Kinetic Turbulence and Particle Transport in Astrophysical Plasmas

在像太阳风或星际介质这样的天体物理等离子体中，已经观察到湍流。这意味着等离子体中的能量从大尺度级联到小尺度。到目前为止，已经考虑了频谱的低频部分（即质子回旋频率以下）。在这个频率范围内，等离子体湍流可以用磁流体力学理论和Kolmogorov或Goldreich和Sridhar的湍流模型来描述。这种方法的先决条件是对等离子体进行流体描述。最近由Cluster和MMS进行的太阳风观测以及THOR任务的未来观测将使我们对质子回旋频率以外的湍流有更详细的了解。在这个范围内，等离子体中的动力学效应变得相关，流体描述不再是一种选择。从观察和理论考虑中还不可能找到波的模式和结构在动力学状态下的详细描述。当前的项目旨在更好地理解这一机制：使用数值模拟，将研究湍流如何在更大的尺度上演变。这里的核心问题是，没有一种单一的模拟技术能够捕获所有波数范围。有些技术不能描述每一种状态下的物理特性，而另一些技术则需要大量的数值计算。该项目的目的是将不同的技术结合起来，以一种从模拟中得出多尺度模型的方式，该模型涵盖了从MHD湍流到最小动力学尺度的范围。将使用的技术有MHD模拟、混合和细胞内粒子编码。本项目需要解决以下问题：-质子回旋频率之外的湍流本质是什么？动力学湍流对粒子输运（尤其是电子输运）有什么影响？最后一个问题在这个项目中非常重要。粒子输运本身不仅具有科学意义，而且在空间天气中的应用也具有技术意义。在这个项目中，将特别关注电子的输运。由于电子在动力学体系中主要与湍流共振，因此质子输运应该存在可观察到的差异。电子输运是否可以用来探测湍流，应该进行研究。

In astrophysical plasmas like the solar wind or the interstellar medium turbulence has been observed. This means that energy in the plasma is cascading from large scales to smaller scales. Up to now the low frequency part of the spectrum (i.e., below the proton gyrofrequency) has been considered. In this frequency range plasma turbulence can be described using magnetohydrodynamics theory and the turbulence models by Kolmogorov or Goldreich and Sridhar. A prerequisite for this approach is a fluid description for the plasma.Recent observations in the solar wind as performed by Cluster and MMS and future observations by the THOR mission will grant a more detailed insight into turbulence beyond the proton gyrofrequency. In that range kinetic effect in the plasma become relevant and fluid descriptions are no longer an option. It has not been possible from observations and theoretical considerations to find a detailed description of wave modes and structures presents in the kinetic regime.The current project aims at a better understanding of this regime: Using numerical simulations, it shall be investigated how turbulence evolves over even larger scales. The core problem here is that there is not a single simulation technique capable of capturing all wave number ranges. Some techniques do not describe the physics in every regime, while other would incur a too large numerical effort.The ansatz of this project is to couple different techniques in a way that a multiscale model can be derived from the simulations, which spans a range from MHD turbulence to smallest kinetic scales. The techniques to be used are MHD simulations, hybrid and particle-in-cell codes.The following problems shall be approached in this project:- What is the nature of turbulence beyond the proton gyrofrequency?- Which influence does kinetic turbulence have on the particle transport (especially of electrons)?The last question is very important in this project. Particle transport itself is not only scientifically interesting, but the application to space weather have also technical applications. In this project there will be a special focus on the transport of electrons. Since electrons resonate mostly with turbulence in the kinetic regime, there should be an observable difference to proton transport. It shall be investigated if electron transport can be used to probe turbulence.