MHD SIMULATIONS OF OPPOSITELY PROPAGATING ALFVEN WAVES IN THE MAGNETOSHEATH AND SOLAR-WIND

Recent WIND and GEOTAIL observations indicate that Alfven waves can re verse their direction of propagation relative to the ambient magnetic field when they cross the bow shock [Sibeck et al., 1997]. To investig ate this effect, we use three-dimensional magnetohydrodynamic (MHD) si mulations to study an Alfven wave (AW) propagating into the magnetoshe ath. Our results are consistent with the observations. The AW is set t o propagate with a wave vector k pointing anti-sunward and largely opp osite to the magnetic field direction, i.e. B.k < 0. The interplanetar y magnetic field (IMF) is given a typical Parker spiral angle. The pri mary Alfven wave that is transmitted into the magnetosheath (AW') cont inues to propagate anti-sunward. In the dusk magnetosheath, AW' largel y maintains this propagation direction through its entire passage thro ugh the magnetosheath. On the dawn side, however, AW' changes propagat ion direction relative to the magnetic field so that it propagates alo ng the magnetic field instead of against it, i.e. B.k becomes positive . This effect is strongest in the region of the magnetosheath where fi eld line draping forces the x component of the ambient IMF to change s ign. The reversal of B.k is the end result of a phase shift between th e velocity and magnetic field perturbations that occurs smoothly as th e wave propagates toward the magnetopause.