Propagation sense of low-frequency MHD waves in the magnetosheath observedby Geotail

Using Geotail data from the months of December 1994 and January 1995, we ha ve investigated transverse fluctuations in the magnetic field and in the pl asma velocity in the magnetosheath. In the frequency range below about 10 m Hz, a change in the magnetic field is often linearly correlated with a chan ge in plasma velocity, satisfying the Walen relation expected from the theo ry of MHD wave propagation. We found well-correlated events in a wide range of radial distances of the magnetosheath, -42 R-E < GSM-X < 6 R-E, where R -E is the radius of the Earth. The correlation is sometimes positive and so metimes negative, depending upon the angle between the magnetic field and t he plasma flow direction. The results indicate that, in a frame moving with the plasma, the waves propagate unidirectionally in the same direction as the plasma flow in a draping field. In other words, the wave energy is alwa ys propagating simultaneously antisunward and outward. Moreover, our result s suggest two possibilities for the origin of the signals in the magnetushe ath: the magnetopause or the nose region of the bow shock.