RAPID VARIATIONS OF THE PLASMA BULK FLOW DIRECTION OBSERVED IN THE PLASMA SHEET AT X(GSE)SIMILAR-TO-60RE

Three-dimensional variations of the plasma bulk flow direction in the plasma sheet at X(GSE) approximately -60Re were observed by the Low En ergy Particle (LEP) instrument on board the GEOTAIL satellite. These v ariations have two components, namely a rapidly varying component of w hich the period is about 8 minutes and a more slowly varying component . Separating these two components, we have found a period when the rap idly varying component showed clear rotations while the direction of t he slowly varying component nearly corresponded with the direction of the magnetic field. We have calculated the rotation axis of the rapidl y varying component during this period and have found that the directi on of the rotation axis changed by nearly 900 in a step-like motion wh ile the vortex motion remained coherent in the plane perpendicular to the moving rotation axis. We discuss several possible mechanisms for t hese variations, MHD waves, vortex structures, and tail flapping motio n with velocity shears.