A statistical study of variations in the near and middistant magnetotail associated with substorm onsets: GEOTAIL observations

We have studied the three-dimensional structure of substorm-associated vari ations in the magnetotail with GEOTAIL; data. For this study we selected 34 2 substorm events from the Pi2 pulsation and applied the method of superpos ed epoch analysis. We divided the data into those in the plasma sheet, the plasma sheet boundary layer, and the lobe by the ion beta. It was found tha t the fast tailward hows start to develop in the premidnight plasma sheet a round X similar to -28 R-E (GSM) about 0 similar to 2 min before onset, ass ociated with the plasmoid formation. Immediately after onset, the fast tail ward hows develop further, and the magnetic field substantially increases s outhward. Simultaneously, the northward magnetic field increases around X s imilar to -10 R-E, corresponding to the dipolarization. In the lobe, the pe rpendicular plasma flow toward the plasma sheet, as well as the dawn-dusk e lectric Geld, first starts to be enhanced around (X, Y) similar to (-20, 7) R-E about 0 similar to 2 min before onset and then in the surrounding regi ons successively. The total pressure decrease first occurs around (X,Y) sim ilar to (-18,7) R-E about 0 similar to 2 min before onset, and then propaga tes to the surrounding regions successively. The dawn-dusk electric fields both calculated with E = -V x B and measured directly by the double probe, simultaneously develop around X similar to -10 R-E and X similar to -28 R-E immediately after onset, while those in the plasma sheet around (X, Y) sim ilar to (-20,5) R-E do not develop much even after onset. These observation al results strongly suggest that an efficient magnetic reconnection takes p lace at least about 0 similar to 2 min earlier than the Pi2 onset, that the substantial plasmoid evolution and the dipolarization occur simultaneously immediately after onset, and that, on average, the center of the energy re lease, where the near-Earth neutral line (i.e., the diffusion region) is po ssibly created, is initially located around (X, Y) similar to (-19, 6) R-E. These features are consistent with a thin-current reconnection model.