Correlation between magnetic and density fluctuations in the foreshock MHD turbulence

我们主要对以下几个主题进行了研究。与参数不稳定性相关的磁和密度波动：非相干母波的影响许多关于Alfven波参数不稳定性的理论和数值模拟研究最初假设存在单色（相干）母波，尽管与观测结果不一致。当初始母波为波数有限宽度的非相干湍流时，我们发现在小波数处，由于Alfven波之间的跳动，密度波动会被瞬时触发。由于这种影响，调制型而不是衰减型参数不稳定优先被驱动。利用离子纵向和流体横向的Vlasov描述框架，研究了平行Alfven波的参数不稳定性。纵向离子动力学效应在参数不稳定性中起重要作用。近似简并的Alfven和声波系统的非线性演化由于等离子体的β比接近一致，且Alfven波的磁场幅值与背景场相比较大，因此准线性Alfven和声波在太阳风和前震等离子体中的传播速度接近相等，可以认为是简并的（传播速度接近相等）。我们在上述模型中加入了离子动力学效应，并在该模型中讨论了磁扰动和密度扰动之间的关系。利用常用于确定波数和对阵列信号数据进行功率谱分析的capon和线性预测方法，我们开发了一种评估地球弓形激波前震区观测到的大振幅MHD波双谱（和双相干）的方法。少

We have conducted the research mainly on the following topics.1.Magnetic and density fluctuations associated with parametric instabilities: effects of incoherent parent waves Many theoretical and numerical simulation studies of Alfven wave parametric instabilities assume a presence of monochromatic (coherent) parent wave initially, although not consistent with observations. When the initial parent waves are given as incoherent turbulence with finite width in the wave number, we find that density fluctuations at small wave numbers are triggered instantaneously due to the beating among the Alfven waves. Due to this effect, the modulational, rather than the decay type parametric instabilities are preferentially driven unstable.2. Magnetic and density fluctuations associated with parametric instabilities: effects of incoherent parent waves Parametric instabilities of parallel Alfven waves are studied, using a framework of Vlasov description for ions in the longitudinal direction and fluid … More description of transverse directions. The longitudinal ion kinetic effects play essential roles in the parametric instabilities.3. Nonlinear evolution of a system with Alfven and sound waves which are nearly degenerate The quasi-linear Alfven and the sound waves in the solar wind and the foreshock plasma should be considered degenerate (propagate with nearly equal velocities) since the plasma beta ratio is near unity, and the Alfven wave magnetic field amplitude is large compared with the background field. We have included the ion kinetic effects to the above model and discussed within this model the relationship between the magnetic and the density perturbations.4. Development of a method to analyze turbulence data obtained by multiple-spacecraft experiments By using the capon and the linear prediction method, frequently used for determination of the wave numbers and the power spectra analysis for array signal data, we are developing a method to evaluate bi-spectrum (and bi-coherence) of large amplitude MHD waves typically observed in the foreshock region of the earth's bowshock. Less