10 - Dynamo processes: the interaction of turbulence and magnetic fields

Summary

This chapter reviews recent research on the interaction of magnetic fields with MHD turbulence, with particular application to the question of the influence of Lorentz forces on the efficiency of large-scale field generation.

Scales for solar magnetic fields

The solar magnetic field outside the radiative core exists on a great range of length and time scales; these embrace all sizes from that of the disc itself to that of the diffusion length scales of a few km, well below present observational resolution. While it is the largest scales that force themselves on our attention, due to the visibility of sunspots and associated coronal structures, and the coherence of the solar cycle, it is not clear whether these large-scale fields control, or are controlled by, the small-scale fields that have much greater total energy. While the cycle is clearly global in nature, the “magnetic carpet” of small-scale field structures that appear in quiet regions would seem to be a local manifestation of dynamo action due to turbulent stretching.

Linear dynamo theory, in particular the “mean-field” or “α-effect” models, has proved amazingly successful in predicting aspects of the solar cycle such as the butterfly diagram. In fact some of this ‘success’ has nothing to do with the physics employed, but derives from the symmetry of the underlying geometry.