Equatorial symmetry breaking and the loss of dipolarity in rapidly rotating dynamos

Numerical studies of convection driven dynamos in rotating spherical shells exhibit a transition from steady dipolar to reversing multipolar dynamos as the forcing is increased. The dipolar-multipolar transition has so far been characterized using purely hydrodynamic parameters (Christensen and Aubert, Geophys. J. Int. 2006, 166, 97-114, Soderlund et al., Earth Planet. Sci. Lett. 2012, 333-334, 9-20, Oruba and Dormy, Geophys. Res. Lett. 2014, 41, 7115-7120). Motivated by these earlier descriptions, we investigate the hydrodynamic transitions occurring at the critical parameters. We show that the loss of dipolarity in dynamos is associated with a purely hydrodynamic transition, characterized by a breaking of the flow equatorial symmetry. Contrary to earlier expectations, we show by varying the Prandtl number that the transition is not necessarily associated with a degradation of the flow helicity.