SO IAA course: Dynamo Theory in Astrophysics

Magnetic fields are ubiquitous in the Universe, including the geomagnetic field, the systematic magnetic field responsible for the eleven year solar activity cycle, and the magnetic fields of planets, stars, galaxies and accretion disks. It is now generally accepted that these fields are created by the action of hydromagnetic dynamos; i.e. by the motions of conductive fluids or plasmas that sustain the field against the action of Ohmic dissipation. For all these objects there is no ab initio predictive theory for the origin of their magnetic fields. Since the 1950s, mean-field electrodynamics explains sustained large-scale astrophysical magnetic fields through systematic stretching and twisting of magnetic field lines by turbulent flows. This process is efficient when velocity and magnetic field fluctuations correlate well, so that net electromotive force is non-zero. This is possible for flows which possess a high degree of asymmetry and linkedness. Astrophysical rotating turbulence often complies with this assumption, however, high flow complexity of the flow means that analytical dynamo solutions are very challenging to derive. 3D numerical simulations became and remain the main tool of dynamo modelling.