Surface current balance and thermoelectric whistler wings at airless astrophysical bodies: Cassini at Rhea

Abstract

Sharp magnetic perturbations found by the Cassini spacecraft at the edge of the Rhea flux tube are consistent with field-aligned flux tube currents. The current system results from the difference of ion and electron gyroradii and the requirement to balance currents on the sharp Rhea surface. Differential-type hybrid codes that solve for ion velocity and magnetic field have an intrinsic difficulty modeling the plasma absorber's sharp surface. We overcome this problem by instead using integral equations to solve for ion and electron currents and obtain agreement with the magnetic perturbations at Rhea's flux tube edge. An analysis of the plasma dispersion relations and Cassini data reveals that field-guided whistler waves initiated by (1) the electron velocity anisotropy in the flux tube and (2) interaction with surface sheath electrostatic waves on topographic scales may facilitate propagation of the current system to large distances from Rhea. Current systems like those at Rhea should occur generally, for plasma absorbers of any size such as spacecraft or planetary bodies, in a wide range of space plasma environments. Motion through the plasma is not essential since the current system is thermodynamic in origin, excited by heat flow into the object. The requirements are a difference of ion and electron gyroradii and a sharp surface, i.e., without a significant thick atmosphere.Key PointsSurface current balance condition yields a current system at astronomical bodiesCurrent system possible for sharp (airless) objects of any sizeCurrent system is thermoelectric and motion through the plasma nonessential

The role of plasma slowdown in the generation of Rhea's Alfvén wings

“…[] and recently by Teolis et al . []. These flybys confirmed that the field is indeed strengthened on field lines that are in contact with Rhea and have lost plasma.…”

“…Teolis et al . [] have shown that the sharp current system results from differences in ion and electron gyroradii and the requirement to balance currents on the sharp Rhea surface. The ions, because of their large gyroradii, precipitate (almost) uniformly on to Rhea from all directions, while the electrons are constrained to move north/south along the field lines.…”

“…Similarly, to compensate for the ion deposition on Rhea, electrons move along the field lines that are in direct contact with Rhea creating an oppositely directed current to that of ions (directed away from Rhea's equatorial plane). The current system is closed at large distances from Rhea by whistler type waves [ Teolis et al ., ]. Indeed, Santolík et al .…”

“…These flybys confirmed that the field is indeed strengthened on field lines that are in contact with Rhea and have lost plasma. In addition, two new surprises emerged from these flybys, which are as follows: (1) an Alfvénic perturbation is formed in the wake side of Rhea [ Simon et al ., ], a topic we revisit in this work, and (2) near the equatorial edges of Rhea, a sharp field‐aligned current system forms from the asymmetric precipitation of ions and electrons onto Rhea because of their differing gyroradii [ Teolis et al ., ]. Ions precipitate more uniformly onto Rhea because of their large gyroradii, while the electrons are constrained to flow north/south along the field lines.…”

“…This difference creates a flow of ions toward the equatorial plane of Rhea to replenish the ions lost in that region. The resulting charge imbalance on Rhea's surface is remedied by field‐guided electrons that precipitate onto Rhea's low‐latitude rim [ Teolis et al ., ].…”