GEOTAIL observations of flow velocity and north-south magnetic field variations in the near and mid-distant tail associated with substorm onsets

Temporal changes in the plasma flow and magnetic fields of the Earth's magn etotail in the region of -7.5 > X(R-E) > -52.5 and -15 < Y(R-E) < 15 in the CSM coordinates were studied by applying a superposed epoch analysis metho d to 263 substorm events observed by GEOTAIL spacecraft. The onset times of the substorms were determined with Pi-2 pulsations at Kakioka station. It was found that the tailward flow associated with plasmoids started about 2 or 3 min before the Pi-2 onsets, and the earthward flow became somewhat coh erent 1 min before the onsets. The variation of the northward magnetic fiel d component was investigated by obtaining: the deviation of this component from that in the interval from 10.5 to 7.5 min before the Pi-2 onsets. We f ound that the north ward magnetic field increased at X similar to -10R(E) a nd decreased at X similar to -28R(E) simultaneously in the premidnight tail immediately after the onsets. These variations definetely correspond to th e dipolarization and plasmoid formation, respectively.