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 variations in the magnetotail with GEOTAIL data. For this study we selected 342 substorm events from the Pi2 pulsation and applied the method of superposed epoch analysis. We divided the data into those in the plasma sheet, the plasma sheet boundary layer, and the lobe by the ion β. It was found that the fast tailward flows start to develop in the premidnight plasma sheet around X ∼ −28 R E (GSM) about 0 ∼ 2 min before onset, associated with the plasmoid formation. Immediately after onset, the fast tailward flows develop further, and the magnetic field substantially increases southward. Simultaneously, the northward magnetic field increases around X ∼ −10 R E , corresponding to the dipolarization. In the lobe, the perpendicular plasma flow toward the plasma sheet, as well as the dawn‐dusk electric field, first starts to be enhanced around ( X , Y ) ∼ (−20,7) R E about O ∼ 2 min before onset and then in the surrounding regions successively. The total pressure decrease first occurs around ( X , Y ) ∼ (−18,7) R E about 0 ∼ 2 min before onset, and then propagates to the surrounding regions successively. The dawn‐dusk electric fields both calculated with E = − V × B and measured directly by the double probe, simultaneously develop around X ∼ −10 R E and X ∼ −28 R E immediately after onset, while those in the plasma sheet around.( X , Y ) ∼ (−20,5) R E do not develop much even after onset. These observational results strongly suggest that an efficient magnetic reconnection takes place at least about 0 ∼ 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 release, where the near‐Earth neutral line (i.e., the diffusion region) is possibly created, is initially located around ( X , Y ) ∼ (−19,6) R E . These features are consistent with a thin‐current reconnection model.