Magnetic field observations by IMP 6 at about 20 RE tailward from the earth show that magnetic field in the neutral sheet sometimes takes north and south polarities alternately at successive crossings. This can be understood by wavy motion of the neutral sheet that propagates in the dawn-dusk direction in the presence of the dawn-dusk component of the magnetic field. Such wavy motion of the neutral sheet can be generated by the Kelvin-Helmholtz instability if the dawn-dust flow of only several tens of km/s is present. ¿ American Geophysical Union 1989