Simulation of kinetic effects in turbulence and collisionless shocks

这个项目探索了我们地球周围空间环境的一些关键部分：太阳风和地球的弓形激波。了解地球的弓形震波有助于我们理解太阳和地球之间的关系。如果我们想要了解其他天体物理环境，其他恒星周围或超新星爆炸，它也很有用，因为在那里也发现了冲击波。激波很重要，因为在激波中，流动能量既可以通过提高气体温度转化为热能，也可以转化为高能粒子（如宇宙射线）的能量。太阳的外层，也就是日冕，因为它太热了，所以以高速流出太阳。这就是太阳风。它是一种等离子体或由带电粒子构成的完全电离的气体，因此磁场和电场在它的演变中很重要。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。一种冲击波，就像超音速飞机产生的冲击波一样，是在地球前方的太阳风中产生的。（实际上是地球磁场造成的障碍——磁层。）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。事实证明，气流是湍流的，所以一些能量流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。天体物理冲击复杂且难以理解，因为与地球上的气体不同，等离子体中的粒子几乎不会碰撞。然而，宇宙飞船的观测表明，即使整个激波的大小超过数千公里，弓形激波中也可能存在小至100公里的结构。该项目将使用计算机模拟等离子体，如太阳风，来研究粒子和场在碰撞激波中的行为。这种模拟是一种被称为动力学的模拟，这样计算机就有了等离子体的数学模型，等离子体由数百万个具有位置和速度、磁场和电场的单个粒子组成。该项目使用最新的计算机模拟技术，使用大型PC集群。无碰撞冲击具有湍流过渡，即使它们可能在不同的尺度长度上。我们将使用模拟来详细研究不同类型冲击下的波和湍流的类型。有时这些波是小波纹，我们的模拟将能够阐明它们是否与观测结果一致。对于其他类型的激波，湍流占据了很大的空间区域，并且有大量的高能粒子影响波和湍流。我们将进行非常大的计算机模拟来研究可能观察到的波的类型以及它们如何产生高能粒子。我们还将研究太阳风中的湍流，以及它是如何通过包括电子和质子在内的大型模拟来阻尼和加热粒子的。有一些关于阻尼的理论，我们将用仿真来与这些理论进行比较。理解太阳风流动的问题之一是解释粒子分布函数（如光束和其他扭曲）特征的发展。这些特征只能用动力学物理学（即，包括粒子的动力学）来解释，所以粒子模拟对于理解这些过程至关重要。

This project explores some of the key parts of the space environment around our Earth: the solar wind and the Earth's bow shock. Finding out about the Earth's bow shock helps us to understand the relationships between the Sun and the Earth. It is also useful if we want to understand other astrophysical environments, around other stars or in super novae explosions, because shock waves are also found there. Shock waves are important because they are places where flow energy can be turned into both thermal energy by increasing the temperature of the gas, and also into energy in energetic particles such as cosmic rays. The outer layer of the sun, the corona, because it is so hot, streams away from the sun at a high speed. This is called the solar wind. It is a plasma or a completely ionized gas made from charged particles, so that magnetic and electric fields are important in its evolution. The solar wind travels faster than any wave (like a sound wave), so it is a supersonic flow. A shock wave, like that made by supersonic aircraft, is made in the solar wind just in front of the Earth. (In fact the obstacle created by the Earth's magnetic field - the magnetosphere.) 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. It turns out that the flow 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. Astrophysical shocks are complex and difficult to understand because, unlike gases on the Earth, the particles in the plasma hardly ever collide. Nevertheless, observations by spacecraft show that there can be structures in the bow shock as small as 100km, even though the whole shock has a size across of many thousands of km. The project will use computer simulations of plasmas, such as the solar wind, to study how the particles and fields behave at collision shocks. The simulations are a type known as kinetic, so that the computer has a mathematical model of the plasma as consisting of many millions of individual particles with position and velocity, and also magnetic and electric fields. The project uses the latest computer simulation techniques, using large clusters of PC's. Collisionless shocks have turbulent transitions, even though they may be on different scale lengths. We will use simulations to look in detail at the types of waves and turbulence at different types of shocks. Sometimes the waves are small ripples, and our simulations will be able to shed light on whether they are consistent with observations. For other types of the shock, the turbulence fills a large region of space, and there are copious amounts of energetic particles which affect the waves and turbulence. We will carry out very large computer simulations to study the types of waves that might be observed and how they can produce the energetic particles. We will also study the turbulence in the solar wind, and how it damps and heats particles, by doing large simulations which include both electrons and protons. There are some theories for the damping, and we will use the simulations to make comparisons with these theories. One of the problems in understanding the solar wind flow is explaining the development of features in the particle distribution functions (such as beams and other distortions). These features can only be explainable using kinetic physics (i.e., including the dynamics of particles), so particle simulations are vital to understanding these processes.