Simulation of kinetic effects in turbulence and collisionless shocks

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.