How are Rhea's Alfven Wings Generated?

Abstract:

Alfven wings are known to arise from the interaction of a conducting or mass-loading object with a flowing magnetized plasma. In such an interaction, the plasma and the magnetic flux are slowed down and diverted around the interacting object. The Alfvenic wing current system facilitates the exchange of momentum between the object and the flowing plasma. The strength of interaction is governed by the effective conductivity of the object as well as the Alfven conductance of the magnetized plasma.

Alfven wings are not expected to be generated when an inert object such as Rhea interacts with the corotating plasma of Saturn's magnetosphere. In such an interaction, the plasma impacting the moon is expected to be absorbed by the moon whereas the magnetic flux passes unimpeded through the moon. However, in two close polar passes of Rhea called R2 and R3, Cassini clearly observed magnetic field signatures consistent with Alfven wings. In addition, observations from a high inclination flyby (CA > 100 R_H) of Rhea on June 3, 2010 showed that the Alfven wing continues to propagate away from Rhea even at this large distance.

We have performed 3-dimension hybrid simulations of Rhea to understand the interaction of Rhea with Saturn's magnetosphere. We show that in the wake region of Rhea, the plasma pressure is much lower than the background and a pressure gradient is generated that is directed in the direction of corotating plasma. The resulting pressure gradient force directs plasma from downstream region towards Rhea in order to refill the wake. This back-filling slows down the plasma in the wake. As the plasma is close to corotational speed outside of the plasma wake, an Alfven wing is generated that bends the magnetic flux tubes and transports momentum directed in the flow direction from regions above and below the wake into the wake. We will illustrate the global current circuit that facilitates this transfer of momentum.