Correlation between magnetic and density fluctuations in the foreshock MHD turbulence

主要研究内容如下：1.与参量不稳定性相关的磁场和密度涨落：非相干母波的影响许多关于Alfven波参量不稳定性的理论和数值模拟研究都假设初始存在单色（相干）母波，尽管这与观测结果并不一致。当初始母波为波数为有限宽度的非相干湍流时，我们发现由于Alfven波之间的拍频，在小波数下的密度涨落是瞬间触发的。由于这种效应，调制型参量不稳定性，而不是衰减型参量不稳定性被优先驱动不稳定.与参数不稳定性相关的磁场和密度涨落：非相干母波的影响本文用纵向离子和流体的Vlasov描述框架研究了平行Alfven波的参数不稳定性 ...更多信息 横向方向的描述。纵向离子动力学效应在参数不稳定性中起重要作用.准线性阿尔芬波和声波在太阳风和前震等离子体中应被视为简并（以几乎相等的速度传播），因为等离子体β比接近1，阿尔芬波磁场振幅与背景场相比很大。我们在上述模型中加入了离子动力学效应，并在此模型中讨论了磁扰动和密度扰动之间的关系.多航天器实验获得的湍流数据的分析方法的开发利用capon和线性预测方法，经常用于确定波数和阵列信号数据的功率谱分析，我们正在开发一种方法来评估在地球弓形激波的前震区典型观测到的大振幅MHD波的双谱（和双相干）。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