Diffusion at the magnetopause: A theoretical perspective

摘要

This paper discusses theoretical aspects of cross-field wave-particle transport at the magnetopause. The basic framework for calculating the transport based on second order-theory is reviewed. Instabilities that give rise to the waves which then lead to particle scattering and diffusion are discussed, and in particular, the lower hybrid drift instability is emphasized. The linear and nonlinear properties of this instability are presented, along with results from revelant computer simulations. Other potential diffusion processes at the magnetopause are also described. The overall conclusion, supported by wave and particle observations, is that wave-particle diffusion is a relatively weak process at the magnetopause. Further, it is argued that this line of research, while illuminating potential new mechanisms, cannot address the more important question of diffusion and transport on a spatial scale of many ion gyroradii over a temporal scale of many ion gyroperiods. A new paradigm to examine the global aspects of magnetopause transport on ion scales using hybrid code methods is presented. Results of two-dimensional hybrid simulations show that the structure of the magnetopause for northward interplanetary magnetic field is reproduced, and the resulting transport in this case is due to waves below the ion gyrofrequency. Diffusion in the presence of viscous transport due to the kinetic Kelvin-Helmholtz instability is then discussed. The potential for the study of particle diffusion, viscous interactions and magnetic reconnection simultaneously through this paradigm is also addressed.