Dynamo Mechanisms

Dynamo theory is reviewed with particular emphasis on recent developments. There now seems to be a strong case for dynamo eeects that are driven by the magnetic eld itself. This is linked to recent interpretations of the observed stellar cycle periods which suggest that the ratio of cycle frequency to rotational frequency increases, up to some point, with stellar chromospheric activity. This ratio may be interpreted as a measure of the alpha-eeect in dynamo theory, which would then increase with magnetic eld strength. The important question of what happens as stars become fully convective is also addressed. It is argued that the dynamo does not work at the bottom of the convection zone, but in the entire convection zone proper. However, because of turbulent pumping eeects the magnetic eld is pushed to the bottom of the convection zone or, in the case of a fully convective star, to its center.