Kinetic plasma turbulence in space and astrophysical flows

太阳的外层，日冕，因为它是如此的热，流远离太阳在高速：所谓的太阳风。它是一种等离子体或由带电粒子组成的完全电离的气体，因此磁场和电场对其演化很重要。太阳风的速度比任何波（如声波）都快，所以它是一种超音速流。太阳风是一个天体物理流动的例子，实际上可以通过现场采样进行研究。这让我们对构成太阳风的波和粒子有了一个非常详细的了解。太阳风是湍流的，使一些能量流在风之间发生波动。能量从长波长到短波长级联，在某个阶段，波能耗散并加热粒子。这一过程影响了风的整体扩张。这是天体物理学中一个尚未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和天体物理的主要挑战之一。在湍流级联中，大驱动尺度的波动包含了大部分能量，但也存在向更小尺度的波动的能量转移。湍流波动的耗散发生在尺度上，例如拉莫尔半径，其中粒子动力学主导行为。粘性流体阻尼的标准思想是不够的，发生什么取决于无碰撞等离子体的基本性质，如非麦克斯韦粒子分布（各向异性，梁），微观不稳定性和波粒耦合。这项建议是为了制定一项研究方案，应用动力学等离子体物理学和自洽等离子体模拟工具，了解天体物理学湍流如何在最小尺度上消散，以及无碰撞等离子体的基本特性如何控制湍流的演变或对其作出反应。我们的工作将解决一些基本问题：为什么无碰撞的太阳风在膨胀时仍然很热？在无碰撞等离子体中湍流加热是如何运作的？离子和电子布居的行为是否不同？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实例，但我们的结果将适用于从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、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，并影响它们的传输和加速。将要使用的模拟代码是专门开发的，可以使用数百个计算节点的集群，并将遵循数十亿模拟粒子的工作原理。拟议的方案符合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有关的知识的进步。它与欧空局太阳轨道飞行器使命的关键科学密切相关："确定近太阳日光层中等离子体、场和粒子的性质、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''.