Particle transport in space plasma : beyond the quasi-linear theories

带电粒子在无碰撞等离子体中的扩散输运是空间等离子体研究的中心问题之一。大多数传统的论点都是基于经典的扩散模型，即具有给定扩散系数的二阶线性扩散方程。然而，航天器观测和数值模拟研究的最新进展表明，有必要对该模型进行几项必要的修改。特别是，Levy统计量的使用似乎非常有用，其中输运性质与相关的空间和时间尺度有明确的依赖关系。这是一个直接的结果：(1)空间中观测到的MHD湍流具有很大的振幅；(2)MHD波具有有限的相位相干性。在本研究项目中，我们通过数值模拟(主要是测试粒子模拟)评估了高能粒子在MHD湍流中的各种输运系数，并将扩散特性与湍流的统计特性进行了关联。MHD波之间的有限位相关联的存在使得输运性质大大偏离准线性预测。

Diffusive transport of charged particles in a collisionless plasma is one of the central issues in space plasma research. Most of the conventional arguments are based on a classical diffusion model, a second order linear diffusion equation with a prescribed diffusion coefficient. However, recent progress in spacecraft observations as well as numerical simulation studies imply that several essential modifications are necessary to the model. In particular, the use of the Levy statistics appears to be quite useful, in which the transport properties have explicit dependence to relevant spatial and temporal scales. This is a direct consequence of that (1)the observed MHD turbulence in space has large amplitude and (2)the MHD waves have finite phase coherence. In this research project we evaluated various transport coefficients of energetic particles in MHD turbulence by performing numerical simulations (mainly test particle simulations), and related the diffusion properties to statistics of the turbulence. The presence of finite phase correlation among the MHD waves make the transport properties drastically deviate from quasi-linear predictions.