Anomalous plasma diffusion and the magnetopause boundary layer

The current state of anomalous diffusion research at the magnetopause and its responsibility for the formation of the magnetopause boundary layer (MPBL) are reviewed. Most of the relevant unstable wave modes contribute negligibly to the diffusion process at the magnetopause under magnetically undisturbed southward IMF conditions. Global stationary diffusion due to wave-particle interactions does not take place at the magnetopause. Resonant diffusion due to electrostatic ion cyclotron waves is probably also unlikely to explain the existence of the boundary layer. There is a relation between phase space diffusion and spatial diffusion due to resonant interactions and the diffusion coefficients may be high, but diffusion affects only a narrow velocity interval of resonant particles injecting mainly monochromatic particles into the magnetosphere. Bulk diffusion can probably not be caused by this mechanism. Eddy diffusion or time-dependent processes to feed the MPBL are favored. Microscopic wave-particle interactions and anomalous diffusion may contribute to locally break the MHD frozen-in conditions and help in transporting large amounts of magnetosheath plasma across the magnetospheric boundary