Magnetic field fluctuations during substorm-associated dipolarizations in the nightside plasma sheet around X=-10 RE -: art. no. A05212

Using high-time-resolution data for the magnetic field (1/16 s), plasma moment (12 s), and electric field (3 s) obtained by Geotail, we have investigated various characteristics of magnetic field fluctuations for 21 events of substorm-associated near-Earth dipolarization at X-GSM = -8 similar to -11 R-E and vertical bar Y(GSM)vertical bar < 5 R-E. The dipolarizations were always accompanied by fluctuations of the magnetic field. The amplitude of the fluctuations tends to be larger for smaller ambient magnetic field. For several events we found that magnetic field elevation angles show rapid decrease prior to the rapid increase (features like explosive growth phase). Compressional fluctuations of magnetic field intensity B show characteristic spikes of a sudden decrease and increase with a timescale of seconds. For quite a few cases the fluctuations are accompanied by enhancements of earthward plasma velocity. The spectral shape of the B fluctuations had a steeper slope (f(-2.7)) at higher frequency range at 1.0-0.2 Hz than at 0.01-0.2 Hz (f(-2.0)) for the events closer to the cross-tail current sheet (smaller ambient-B region). We found that the steeper slope extends to lower frequency range below 0.2 Hz for the events away from the current sheet (larger ambient-B region). This fact may suggest transition from three-dimensional to two-dimensional turbulence from smaller B to larger B region. We also investigated correlation of fluctuations in plasma velocity and magnetic field for waves with periods around 60 s. The correlations were rather weak, suggesting that non-MHD plasma processes are going on. For these waves, the magnetic and plasma pressure variations were clearly antiphase.