Morphology of field reversals in turbulent dynamos

¹  Laboratoire de Physique Statistique, Ecole Normale Supérieure, UPMC Université Paris 6, Université Paris Diderot, CNRS - 24 rue Lhomond, 75005 Paris, France, EU
²  MAG (IPGP/ENS), CNRS UMR 7154, LRA, Ecole Normale Supérieure - 24 Rue Lhomond, 75231 Paris Cedex 05, France, EU

Received: 17 March 2010
Accepted: 20 May 2010

Abstract

We show that the modes involved in the dynamics of reversals of the magnetic field generated by the flow of an electrically conducting fluid in a spherical domain, strongly depend on the magnetic Prandtl number P_m, i.e., the ratio between viscous and magnetic diffusivities. For P_m ~ 1, the axial dipolar field reverses in the presence of a strong equatorial dipolar component, whereas for P_m < 0.5, field reversals mostly involve axisymmetric modes, axial magnetic dipole and quadrupole coupled through a broken symmetry of the flow. Using symmetry arguments, we write a dynamical system for these three modes that qualitatively reproduces the main features of the reversals observed in direct simulations for small P_m.