Analysis of mirror modes convected from the bow shock to the magnetopause

Spacecraft observations confirm the existence of mirror fluctuations in the magnetosheath. The mirror instability occurs in an anisotropic magnetized plasma when the difference between perpendicular and parallel (with respect to the magnetic field) plasma pressure exceeds a threshold depending on the perpendicular plasma beta. The anisotropy of the plasma pressure increases from the shock to the magnetopause as a result of magnetic field line stretching. This gives rise to plasma fluctuations which in turn lead to a relaxation between parallel and perpendicular temperatures. Mirror perturbations do not propagate and are convected with plasma flow along the streamlines. Using an anisotropic steady-state MHD flow model, we calculate the growth of mirror fluctuations from the bow shock to the magnetopause along the subsolar streamline. For the anisotropic MHD model, we use the empirical closure equation suitable for the AMPTE/IRM observations. The amplitudes of mirror fluctuations, which are obtained as a function of distance from the magnetopause, are directly compared with AMPTE/IRM observations on October 24, 1985. With regard to both the amplification of the magnetic field and the plasma density oscillations, as well as the location of maximum amplitudes, model calculations are in good agreement with values obtained from the AMPTE/IRM data.