Anisotropy Reversals in the Distant Magnetotail and Their Association with Magnetospheric Substorms

Abstract

We present a series of tailward-to-Earthward particle anisotropy reversals observed by the GEOTAIL spacecraft in the distant magnetotail plasma sheet at a distance of | x |≈ -90R_E during the 2.5-day period of 11/01/94 1200 UT to 13/01/94 2400 UT. The reversals were detected in the first order anisotropy of energetic protons and the first moment of the low energy plasma distribution function. The X-component of the cross-field flow exhibits the clearest indicator of a particle anisotropy reversal. A comprehensive examination of ground magnetometer and geosynchronous, satellite data reveals that each distant tall anisotropy reversal occurred during the late expansion or recovery phase of a magnetospheric substorm. Out of 14 substorms during which adequate monitoring of the geomagnetic activity and the plasma sheet was possible all but possibly one exhibited a tailward-to-Earthward reversal in the cross field flow ∼1-2 hours after substorm onset. The median time delay between substorm onset and the beginning of the subsequent Earthward convective flow was 94 min. Some uncertainty is associated with this time delay due to the commonly observed exits of the spacecraft to the lobe/mantle. The lower limit on the time delay based on the time of exit of the spacecraft to the lobe/mantle is 61 min. The implications of the particle anisotropy reversals for substorm dynamics are considered.