In the outer layers of the solar interior, vigorous convective motions interact with complex magnetic field structures. Regions of quiet Sun (which, by definition, contain no large-scale magnetic features such as sunspots) are characterized by a spatially intermittent distribution of small-scale magnetic fields. Explaining the origin and evolution of quiet Sun magnetic fields is one of the most important challenges in modern solar physics.
It is believed that a significant fraction of quiet Sun magnetism is generated in the surface layers by a convectively-driven dynamo. To simulate this dynamo process, it is necessary to solve the governing equations of Magneto HydroDynamics (MHD) for a compressible, electrically-conducting gas in three spatial dimensions and time. This is a computationally challenging problem that can only be tackled by using massively parallel simulations on the Dirac High Performance Computing facilities.
Using simulations of this type, we have confirmed that a dynamo mechanism could indeed account for the observations. In the absence of magnetic fields, the simulated convective flows are solar-like in many respects, with a “granular” pattern of broad warm upflows, surrounded by a network of cooler, narrow downflows. Furthermore, as suggested by the observations, these granules are organized on a larger (“mesogranular”) scale. When a seed magnetic field is introduced into this system, the convective motions drive a dynamo with a complex, intermittent field distribution that is comparable to that observed in the quiet Sun, with the most persistent magnetic structures accumulating preferentially at the mesogranular boundaries.