Initial GEOTAIL survey of magnetic substorm signatures in the magnetotail

Abstract

Substorm signatures in the magnetotail at radial distances from −30 to −210 Re are studied using magnetic field data acquired with the GEOTAIL spacecraft. The current data set consists of GEOTAIL magnetic field data transmitted in real time to Japan from October 1992 to September 1993, and the corresponding GMS‐4 (140°E geographic, geostationary) energetic particle data and 1‐s Kakioka (L=1.2, 140°E) magnetic field record. We first identify substorms from major increases in the particle flux at the geostationary satellite, then determine accurate onset times from Pi 2 pulsations at the Kakioka ground station, and finally examine the corresponding magnetic field signatures at GEOTAIL. Eighty‐nine substorm events occurred when GEOTAIL was either in the plasma sheet or in the tail lobe, and these events were analyzed with a special emphasis on the temporal development of the north‐south magnetic field component Bz. A clear bipolar signature (northward perturbation followed by southward) is observed in Bz for 53 events. The bipolar events have characteristics consistent with the plasmoid or the traveling compression region reported in earlier studies. In the middle and distant tail (X<−60 Re), most of events seem to indicate the passage of a plasmoid. The present results provide evidence for near‐Earth reconnection and the formation and release of plasmoids in association with substorm onsets. A simple model yields the following plasmoid parameters: near‐Earth reconnection around X=−30 Re, the center of the newly‐created plasmoid around X=−60 Re, and the expansion of the tailward‐moving plasmoid to a dimension of 100 Re in the X direction and almost fully across the distant tail (X=−200 Re).