Kinetic plasma turbulence in space and astrophysical flows

太阳的外层，日冕，因为它太热了，所以以高速流出太阳：即所谓的太阳风。它是一种等离子体或由带电粒子构成的完全电离的气体，因此磁场和电场对它的演变很重要。太阳风的传播速度比任何波（比如声波）都快，所以它是一种超音速流。太阳风是天体物理流的一个例子，实际上可以通过现场采样来研究。这让我们对构成太阳风的波和粒子有了非常详细的了解。太阳风是紊流的，所以一些能量的流动发生在风的波动之间。能量从长波级联到短波，在某个阶段，波能消散并加热粒子。这个过程会影响风的整体膨胀。这是天体物理学中一个尚未被完全理解的关键过程。磁化无碰撞等离子体中的湍流是空间物理学和天体物理学的主要挑战之一。在湍流级联中，大驱动尺度上的波动包含了大部分能量，但存在能量向更小尺度上的波动的转移。湍流波动的耗散发生在尺度上，比如拉莫尔半径，在那里粒子动力学主导着行为。粘性流体阻尼的标准概念是不充分的，发生的情况取决于无碰撞等离子体的基本特性，如非麦克斯韦粒子分布（各向异性、光束）、微不稳定性和波粒耦合。这是一个应用动力学等离子体物理和自洽等离子体模拟工具来理解天体物理湍流如何在最小尺度上消散的研究项目，以及无碰撞等离子体的基本特性如何控制或反应湍流的演变。我们的工作将解决一些基本问题：为什么没有碰撞的太阳风在膨胀时仍然保持高温？湍流加热在无碰撞等离子体中是如何运作的？离子和电子居群的行为不同吗？我们的工作将主要是在太阳风的背景下，作为天体物理流的最佳观测例子，但我们的结果将适用于从太阳日冕到星系间介质的环境。我们将使用大规模并行计算机模拟来研究天体物理等离子体中的波动、不稳定性和湍流。我们还将研究这种湍流如何与高能粒子耦合并影响它们的输运和加速度。将使用的模拟代码是专门为使用数百个计算节点的集群而开发的，并将遵循数十亿个模拟粒子的工作原理。拟议的项目符合STFC的使命，即支持空间科学的基础研究和培训，并支持与先进等离子体模拟技术相关的知识的进步。它与欧空局太阳轨道飞行器任务的关键科学密切相关：“确定近太阳日球层中等离子体、场和粒子的特性、动力学和相互作用”。

The outer layer of the sun, the corona, because it is so hot, streams away from the sun at a high speed: the so-called solar wind. It is a plasma or a completely ionized gas made from charged particles, so that magnetic and electric fields are important for its evolution. The solar wind travels faster than any wave (like a sound wave), so it is a supersonic flow. The solar wind is an example of an astrophysical flow that can actually be studied by in-situ sampling. This has given us an incredibly detailed view of the waves and particles that make up the solar wind. The solar wind is turbulent, so that some of the energy flow occurs between the fluctuations in the wind. Energy cascades from long wavelengths to short, and at some stage the wave energy dissipates and heats the particles. This process affects the overall expansion of the wind. It is a key process in astrophysics which is not fully understood. Turbulence in magnetized collisionless plasmas is one of the major challenges of space physics and astrophysics. In a turbulent cascade fluctuations at a large driving scale contain most of the energy, but there is an energy transfer to fluctuations with ever smaller scales. The dissipation of turbulent fluctuations happens at scales, such as the Larmor radius, where particle kinetics dominates behaviour. Standard ideas of viscous fluid damping are inadequate, and what happens depends on fundamental properties of collisionless plasmas such as non-Maxwellian particle distributions (anisotropies, beams), micro-instabilities and wave-particle coupling. This proposal is for a programme of research which applies the tools of kinetic plasma physics and self-consistent plasma simulations to understanding how astrophysical turbulence dissipates at the smallest scales, and how fundamental properties of collisionless plasmas control or react to the evolution of turbulence. Our work will address some fundamental questions: Why does the collisionless solar wind remain hot as it expands? How does turbulent heating operate in a collisionless plasma? Do ions and electron populations behave differently? Our work will be mostly in the context of the solar wind, as the best observed example of an astrophysical flow, but our results will be applicable over environments from the solar corona to the inter-galactic medium. We will use massively parallel computer simulations to study waves, instabilities and turbulence in astrophysical plasmas. We will also study how this turbulence couplles to energetic particles and affects their transport and acceleration. The simulation codes which will be used have been specially developed to use clusters of hundreds of computational nodes, and will follow the workings of billions of simulational particles. The proposed programme fits into the STFC's mission to support basic research and training in Space Science, and to support advancement of knowledge related to advanced plasma simulation technology. It is closely aligned with key science of the ESA Solar Orbiter mission: ``Determine the properties, dynamics and interactions of plasma, fields and particles in the near-Sun heliosphere''.