Analysis of Cassini magnetic field observations over the poles of Rhea

Abstract

We analyze Cassini magnetic field observations from the only two polar flybys of Saturn's largest icy satellite Rhea (R2 on 02 March 2010 and R3 on 11 January 2011) which are scheduled between Saturn Orbit Insertion and the end of the mission in 2017. For the interpretation of these data, we apply estimations from simple analytical models as well as results from numerical hybrid simulations (kinetic ions, fluid electrons) of Rhea's interaction with the incident magnetospheric plasma. In‐situ observations of exospheric neutral gas and pick‐up ions suggest Rhea to be embedded in a tenuous gas envelope. However, the interaction of this gas with the magnetospheric flow does not make any measurable contributions to the magnetic field perturbations detected above the poles of the moon. Instead, the field perturbations observed in these regions mainly arise from the absorption of magnetospheric particles with large field‐aligned velocities, impinging on the north and south polar surface of Rhea. In addition to numerous interaction features known from preceding Cassini flybys of Saturn's plasma‐absorbing moons, the magnetic field data acquired above Rhea's poles reveal perturbations of the flow‐aligned field component, corresponding to a draping/Alfvén wing pattern. Based on our hybrid simulations, we suggest that these signatures arise from the finite extension of Rhea's wakeside plasma void along the corotational flow direction, yielding a density gradient in corotation direction, and thereby generating a diamagnetic current from the Saturn‐facing into the Saturn‐averted hemisphere of the moon. This transverse current is responsible for generating a weak Alfvén wing pattern at Rhea which has been detected by the Cassini spacecraft during the R2 and R3 flybys. Due to the large gyroradii of the incident magnetospheric ions, this structure features a pronounced asymmetry with respect to the direction of the convective electric field. Results of our simulation, considering only plasma absorption on the moon, are in good agreement with Cassini magnetometer data from both flybys. At Saturn's icy satellites Tethys and Dione, the low value of the magnetospheric plasma beta most likely prevents the formation of similar currents and measurable flow‐aligned magnetic field distortions.