Electron dynamics in the minimagnetosphere above a lunar magnetic anomaly

Abstract

We consider a three-dimensional electromagnetic particle-in-cell simulation of the boundary layer current in a minimagnetosphere created by the interaction between a magnetized plasma flow, which models the typical solar wind, and a small-scale magnetic dipole, which represents the Reiner Gamma magnetic anomaly on the lunar surface. The size of this magnetic anomaly (measured as the distance from the dipole center to the position where the pressure of the local magnetic field equals the dynamic pressure of the solar wind) is one quarter that of the Larmor radius of the solar wind ions. In spite of the weak magnetization of the ions, a minimagnetosphere is formed above the magnetic anomaly. In the boundary layer of the minimagnetosphere, the electron current is dominant. Due to the intense electric field induced by charge separation, electrons entering the boundary layer undergo E × B drift. In each hemisphere, the electron boundary current due to the drift shows a structure where the convection reverses; these structures are symmetric with respect to the magnetic equator. Detailed analysis of the electron cyclotron motion shows that electrons at the edge of the inner boundary layer obtain maximum velocity by the electric field acceleration due to the charge separation, not due to the drift of the electron's guiding center. The maximum electron velocity is approximately 8 times that of the upstream plasma. The width of the boundary layer current becomes approximately equal to the radius of the local electron cyclotron.