High-speed ion flow, substorm current wedge, and multiple Pi 2 pulsations

We have studied the onset timing of earthward high‐speed ion flow observed by the AMPTE/IRM satellite at 12.3 Earth radii (RE) and 0100 MLT in the central plasma sheet during an isolated substorm event on March 1, 1985. The timing of this onset is compared with that of the substorm current wedge and Pi 2 magnetic pulsations observed by a large number of ground‐based stations and the AMPTE/CCE, GOES 5, and ISEE 1 satellites and with that of high‐energy particle injection observed at Los Alamos geosynchronous satellite 1982‐019. The onset of earthward high‐speed flow is observed 3 min before the onset of the global current wedge formation and 6 min before the onset of high‐energy particle injection. The three bursts of the high‐speed flow observed at AMPTE/IRM are likely to correspond to three compressional pulses observed at AMPTE/CCE at 6 RE and three Pi 2 wave packets observed at midlatitude ground stations. On the basis of these observations we conclude that the substorm current wedge is caused by inertia current and the current due to flow shear at the braking point of the earthward high‐speed flow during the initial stage of the substorm expansion phase. The braking point is well separated from the near‐Earth neutral line. It is also suggested that the compressional pulses and fluctuations of field‐aligned currents generated at the flow braking point can be the initial cause of the Pi 2 magnetic pulsations in the inner magnetosphere.