Bypassing Cowling's Theorem in Axisymmetric Fluid Dynamos

Christophe Gissinger¹, Emmanuel Dormy², and Stephan Fauve¹

¹ Laboratoire de Physique Statistique de l'Ecole Normale Supérieure, CNRS UMR 8550, 24 Rue Lhomond, 75231 Paris Cedex 05, France
² MAG (IPGP/ENS), CNRS UMR 7154, LRA, Ecole Normale Supérieure, 24 Rue Lhomond, 75231 Paris Cedex 05, France

Received 9 May 2008; published 1 October 2008

PRL 101, 144502 (2008)

Abstract. We present a numerical study of the magnetic field generated by an axisymmetrically forced flow in a spherical domain. We show that, even in the absence of nonaxisymmetric velocity fluctuations, a mean magnetic field with a dominant axisymmetric dipolar component can be generated via a secondary bifurcation from an equatorial dipole. We understand the dynamical behaviors that result from the interaction of equatorial and axial dipolar modes using simple model equations for their amplitudes derived from symmetry arguments.