Effects of the fast plasma sheet flow on the geosynchronous magnetic configuration: Geotail and GOES coordinated study

[1] The present study statistically examines how ( or if) the geosynchronous ( GOES) magnetic field responds to fast earthward flow observed by the Geotail satellite in the plasma sheet. The change of the GOES H (north-south) component within 15 min of the detection of fast flows, Delta H, is used as a primary measure of the geosynchronous response. It is found that following the detection of fast flows, the geosynchronous magnetic field rarely dipolarizes, but it often becomes more stretched, which is manifested by negative Delta H. This H decrease is not accompanied by any correlated variation of the D ( azimuthal) component, suggesting that the associated stretching is not an edge effect of the substorm current wedge formation, but it can be attributed to the intensification of the local tail current. No systematic dependence of Delta H on the satellite separation can be found. On the other hand, the geosynchronous magnetic field tends to dipolarize if it is already stretched significantly, although the associated changes in the H and V ( radial) components are not much larger than those in events that are not preconditioned. The flow intensity does not seem to be a controlling factor, either. However, caution needs to be exercised because the present study is not able to address the azimuthal structure of the fast flow. It is concluded that in most events the fast plasma flow does not reach geosynchronous orbit and that the generation of the fast plasma flow in the plasma sheet is not sufficient for causing geosynchronous dipolarization.