GEOTAIL OBSERVATIONS OF NORTH-SOUTH PLASMA VELOCITY ENHANCEMENTS IN THE LOBE NEAR SUBSTORM EXPANSION PHASE ONSET

This paper reports on the enhancement of the lobe plasma velocity towa rds the neutral sheet. Analyzing the Geotail plasma and magnetic field data in the lobes earthward of X = -45 R-E, we identified 30 events o f several-minute magnetic field increase-and-decrease during which the field tilts only southward; that is, the B-Z variation is unipolar. F or most of these events, we found that the lobe plasma velocity toward s the neutral sheet is enhanced and peaked around the most southward t ilting of the field. The average of the peak magnitude is about 130 km /s in a plane perpendicular to the magnetic field. For each case, this magnitude is roughly 1 - 10 % of the lobe Alfven speed. We interpret the velocity enhancement having these characteristics as sig-natures o f the transition from plasma sheet reconnection to lobe reconnection a t the near-Earth neutral line.