FAST DYNAMO ACTION IN A STEADY CHAOTIC FLOW

THE observation of rapid variations in the Sun's magnetic field motivates the search for 'fast dynamos' 1-3 - flows of highly conducting fluid that amplify magnetic fields on the typically rapid timescales of convection, rather than the longer timescales of diffusion. Certain helical flows 4-6 have been proposed as possible fast dynamos, but numerical studies 7,8 of such flows have shown no conclusive evidence for this. Here I examine the evolution of a magnetic field in one such flow, which possesses a web of chaotic streamlines mingled with tubes of regular streamlines 9. In the case of no magnetic diffusion, I observe intense stretching and folding of the magnetic field in the chaotic regions of the flow. The folding brings together field that is largely aligned in the same direction, and the average field in a chaotic region therefore grows exponentially with time. This provides evidence for fast dynamo action, the main effect of weak diffusion being to average the field locally 10-13, and indicates that smooth, steady chaotic flows can be fast dynamos.