Transport Mechanisms at the Magnetopause

Abstract

Mass, momentum, and energy transferred from the solar wind and magnetosheath into the magnetosphere must cross the boundary of the earth’s magnetic field, the magnetopause. This paper reviews the possibilities for convective and diffusive transport across the boundary and discusses the dependence of such processes on the orientation of the interplanetary magnetic field. It is first shown that, in the absence of transport processes, the magnetosheath magnetic field just outside the magnetopause always builds up to a dynamically significant level. The possibility of macroscopic plasma flow (convection) across the magnetopause is then discussed both for the case of a vanishing and a nonvanishing magnetic field component normal to the boundary. In the former case, convective flow does not appear possible, unless substantial electric fields occur parallel to the magnetic field. The latter case includes exactly field-aligned flows as well as magnetic-field reconnection. Particular attention is given to the problem of particle energization during reconnection. Finally, a brief discussion is given of diffusion processes and the constraints placed upon them by existing observations. It is shown that the efficiency of diffusive as well as convective transfer should be expected to have a strong dependence on the angle θ between the magnetospheric and the magnetosheath magnetic fields with a maximum for θ = π, a minimum for θ = 0.