
the comparable thermal and bulk plasma velocities at Rhea. Perpendicular to
the magnetic field lines the wake's extension is constrained by the magnetic
field. A magnetic field compression in the wake and the rarefaction in the
wake sides is also observed in our results. This configuration reproduces
well the signature in the Cassini magnetometer data, acquired during the
close flyby to Rhea on November 2005. Almost all plasma and field parameters
show an asymmetric distribution along the plane where the corotational
electric field is contained. A diamagnetic current system is found running
parallel to the wake boundaries. The presence of this current system shows a
direct corelation with the magnetic field configuration downstream of Rhea,
while the resulting j×B forces on the ions are
responsible for the asymmetric structures seen in the velocity and electric
field vector fields in the equatorial plane. As Rhea is one of the many
plasma absorbing moons of Saturn, we expect that this case study should be
relevant for most lunar-type interactions at Saturn.
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