Mechanisms of planetary and stellar dynamos

Emmanuel Dormy, dormy@phys.ens.fr, Departement de Physique, Ecole Normale Supérieure, France

Abstract
I will review recent progress on modeling planetary and stellar dynamos. Particular attention will be given to the dynamo mechanisms and the resulting properties of the field. Direct numerical simulations will be interpreted using the classical meanfield formalism. I will investigate the transition from steady dipolar to multipolar dynamo waves solutions varying different control parameters, and discuss the relevance to stellar magnetic fields. I will show that owing to the role of the strong zonal flow, this transition is hysteretic. Finally, I will investigate the extent to which the weak-field versus strong-field bistability predicted for the geodynamo can apply to recent observations (spectropolarimetric and spectroscopic) of two groups of very low mass fully-convective stars sharing similar stellar parameters but generating radically different types of magnetic fields.