On the relevance of the MHD approach to study the Kelvin-Helmholtz instability of the terrestrial magnetopause

The usual MHD approximation is not suitable for the description of the Kelvin-Helmholtz instability of the terrestrial magnetopause. A description implies an ideal magnetopause with a smooth continuous variation of the plasma and field parameters which occurs in an extended current sheet. The actual magnetopause and its adjacent plasma boundary layer are highly irregular, both spatially and temporally. It is found that finite ion Larmor radius effects do not represent small corrections. It is argued that these small scale inhomogeneities can change drastically the usual large scale description of the magnetopause oscillations.