Thermodynamics of Astrophysical Plasmas: Macroscopic Effects of Collisionless Microphysics

等离子体是继固态、液态和气态之后的第四种物质状态。它是一种由未结合的带电粒子组成的介质。由于它们的电荷，这些粒子对电场和磁场的反应方式与普通的电中性粒子气体非常不同。宇宙中绝大多数普通物质都处于等离子体状态。因此，更好地理解等离子体物理对我们理解宇宙是至关重要的。太阳本身就是一个等离子球体，它不断发出等离子体流，即所谓的太阳风，填充在我们太阳系行星之间的空间。它的典型流动速度为每秒几百公里，其典型温度约为一百万度。它让我们有机会利用宇宙飞船上的仪器探索天体物理等离子体。这些测量表明，太阳-风粒子并不处于局部热力学平衡状态，这意味着粒子碰撞是无效的。这种“无碰撞”的等离子体可以承载许多物理过程，这些物理过程是我们日常与空气等碰撞气体的经验所未知的。此外，我们看到电场和磁场在许多空间尺度和时间尺度上波动，表明太阳风处于湍流状态，携带不同类型的等离子波。太阳风是如何产生的，如何加速，以及如何将其加热到远远高于太阳表面温度的温度，目前尚不清楚。这些重要的悬而未决的问题的答案必须在于起作用的基本等离子体过程。通过发展新的理论想法和利用数值模拟，该奖学金旨在更好地了解等离子体粒子的行为，并将这些想法应用于整个太阳风。该项目专注于三个具体的主题：第一个主题解决了无碰撞等离子体中波、湍流和不稳定性的演化，这是一个对所有类型的天体物理等离子体具有广泛影响的基本问题。第二个主题是研究电子对太阳风整体演化的影响，电子是太阳风中最丰富的粒子物种。第三个主题研究湍流对粒子的影响，目的是解释太阳风的加热和加速。2018年，欧空局和NASA计划发射太阳轨道器和太阳探测器Plus航天器，这两个航天器都将非常近地围绕太阳运行，达到以前没有其他航天器进行过测量的距离。这些航天器将以前所未有的分辨率测量太阳风中的粒子和电磁场。这个项目将新的理论结果与这些和其他观测结果联系起来，以促进我们对太阳风和天体物理等离子体的总体理解。这种方法对于将观察结果放在背景中并指导新的测量是必要的。只有通过理论和观察的直接比较，我们才能了解这些媒体的独特行为。太阳风是实现这一目标的唯一等离子体实验室，因为它可以与航天器相连。对这些过程的理解对空间天气的预测具有重要意义，空间天气是太阳粒子和太阳风对地球影响的集合。空间天气效应可能导致停电、GPS导航偏差、对人类航天的危害、对卫星的破坏、无线电通信失真等人类社会的重大问题。除了理论和观测之间的协同联系外，该项目位于空间物理学和天体物理学之间的交汇点，通过弥合这两个领域之间的差距并将太阳环境中的知识转化为一般天体物理学。

Plasma is the fourth state of matter after solid, liquid, and gaseous. It is a medium consisting of unbound electrically charged particles. Due to their charge, these particles react to electric and magnetic fields in very different ways than a regular gas of electrically neutral particles. The vast majority of the ordinary matter in the universe is in the plasma state. Therefore, it is critical for our understanding of the universe to understand the physics of plasmas better.The Sun itself is a plasma ball that continuously emits a plasma flow, the so-called "solar wind", which fills the space between the planets of our solar system. Its typical flow speed is a few hundreds of kilometres per second, and its typical temperature is about a million degrees. It gives us the opportunity to explore an astrophysical plasma with instruments onboard spacecraft. These measurements show us that the solar-wind particles are not in a state of local thermodynamic equilibrium, which means that particle collisions are ineffective. Such a "collisionless" plasma can host many physical processes that are unknown from our everyday experience with collisional gases like the air. In addition, we see that the electric and magnetic fields fluctuate on many spatial scales and timescales, suggesting that the solar wind is in a turbulent state and carries different types of plasma waves.It is still unknown how the solar wind is generated, how it is accelerated, and how it is heated to temperatures that are much greater than the temperature of the surface of the Sun. The answers to these important open questions must lie in the fundamental plasma processes at work. By developing novel theoretical ideas and exploiting numerical simulations, this fellowship aims to a better understanding of the behaviour of the plasma particles and to apply these ideas to the solar wind as a whole. The project focuses on three specific topics: The first topic addresses the evolution of waves, turbulence, and instabilities in a collisionless plasma, which is a fundamental problem with broad implications for all kinds of astrophysical plasmas. The second topic investigates the effects of electrons, which are the most abundant solar-wind particle species, on the overall evolution of the solar wind. The third topic investigates the effects of turbulence on the particles with the goal to explain the heating and the acceleration of the solar wind. In 2018, ESA and NASA plan to launch the spacecraft Solar Orbiter and Solar Probe Plus, which will each orbit very closely around the Sun, reaching distances at which no other spacecraft have taken measurements before. These spacecraft will measure both the particles and the electromagnetic fields in the solar wind with an unprecedented resolution. This project connects the novel theoretical results with these and other observations to advance our understanding of the solar wind and of astrophysical plasmas in general. This approach is necessary to put the observations into context and to direct new measurements. Only through the direct comparison of theory with observations, can we learn about the peculiar behaviour of these media. The solar wind is a unique plasma laboratory for this goal due to its accessibility with spacecraft.The understanding of these processes has important implications for the prediction of space weather, which is the collection of effects that solar particles and the solar wind have on the Earth. Space-weather effects can lead to power outages, deviations in GPS navigation, hazards for human spaceflight, damage to satellites, distortion of radio communication, and other major problems for the human society. In addition to the synergistic connection between theory and observations, this project is located at the intersection between space physics and astrophysics by bridging the gap between these two fields and transferring knowledge from the solar environment into general astrophysics.