Solar and Stellar Dynamos.

The Sun, solar and later spectral type stars show cyclical magnetic activity that manifests itself through the presence of spots on their surfaces. The characteristics of this activity show broad variations both in spot periods, which range from a few to more than 20 years, and spot surface coverages, which range from 0.002 for the Sun to more than 0.5 for the most active stars. In the case of the Sun, owing to its proximity, the spots and their cyclic behaviour are directly observable in detail since Galileo’s times. In the case of the point-like stars, indirect methods, as the Doppler imaging or brightness rotational modulation, allow the determination of the spot coverage and its behaviour with time. It is commonly believed that the occurrence of such phenomena in the Sun and similar stars with surface convection zones is governed by a dynamo mechanism which acts in the deepest convective layers at the interface with the underlying radiative envelope. This mechanism is based on the mutual interaction between rotation, convection and magnetic fields, which produces non uniform rotation (ω-effect), able to intensify magnetic field, and cyclonic turbulence (α-effect), able to regenerate magnetic field against diffusion. These are the basic working principles of the α-ω dynamo, that is essentially governed by the motion and induction equations. The competitive role of the convective and diffusive terms in the induction equation determines the time evolution of magnetic field. This is a formidable, highly non-linear problem of magneto-hydrodynamics of stellar plasmas, and various approaches have been experienced for its solution, from the linearized dynamos, to the asymptotic dynamos, useful for stars in that simple relationships between the relevant parameters are derived, to the non linear and eventually fully hydromagnetic dynamos in which the whole set of equations are solved at once, by means of very sophisticated two- or three-dimensional numerical codes. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]