Structured Turbulence Collisionless Dynamics and Magnetic Field Generation in Astrophysical Plasmas

J. C. McWilliams和S. Cowley各向异性天体物理等离子体中的结构湍流、无碰撞动力学和磁场产生本研究项目是对与天体物理动力学特别相关的等离子体湍流的一些基本问题的研究。这项研究将使用星系发电机作为主要的天体物理学例子，但其结果也将与原星系和吸积盘发电机相关。特别是，在大普朗特数等离子体中非常小尺度的磁放大的早期阶段和大尺度的反级联磁能积累的后期阶段将进行以下目标的研究：(1)在流体湍流的粘性耗散范围内的尺度上，通过双极扩散，各向异性粘度或自一致非线性动力学确定磁能尺度增长的机制。(2)探索等离子体在相对于碰撞平均自由程较短但相对于碰撞频率演化较慢的尺度上的物理特性，并推导出描述这种物理特性的类流体方程。(3)研究小尺度强迫下旋转圆盘内的逆叶栅，各向异性环境为螺旋的产生提供了物理机制。这些问题在过去还没有得到彻底的研究，但它们解决了必须在实际物理发电机中发生的关键过程。解析理论、湍流闭合模型计算和高雷诺数流体方程的直接数值解将用于解决这些问题。动力学方程的渐近分析将用于描述低碰撞行为。将通过闭合计算和高普朗特数下的直接数值解来探索耗散范围磁流体动力学（MHD），包括非MHD效应，如部分电离和低碰撞。还将计算各向异性环境下湍流MHD的直接数值解，在可能的情况下使用渐近约简来降低计算复杂性并增加可解析尺度的范围，同时保留自洽发电机作用所需的成分。这项理论研究将解决小型和大型发电机经常被忽视的问题，并提供对高普朗特数和各向异性等离子体湍流的基本理解。＊＊＊

ABSTRACT - J. C. McWilliams and S. Cowley Structured Turbulence, Collisionless Dynamics, and Magnetic Field Generation in Anisotropic Astrophysical Plasmas This research project is an investigation of some basic problems of plasma turbulence which are particularly relevant to astrophysical dynamos. The study will use the galactic dynamo as the primary astrophysical example, but the results will be relevant also to protogalactic and accretion-disk dynamos. In particular, the early stages of magnetic amplification at very small scales in large-Prandtl-number plasmas and the late stages of magnetic energy buildup at large scale by inverse cascade will be studied with the following objectives: (1) To determine mechanisms for magnetic energy scale growth at scales within the viscous dissipation range of fluid turbulence, by ambipolar diffusion, anisotropic viscosity, or self-consistent nonlinear dynamics. (2) To explore the physics of plasmas on scales short compared to the collisional mean free path, but when the evolution is slow compared to the collision frequency, and to derive fluid-like equations that describe this physics. (3) To study the inverse cascade in a rotating disk with small-scale forcing, such that the anisotropic environment provides a physical mechanism for helicity generation. These problems have not been thoroughly studied in the past, yet they address key processes which must occur in actual physical dynamos. Analytical theory, turbulent closure model calculations, and direct numerical solutions of high Reynolds number fluid equations will be used to attack these problems. Asymptotic analysis of kinetic equations will be used to describe low collisionality behavior. Dissipation-range magnetohydrodynamics (MHD) will be explored by closure calculations and direct numerical solutions at high Prandtl number, including non-MHD effects such as partial ionization and low collisionality. Direct numerical solutions of turbulent MHD in an anisotropic environment w ill be also be calculated, making use where possible of asymptotic reductions to decrease computational complexity and increase the range of resolvable scales while retaining the ingredients necessary for self-consistent dynamo action. This theoretical study will address often overlooked questions of small- and large-scale dynamos and provide basic understanding of high Prandtl-number and anisotropic plasma turbulence. ***