Diffusion at the magnetopause: Hybrid simulations

Electromagnetic wave generation and resulting cross-field diffusion of plasma are considered at a tangential discontinuity, which characterizes the magnetopause for northward interplanetary magnetic field. Two-dimensional hybrid (particle ions, massless fluid electrons) simulations, in which the tangential discontinuity is generated self-consistently via a stream-stream interaction, are used to show that wave growth occurs when the ambient magnetic field is predominantly perpendicular to the direction of the density gradient. Low-frequency (much less than ion gyrofrequency) waves, with amplitudes delta B/B less than or equal to 0.2 and anticorrelated density fluctuations delta n/n less than or equal to 0.6, are generated at the discontinuity, resulting in cross-field diffusion that is comparable to the Bohm rate. Both the fluctuation level and the lack of ion heating in the calculations are consistent with observations at the magnetopause. The magnitude of the diffusion is considered in the presence of numerical effects and in the context of the inferred diffusion rate at the magnetopause. The relation of the low-frequency waves and their consequences to faster growing, short-wavelength waves due to the lower hybrid drift instability is also addressed. The overall conclusion of this initial study is that diffusion due to low frequency waves is not likely to be a major effect at the magnetopause.