WHAT IS A STELLAR DYNAMO

Numerical simulations of turbulent stellar dynamos are now feasible. The characteristic time-scale for kinematic behaviour is related to the turnover time of the turbulent eddies. Results from idealized two-dimensional models show that the Lorentz force alters the velocity field, allowing transient magnetic activity to persist for intervals much longer than the expected turbulent decay time. For three-dimensional flows we define a characteristic time T(e) for turbulent diffusion to act, based on the rate at which magnetic energy is dissipated, and assert that there is a dynamo only if the field survives for times much longer than T(e). This criterion is then applied to cyclic magnetic activity in late-type stars.