MHD turbulence and the generation of large-scale fields in the Sun.

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The well-known eleven-year sunspot cycle is a manifestation of solar magnetic activity. Sunspots are cool dark patches on the solar surface which are associated with strong magnetic fields. They appear at mid-latitudes at the start of a cycle and migrate to the equator. Solar magnetic activity has been recorded since the early 17th Century following the invention of the telescope and the activity record can be extended back thousands of years using proxy data. Explaining the origin of the sun's magnetic field is the fundamental problem of solar magnetohydrodynamics. This field underlies all solar magnetic phenomena such as solar flares, coronal mass ejections and the solar wind, and is responsible for heating the solar corona to such high temperatures. These phenomena may all have important terrestrial impacts, causing severe magnetic storms and major disruption to satellites, as well as having a possible impact on the terrestrial climate. This proposal is designed to give an insight into the physical processes that lead to the generation of the magnetic field in the Sun. This is a complicated problem that involves the interaction between the turbulent fluid flows in an extremely hot plasma and the magnetic field that is generated by motions of charged particles within the fluid. The outstanding question is how a coherent systematic phenomenon such as the large-scale eleven-year solar magnetic cycle can arise from such a turbulent system under such extreme conditions as exist in the interior of the Sun. As it is impossible to simulate the whole Sun on a computer, the traditional approach is to model the action of the small-scales using parameters. These are chosen in order to fit the observations without taking the details of the physical processes involved into account. Here we propose to use numerical simulations of the interaction between turbulent flows and magnetic field to investigate how systematic large-scale behaviour can emerge. These numerical computations, which will be carried out in two and three-dimensions, will use state-of-the-art computational methods optimised for use on powerful modern computers, which will enable us to reach high degrees of turbulence and extreme values of the parameters. These simulations will be carried out using the parallel computational facility at the University of Leeds. The computations will be compared and contrasted with the results of analysis in parameter regimes where analytical progress is possible. This approach will enable the identification of the limitations of the range of applicability of analysis as applied to such turbulent phenomena and will also yield an understanding of the processes that can lead to ordered behaviour emerging from incoherent turbulent systems. In this way we propose to enhance our understanding of the mechanisms for generation of the solar magnetic cycle and of the important dynamics of the solar magnetic field.