Ion Heating by Turbulence in the Solar Wind

The solar wind is supersonic plasma ejected by the Sun. It fills the solar system and pushes against the Earth's magnetosphere causing geomagnetic storms. The solar wind is very variable, its speed changes from 275 km/s up to an even more staggering 800 km/s. Carried along in the plasma flow is the solar magnetic field. On large scales (many thousands of Earth radii) the plasma and magnetic field move together, acting as a magnetofluid. This magnetofluid is turbulent and the changes in velocity and magnetic field drive a continual cascade of energy from these large scales to smaller scales. The turbulence is a chaotic, stochastic process that alters the characteristics of the plasma fluctuations. It is these fluctuations through which cosmic rays and solar flare particles travel and that modify the direction and speed of other solar ejecta. The solar wind is an 'astrophysical laboratory'; the vast majority of the Universe is full of turbulent plasma and observing the solar wind with spacecraft can give us insight into the processes going on throughout the Universe.

The turbulent cascade transports energy from the large scales to smaller scales and eventually to such small scales that the plasma no longer behaves like a fluid. Instead we must consider the collective particle dynamics that characterise kinetic motions of plasma. At this point the magnetic field can interact with the particle distribution and heat it directly, either through resonant interactions or through stochastic 'jumps'.

There are now very long and very high quality data sets that can be used to study plasma turbulence and new missions launching soon that will provide unparalleled observations to test theories and develop new ideas about the dynamics of the solar wind. These data can be used to examine a large number of open questions relating to solar wind turbulence. In this project the student will use magnetic field and plasma data from the Wind, ACE and Cluster missions to investigate how the turbulent cascade dissipates into heat at small scales. Wavelet techniques will be used to measure the magnetic helicity, coherency and stochasticity of fluctuations that are at scales that interact with protons and alpha particles to determine through correlation with proton and alpha particle distribution data the most prevalent, and most effective heating mechanisms in the solar wind (these may not be the same). The same data can be used to investigate why alpha particles are hotter than protons in the solar wind? The final conclusion of the work will be a quantified statement about what is more important for energy dissipation in solar wind plasma turbulence: non-linear stochastic heating or resonant wave-particle interactions?