Quiet time magnetotail dynamics and their implications for the substorm trigger

[1] The present study seeks to systematically examine the fast plasma flow in the plasma sheet at geomagnetically quiet time. The study uses plasma measurements made by the Geotail satellite in the midnight sector at x > −50 RE during quiet intervals with a total duration of 446 hours. Comparison with the results of previous studies suggests that the occurrence frequency of the perpendicular flow velocity depends more clearly on the radial distance than on geomagnetic activity. Two extreme events were selected for detailed studies; they occurred on 23 November and 10–11 December 1994. In the 23 November event, Geotail was located ∼37 RE from Earth and observed a fast tailward flow ( ∼ −1250 km s−1) with a strongly southward magnetic field (BZ ∼ −8.9 nT). The signature indicates that a near-Earth neutral line was formed earthward of the satellite and the reconnection reached the lobe magnetic field. On the ground, however, only weak (<100 nT) magnetic disturbances were observed at high (∼75°) latitudes but not at auroral zone stations. The result strongly suggests that lobe reconnection is not sufficient for the global development of a substorm. The 10–11 December 1994 event is very similar to the 23 November 1994 event except that Geotail observed a fast ( ∼ +1500 km s−1) earthward flow rather than a tailward flow, along with the dipolarization of the local magnetic field. It is asserted that the near-Earth substorm process, that is, tail current disruption, controls the development of a substorm. The fast plasma flow may set a favorable condition for this process to proceed, and therefore the substorm may tend to develop following generation of the fast flow, but the result of the present study indicates that near-Earth reconnection does not necessarily trigger the global substorm.