FORMATION OF THIN CURRENT SHEETS DURING PLASMA SHEET CONVECTION

Fully kinetic simulations in two spatial dimensions are used to demonstrate that the attempt to establish convection in the near-Earth magnetotail can drive the cusp or hinge region to form a thin current sheet which is embedded within a thicker plasma sheet. The simulation model incorporates a realistic near-Earth magnetic field configuration including dipolelike and taillike regions and allows plasma to precipitate at the earthward boundary. The imposition of an electrostatic convection field leads to a slow earthward convection of plasma and a tailward transfer of magnetic flux, with the inductive electric field opposing and almost canceling the external field. This flux transfer leads to the formation of a deep minimum in the equatorial B-z field in the near-Earth plasma sheet. As a consequence of the difference in dynamics between electrons and ions, the central plasma sheet in the cusp region becomes negatively charged, which leads to the formation of a strong polarization electric field (E(z)). The growth of the polarization field causes the electrons to carry the dominant current in the thin current sheet, whose characteristic half thickness is of the order of the local ion gyroradius and whose current density is an order of magnitude larger than for the usual tail current sheet. The negative space charge electric field associated with the thin current sheet and the related field-aligned currents are consistent with the polarization Harang equatorward electric field.