Azimuthal pressure gradient as driving force of substorm currents



We have studied the azimuthal pressure gradient in the central plasma sheet during substorms using plasma and magnetic field data obtained by the AMPTE/IRM satellite at nightside in radial distances of 9–15 RE. The pressure gradient is statistically estimated for the interval when the magnetic field shows a dipolar configuration (elevation angle >45°). It is found that by this criterion, most data are obtained during and after the passage of high-speed ion flow in the vicinity of the neutral sheet during magnetically active times. We show that there is an azimuthal gradient of plasma pressure in the dipolar field region. The pressure gradient can drive a substantial amount of field-aligned current (4.1×105 A per 2-hour local time sector). We suggest that this current is a source of the substorm current system after the high-speed ion flow stops.