Abstract
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.
Highlights
Since the Saturn Orbit Insertion (SOI) of the Cassini/Huygens spacecraft (July 2004), moonmagnetosphere studies have been primarily focused on two saturnian moons, Titan and Enceladus
The opening angle of the rarefaction region in our simulation is about 45◦, almost in excellent agreement with the expected value. All these results show that the sonic Mach number is important for lunar-type interactions and that the one-dimensional approach of the plasma expansion into vacuum is fundamentally valid, at least for the wake evolution in the plane that contains the bulk plasma velocity and the magnetic field
As along the y=0 cut electric field disturbances are insignificant and the wake refilling is driven by the expansion of the along the magnetic field lines, we can test the onedimensional, analytical approach that we applied in Sect. 4.1 for the plasma density
Summary
Since the Saturn Orbit Insertion (SOI) of the Cassini/Huygens spacecraft (July 2004), moonmagnetosphere studies have been primarily focused on two saturnian moons, Titan and Enceladus. Since many of Saturn’s inert moons orbit within the plasmasphere of the planet, the upstream plasma moments can be considered quasi-stable for large time scales compared to the duration of a Cassini flyby or a simulation run. Such a configuration eliminates many complications which can otherwise mask important physical processes associated with the dynamics of a lunar wake. In the present study we utilize some of the advantages mentioned above to simulate the interaction of Saturn’s moon Rhea with the magnetospheric plasma For this purpose we use a 3-D hybrid code and a high-spatial resolution set-up.
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