Ion-scale current sheet instabilities in kinetic plasma turbulence

无碰撞等离子体（如太阳风和日冕）的物理尚未解决的一个关键问题是宏观能量在没有碰撞粘度和电阻率的情况下的不可逆耗散。它是通过能量从宏观尺度到动能尺度的湍流转移而实现的，在动能尺度上能量最终消散。多航天器（Cluster， MMS）对无碰撞等离子体湍流的原位观测和数值模拟表明，在湍流中自一致形成的动力学尺度电流片内部和周围存在间歇性耗散。这些电流片中的等离子体不稳定性可以提供无碰撞耗散，并决定动力学等离子体湍流的相关性质，例如磁场谱中的离子尺度断裂和波矢量各向异性的非单调尺度依赖性。然而，动力学尺度电流片不稳定性在决定耗散和湍流特性中的作用尚不清楚。本提案的目标是研究离子尺度等离子体不稳定性的作用，离子尺度等离子体不稳定性可能在离子动力学等离子体湍流中形成的电流片中增长，在确定湍流的离子尺度特性方面，特别是离子尺度断裂的位置和波矢量各向异性的非单调尺度依赖性。首先，我们将使用不伦瑞克工业大学的A.I.K.E.F.代码对离子动力学等离子体湍流进行三维混合模拟（离子作为粒子，电子作为无惯性流体），以表征(1)通过变量的功率谱、断裂位置和波矢量各向异性来完全开发的三维湍流；(2)由其峰值电流密度、厚度、宽度、电子和离子电流的高度和分数以及(3)这些电流片中的自由能量来源的梯度尺度长度密度和离子和电子体速度和离子温度各向异性。接下来，将通过适当的一维电流片平衡初始化的混合代码模拟，研究具有自由能量源和典型湍流电流片参数的单个层流片中等离子体不稳定性的非线性演化。将通过比较湍流的性质与单个电流片的非线性演化状态的相应性质来探讨单个电流片中湍流和等离子体不稳定性之间的联系（如果有的话）。我们对0.1-10范围内的8个离子等离子体β值的研究将与Z. Vörös和Y. Narita在格拉茨的小组合作进行的电流片和湍流的MMS/CLUSTER观测进行比较。我们的研究是及时的，因为从2018年开始，MMS太空任务和Greifswald的VINETA-II实验室实验和普林斯顿的FLARE实验室实验将进行磁重联、电流片和湍流的专门动力学尺度测量。

A key unsolved problem of the physics of collisionless plasmas, e.g., solar wind and solar corona, is the irreversible dissipation of macroscopic energy without collisional viscosity and electrical resistivity. It is enabled by turbulent transfer of energy from macroscopic to kinetic scales where the energy is finally dissipated. In-situ multi-spacecraft (Cluster, MMS) space observations and numerical simulations of collisionless plasma turbulence show evidences of intermittent dissipation localized in and around kinetic scale current sheets self-consistently formed in the turbulence. Plasma instabilities in these current sheets can provide the collisionless dissipation and determine the associated properties of kinetic plasma turbulence, e.g., the ion-scale break in magnetic field spectra and non-monotonous scale dependence of wave-vector anisotropy. Role of kinetic scale current sheet instabilities in determining the dissipation and the turbulence properties is, however, not clear yet. Goal of this proposal is to investigate the role of ion-scale plasma instabilities, which are likely to grow in current sheets formed in ion kinetic plasma turbulence, in determining the ion-scale properties of the turbulence, in particular, location of the ion-scale break and non-monotonous scale dependence of wave-vector anisotropy. First, we will carry out 3-D hybrid simulations (ions as particles and electrons as inertia-less fluid) of ion-kinetic plasma turbulence using the A.I.K.E.F. code of the TU Braunschweig to characterize (1) the fully developed 3-D turbulence by power spectra of variables, location of the break and wave-vector anisotropy, (2) the current sheets formed thereof by their peak current density, thicknesses, widths, heights and fractions of electron and ion currents and (3) free energy sources in these current sheets by gradient scale lengths of density and ions and electrons bulk velocities and ion temperature anisotropy. Next, nonlinear evolution of plasma instabilities in individual laminar current sheets with free energy sources and parameters typical of the current sheets of the turbulence will be studied by the hybrid code simulations initialized with appropriate one dimensional current sheet equilibria. Links, if any, between the turbulence and plasma instabilities in individual current sheets will be explored by comparing the properties of the turbulence with the corresponding properties of the nonlinear state of the evolution of individual current sheets. Our studies for eight values of ion plasma beta in the range 0.1-10 will be compared with MMS/CLUSTER observations of current sheets and turbulence in collaboration with the group of Z. Vörös and Y. Narita at Graz. Our studies are just timely since dedicated kinetic scale measurements of magnetic reconnection, current sheets and turbulence will be carried out by the MMS space mission and by laboratory experiments VINETA-II at Greifswald and FLARE at Princeton starting 2018.