Turbulent Heating of Space Plasmas

Plasma, the state of matter formed when a gas is heated to sufficiently high temperatures, is by far the dominant form of ordinary matter in the universe. How this plasma gets to be so hot, however, is currently one of the major mysteries in space and astrophysics. For example, the solar corona, the plasma atmosphere of the Sun, is hundreds of times hotter than the Sun's surface, but the reason for this has not yet been established. Understanding this heating is important for determining the origin of the solar wind, the continuous fast stream of plasma emitted from the Sun due to the high temperature in the corona. Furthermore, the solar wind itself is observed to undergo continuous heating as it expands to fill the solar system.

The solar wind and solar corona, as well as most other astrophysical plasmas, are turbulent, meaning that they display complex chaotic motions at a broad range of scales. These motions are a possible source of energy for the observed heating, but exactly how this turbulence leads to heating remains to be understood, due to the previous lack of high resolution in situ measurements. In the next few years, a series of new spacecraft (DSCOVR, Solar Probe Plus, Solar Orbiter) will fly through the solar wind, measuring its properties, such as density, velocity, temperature, and electromagnetic fields, in far greater resolution than has ever been achieved before. This offers a unique opportunity to study turbulent heating, since for the first time we will be able to probe the plasma at the scales at which the heating is thought to occur.

In my proposed research I will use this newly available data to answer some of the key questions about how turbulent energy leads to the heating of the solar wind and astrophysical plasmas in general. I will do this by looking for the characteristic signatures of the possible heating mechanisms in the measured solar wind particles, determining how the heating is distributed throughout the plasma in relation to the structures generated by the turbulence, and measuring how different types of particle in the solar wind are heated differently. By analysing data from Solar Probe Plus and Solar Orbiter, which will travel far closer to the Sun than ever before, I will investigate, in the later years of the project, how turbulent heating operates in different space environments and how the solar wind is heated close to its origin. This information about the dependence on plasma conditions will allow the results to be applied to other astrophysical plasmas in which heating is poorly understood, such as accretion disks and galaxy clusters. The research will be carried out at Imperial College London, in collaboration with colleagues in Europe and the USA.

Although the proposed research is fundamental in nature, the results are potentially important for several applications, for example, space weather and fusion power. Space weather is a term for the changes in the space environment around Earth, which can have a significant impact on society at large, for example, damaging commercial satellites, disrupting flights, and interfering with electrical systems on the ground. Understanding the variability of the solar wind due to turbulence will help contribute to more accurate space weather predictions. Fusion power uses plasmas to generate sustainable clean energy, but is limited by the power needed to heat the plasma, and the difficulty in confining it due to turbulence. By investigating turbulence in space and determining how plasmas are naturally heated in the universe, this knowledge can be transferred to the lab to help achieve the goal of commercial fusion power.