A study of magnetotail dynamics during substorms using high-time resolution data

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We have investigated various characteristics of magnetic and plasma fluctuations for 21 events of substorm-associated near-Earth dipolarization at Xgsm=-8 --11 Re and |Ygsm|<5 Re, using high-time-resolution data for the magnetic field (1/16 s), plasma moment (12 s), and electric field (3 s) measured by the GEOTAIL satellite in 1995-2000. The dipolarizations are always accompanied by intense fluctuations of the magnetic field. The amplitude of the fluctuations is larger for smaller B (closer to the cross-tail current sheet). We found several events in which the magnetic field elevation angles decreased rapidly prior to the dipolarization (explosive growth phase (EGP)-like features). The magnetic fluctuations are not sinusoidal curves but are spikes of a sudden decrease and increase with a time scale of seconds. In quite a few cases, the fluctuations are accompanied by Earthward convective plasma flow. The spectral shape of the fluctuations has a steeper slope (f^-2.7) at 1.0-0.2 Hz than at 0.01-0.2 Hz (f^-2.0). From the correlation between plasma velocity and magnetic field fluctuations, we estimate the parallel scale sizes of waves with periods of 60 s and 15 s to be about 10 Re and 5 Re, respectively. For the 60-s waves, the magnetic and plasma pressure variations are clearly anti-phase. The total pressure tends to decrease gradually before the dipolarization and increase during and after the dipolarization.