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

我们利用1995-2000年GEOTAIL卫星测量的磁场（1/16秒）、等离子体矩（12秒）和电场（3秒）的高时间分辨率数据，研究了在Xgsm=-8—11 Re和|Ygsm|<5 Re的21次亚暴相关近地双极化事件的磁和等离子体波动的各种特征。偶极总是伴随着磁场的剧烈波动。对于较小的B（更接近交叉尾电流片），波动幅度较大。我们发现几个事件中，磁场仰角在双极化之前迅速下降（爆炸生长阶段（EGP）样特征）。磁波动不是正弦曲线，而是以秒为时间尺度突然减少和增加的尖峰。在相当多的情况下，波动伴随着向地球的对流等离子体流。波动的频谱形状在1.0-0.2 Hz处的斜率（f^-2.7）比在0.01-0.2 Hz处的斜率（f^-2.0）更陡。根据等离子体速度与磁场波动的相关性，我们估计周期为60s和15s的波的平行尺度分别约为10re和5re。对于60秒波，磁和等离子体压力变化明显是反相的。总压在双极化前逐渐减小，在双极化过程中和双极化后逐渐增大。

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.