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

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