HF radar observation of field-aligned currents associated with quiet time transient flow bursts in the magnetosphere
The present study reveals the ionospheric signature of low level near-Earth flow burst activity in the central plasma sheet during a relatively quiet magnetosphere. Flow bursts were observed by Geotail in the near-Earth plasma sheet at (X = 17 R-E, Y = -6 R-E)(GSM), between 0300 and 0400 UT on 9 March 1997. Simultaneous observations of the solar wind conditions by the Wind satellite located 235 RE upstream, and of the southern auroral ionosphere, by the Sanae-Halley Super Dual Auroral Radar Network (SuperDARN) HF radar pair, offered a unique opportunity to investigate both the solar wind trigger and the related auroral ionospheric signature of these flow bursts. Although the prevailing solar wind conditions conformed to that expected for a quiescent magnetosphere, the solar wind dynamic pressure showed stepwise decreases, which gave rise to a near instantaneous observation of tailward flow bursts. Also, peak values in the local potential variation associated with the ionospheric convection during the quiet solar wind conditions exceeded 10 kV and therefore are not associated with viscous processes. We believe that the drop in solar wind pressure may have allowed the magnetosphere to relax into a lower metastable energy configuration. The associated reconfiguration of the magnetotail allows for the release or redistribution of stored magnetic energy, via magnetic reconnection, with the consequent observation of tailward flow bursts. The observations of earthward flow bursts, however, are thought to be a natural bimodal response of the magnetosphere to solar wind variations. Concurrent with these flow bursts in the magnetotail, we observed the development of convection vortices near the conjugate midnight auroral ionosphere, which are consistent with the flow of field-aligned currents. The spatial structure and the temporal locale of the convection vortices are suggestive of a small substorm current wedge.