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 ∼07:25 UT on April 18, 1996. WIND and GEOTAIL were located at downstream distances of X=−12 to −14 R e . The separation between these spacecraft was ∼10 R e in the Y direction and less than ∼1 R e 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 ∼25–30 min later. About ∼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.