WIND, GEOTAIL, and GOES 9 observations of magnetic field dipolarization and bursty bulk flows in the near-tail

The WIND, GEOTAIL, and GOES 9 spacecraft were all in the nightside magnetosphere and Interball was in the solar wind when a substorm began at similar to 07:25 UT on April 18, 1996. WIND and GEOTAIL were located at downstream distances of X = -12 to -14 R-c. The separation between these spacecraft was similar to 10 R-c in the Y direction and less than similar to 1 R-c in Z. While the northward field component at GOES 9 began to increase immediately, little or no dipolarization was observed at WIND and GEOTAIL until a series of very rapid B-z increases were observed similar to 25-30 min later. About similar to 1-2 min prior to each dipolarization event at WIND and GEOTAIL an earthward flow burst with peak speeds of 100-500 km/s was initiated. The duration of these bursty bulk flows were 1-7 min. These unique observations are interpreted as strong evidence for the existence of spatially localized, but sometimes temporally overlapping flow bursts in the near-tail during substorm expansion phase which result, cumulatively, in sufficient earthward magnetic flux transport to produce the well known dipolarization at geosynchronous orbit.