The process of accretion plays a fundamental role in astrophysics: it allows the formation of stars and planets and the release of gravitational energy to power some of the most energetic phenomena in the universe. The rate of accretion is controlled by the outward transport of angular momentum; without it the material would just orbit the central object and never accrete. Over a decade ago, it was realized that magnetic fields could lead to the development of turbulence in astrophysical discs, which in turn would enhance the transport of angular momentum and explain the observed accretion rates.
One of the questions that arose concerned the origin of the magnetic fields: were they external to the disc, or could they be generated by dynamo processes by the very turbulence within the disc? The answer to this question has been the subject of considerable controversy.
CMSO researchers have addressed this issue by simulating the type of turbulence that is believed to occur in accretion discs and to show that if the magnetic diffusivity is sufficiently small—a condition that although common in astrophysical situations is extremely difficult to reproduce numerically---dynamo action ensues and the magnetic fields can indeed be generated by the turbulence. Furthermore, the simulations also verified that the resulting self-maintaining motions could efficiently transport angular momentum outwards.
The more numerically demanding simulations were carried out on the Blue-Gene supercomputer at the IBM Watson Facility with up to 64,000 processors. They are, to date, the largest simulations of this type.
Supercomputer simulations of magneto-rotational turbulence in a cylindrical annulus. In this simulations the magnetic field was regenerated by the turbulent motions. The figure shows the azimuthal velocity fluctuations. (Couresy of F. Cattaneo, University of Chicago)