近磁尾TC-1观测到伴随有高速流的ULF波及不稳定性分析

ULF wave accompanied with high-speed flows observed by TC-1 in the near magnetotail and the instability analysis

2004年8月3日近地TC-1卫星在磁尾XGSM^-12RE的等离子体片内,观测到了伴随着高速流的低于离子回旋频率的波,即超低频波(ULF,Ultra Low Frequency).该波垂直分量的振幅在高速流及其振荡减速期间大致相当;而平行分量振幅在高速流时明显大于其振荡减速时.利用一个扰动双流模型对完全磁化离子横场漂移驱动的电磁不稳定性计算后,预测结果表明:(1)对于垂直分量来说,横场漂移速度与Alfvén速度的比值影响不稳定性增长率和激发波频率,随其比值增加,增长率变大,激发波频率从负值增加到正值.(2)对于平行分量来说,温度各向异性时等离子体热速度与Alfvén速度比值只影响不稳定性增长率和激发波频率,未改变不稳定性模类别;而温度各向同性时离子横场漂移速度与Alfvén速度比值既影响不稳定性模的种类及其分支,又影响激发波频率.进一步将卫星观测到的等离子体密度、温度、整体流速和磁场代入模型方程,进行数值计算与上述预测结果对比后发现:卫星观测中垂直分量的功率谱密度(PSD,Power Spectrum Density)增强时间和频段与理论模型中由β//、β⊥和v⊥/VA引起不稳定性激发的波一致;卫星观测中平行分量的功率谱密度增强时间与理论模型基本相符,但是前者的频率明显地低于后者.因此,除了需考虑平行磁场的离子整体流速对不稳定性激发波频率的可能影响,还需要统计上进一步核实伴随有高速流的ULF波与不稳定性的相关性.

On 3 August 2004, the enhancement of Ultra-Low-Frequency （ULF） waves with frequency less than the ion cyclotron frequency and the high-speed flows are observed by TC-1 at XGSM ^-12RE in the plasma sheet of the magnetotail. During the high-speed flows, the amplitude of theperpendicular component is roughly unchanged. For the parallel component, the amplitude accompanied with high peak flow is larger than that accompanied with low peak flow. After numerically analyzing the perturbed two-fluid model for drift-driven electromagnetic instability by the fully magnetized ions, we found. （1） For perpendicular propagation, the ratio of the cross-field drift to the Alfven speed plays a role in the growth rate of the instability and the excited wave frequency. With the increase of the ratio, the growth rate increases and the excited wave frequency turns to positive from negative. （2） For parallel propagation, the ratio of the ion thermal velocity to the Alfven speed plays a role in the growth rate of the instability and the excited wave frequency, but the instability mode keeps unchanged. However, when the temperature is isotropic, the ratio of the cross-field ion drift to the Alfven speed affects the growth rate and the excited wave frequency as well as the instability mode and the corresponding branches. When the plasma density, temperature, bulk velocity and magnetic field observed by TC-1 are input into the model equations, the numerical solutions show that the enhancements of the power spectrum density （PSD） and the excited wave frequency are well consistent with what the theoretical model predicts for perpendicular propagation~ while the enhancements of the power spectrum are consistent with what the theoretical model predicts with the former frequency less than the latter for parallel propagation. In addition to considering whether other factors such as the ion flow parallel to the magnetic field will affect the instability and the excited wave frequency, it is necessary to statistically further verify the correlation of ULF waves with the instabilities accompanied with high-speed flows.