Observations of plasma injection into the ring current during substorm expansive phase

Abstract

The injection of plasmas during the expansive phase of four substorms was investigated with global auroral images and simultaneous measurements of plasmas and magnetic fields in the plasma sheet near the magnetic equator. The high‐resolution images were gained at far‐ultraviolet wavelengths with the Earth Camera on board the Polar spacecraft and the observations of plasmas and magnetic fields were acquired with the Geotail spacecraft at radial distances in the range of 11 to 15 RE. For each of the substorms an ion flow burst was detected with maximum speeds in the range of 200 to 400 km/s and durations of several minutes. The ion bulk flow was directed earthward. For the first time auroral images recorded the position of the flow bursts as the leading edge of the westward or eastward traveling surge during the expansive phase of the substorm. The time intervals from the substorm onset as recorded by the camera until the detection of the flow pulse at the Geotail spacecraft ranged from 3 to 11 min. With the exception of one event which was located near the duskside magnetopause, the ion velocity distributions were characterized by the same densities and temperatures during and after the flow pulse for each of the individual events. For three of the events the electron temperature increased by a factor of about 2 after passage of the flow pulse. For the fourth event the spacecraft was not in the plasma sheet prior to the flow pulse, and hence this temperature increase could not be determined. Substantial fluctuations of the magnetic fields accompany the flow pulses. The electron and ion spectra were smoothly varying during the flow pulses without evidence of the highly structured spectral features due to combined gradient and electric field drifts which are well documented for particle injections near the 6.6‐RE synchronous orbit. The measurements at the Geotail spacecraft position support the existence of the previously proposed “injection front” for plasmas which is due to transient earthward convection during the expansive phase of substorms. This convection occurs during the diversion of the equatorial current sheet into the ionosphere at locations inside the Geotail position.