Formation of a Displaced Plasma Wake at Neptune's Moon Triton

摘要

A prominent feature of the interaction between a planetary moon and its magnetospheric environment is the formation of a wake cavity in the downstream hemisphere, characterized by a significant decrease of the incident plasma density. Using an analytical model of Triton's sub-Alfvénic interaction with Neptune's magnetosphere, we demonstrate that this moon's wake may be rotated away from its downstream hemisphere into a region that would be accessible to the undisturbed upstream flow. Due to the strong tilt of Neptune's magnetospheric field and the low Alfvénic Mach number of the plasma, one of Triton's Alfvén wings can penetrate into the upstream region and intercept the impinging plasma long before it reaches the moon. The interaction with this upstream wing causes the flow to be deflected toward Triton at a steep angle before being absorbed, thereby generating a wake cavity that is significantly displaced with respect to the moon's geometric plasma shadow. Along the downstream-facing wing, the flow is deflected away from Triton and therefore unable to refill the displaced wake. Since the ionospheric Pedersen conductance greatly exceeds the Alfvén conductance, this asymmetric flow deflection is particularly intense at Triton: when the plasma encounters the wings, it is channeled toward or away from the moon along the axes of the wing tubes. The deflection of the streamlines away from their upstream direction peaks in the range of plasma parameters found along Triton's orbit. Therefore, the displaced wake may be a persistent feature of this moon's plasma interaction and observable during future flybys.