Another aspect of the solar wind - magnetosphere interaction is related to diffusion. In the case of a collisionless plasma, the diffusion is not due to collisions but due to the ever-present turbulence in the electromagnetic fields. The magnetopause, in particular, has been observed to be a region characterized by an increased fluctuation level of  electromagnetic waves of ultra-low frequency. 

We have examined the propagation of waves at the magnetopause using a linear magnetohydrodynamic (MHD) description. Even though the mass flux across the magnetopause turns out to be zero, there may very well be an energy flux into the magnetosphere. The figure below shows the wave amplitudes (maximum and minimum modulus of the magnetic field, left frames) and the time-averaged energy flux (right frames) as a function of the distance to the center of the magnetopause (in km). The magnetosphere is to the left, the magnetosheath to the right of the magnetopause. Class 1: a non-vanishing perturbation intensity in the magnetosphere indicates transmission, while the energy flux remains constant as the incident flux equals the transmitted flux plus the reflected one. In the reference frame adopted here, a negative flux indicates energy transfer from the magnetosheath to the magnetosphere. Class 2: the wave does not penetrate into the magnetosphere and is  completely reflected, resulting in a net zero energy flux. Class 3: similar to class 2 but now resonant amplification (infinite wave amplitude) occurs at 1 or 2 points inside the magnetopause; resonant absorption implies a jump in the energy flux profile. Class 4: reflection, transmission and resonant amplification occur simultaneously; again, the energy flux is discontinuous at the resonant points.

Resonant amplification of MHD waves in realistic subsolar magnetopause configurations
J. De Keyser, M. Roth, F. Reberac, L. Rezeau and G. Belmont
J. Geophys. Res., 104(2), 2399-2410, 1999

Broad-band ULF fluctuations are routinely observed throughout the magnetosheath; the fluctuation level peaks at the magnetopause and becomes very small in the magnetosphere. The present paper analyzes the propagation of magnetosheath waves and the transport of energy at the subsolar magnetopause by means of a linear perturbation analysis, in the limit of the MHD approximation. We examine realistic equilibrium magnetopause configurations with a cold and dense magnetosheath and a hot, tenuous magnetosphere, possibly including a trapped magnetopause population.  The effects of magnetic field rotation are examined. Resonant amplification of monochromatic magnetosonic waves at the magnetopause is found to occur under various conditions. For a given frequency several field lines inside the magnetopause layer can resonate simultaneously.
 
 

Author: J. De Keyser and M. Roth   Curator: J. De Keyser   Johan.DeKeyser@oma.be