Thermodynamics of Astrophysical Plasmas: Macroscopic Effects of Collisionless Microphysics

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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.