Phase coherence in the solar wind MHD waves : Geotail data analysis and numerical simulation studies

通过分析GeoTail航天器获得的磁场数据，并通过对模拟MHD波的非线性耦合的各种系统的数值模拟，我们对与波的相位有关的几个问题进行了研究。1.通过比较原始的和它的相位混洗和相位相关的替代物的结构函数，我们提出了一种定量评估波之间的相位相干性的方法。对地尾磁场资料的分析表明，前震MHD波实际上是位相相关的，并且这种相关性随着湍流幅度的增大而增强。2.通过对三重态模型、多重耦合三重态模型、DNLS方程和Hall-MHD方程组等模拟磁流体非线性耦合的系统进行数值时间积分，讨论了相位相干性、非线性色散关系和不同波(或本征模)之间的能量传递之间的关系。通过实验粒子模拟，我们讨论了高能粒子在具有位相相干结构的MHD湍流中的扩散和加速。讨论了粒子输运统计与场涨落统计之间的关系。

By analyzing magnetic field data obtained by Geotail spacecraft, and also by performing numerical simulations of various systems modeling nonlinear coupling of MHD waves, we have made research on several issues related to the wave phases.1.We have developed a method to quantitatively evaluate the phase coherence among waves, by comparing structure functions of the original and its phase shuffled and phase correlated surrogates. Application of the method to Geotail magnetic field data revealed that the MHD waves in the foreshock is in fact phase correlated, and the correlation is stronger as the turbulence amplitude is increased. We have further identified the origin of the phase coherence by a method using the wavelet filtering technique.2.By numerical time integration of various systems which model the nonlinear coupling of MHD waves, such as the triplet model, multiply-coupled triplet model, the DNLS equation, and the Hall-MHD set of equations, we discussed the relationship between the phase coherence, nonlinear dispersion relations, and transfer of energy among the different waves (or eigenmodes). We showed that the phase coherence is self-generated as the energy is transferred between the waves, and the phenomena can be modeled by avalanches.3.By performing test particle simulations, we discussed diffusion and acceleration of energetic particles in the MHD turbulence with phase coherent structures. We argued the relationship between statistics of the particle transport and that of the field fluctuations.