通量传输事件相关的动力学磁场湍流观测研究

摘要

Intense magnetic fluctuations are recorded in the magnetosphere near the magnetopause when Flux Transfer Events (FTE) are passed by the THEMIS satellites.The power spectra of these fluctuations obtained by Fast Fourier Transform (FFT) show that the Power Spectra Density (PSD) peaks around the disturbance frequency of FTE (about 0.1Hz),and decreases from the proton gyrofrequency (about 1 Hz) to 64 Hz following a power law of P0f-α These fluctuations are interpreted as magnetic turbulences in the kinetic regime in the Low Latitude Boundary Layer (LLBL).The results show that both the PSD and the slopes of the power spectra α decrease when the observing satellite position is more and more away from the magnetopause or the FTE location in the LLBL.However,α and the PSD are independent from the azimuthal position of FTE or local time of the low latitude magnetopause.All these observations suggest that the moving FTEs are the source for these magnetic fluctuations.Large scale perturbations on the magnetopause,e.g.FTEs and the associated magnetic turbulences,provide a hint which may reveal the interaction between the magnetosheath and the magnetosphere in a kinetic scale.Whether the magnetic turbulences can provide enough viscosity for the forming of the flow vortices on the magnetospheric side of FTEs or not need to be further confirmed.%THEMIS卫星观测到通量传输事件(FTE)的同时,也在磁层侧涡流区域观测到强磁场扰动现象.利用快速傅里叶变换分析磁场扰动频谱特征发现:大约在FTE的扰动频率(约0.1 Hz)处,功率谱密度达到峰值;在质子回旋频率(约1 Hz)至64 Hz的频段内,功率谱密度随着频率的增大而减小,服从幂律分布Pof-α.因此,可以认为这些磁场扰动为低纬边界层中的动力学磁场湍流.研究结果表明,当低纬边界层(Low Latitude Boundary Layer,LLBL)中卫星相对磁层顶或FTE的位置越来越远时,功率谱密度与功率谱斜率α(幂律指数)降低,但FTE所在的方位角或低纬磁层顶的磁地方时对幂律指数α和功率谱密度没有显著影响.这些观测特征表明移动的FTE是磁场湍流的源.磁层顶上的大规模扰动(如FTE)和相关的磁场湍流从动力学尺度揭示了磁鞘与磁层的类黏滞相互作用.然而低纬边界层中FTE磁层侧涡流形成所需的黏滞性是否可由磁场湍流来提供还需要验证.