 
Summary: Axial vs. equatorial dipolar dynamo models with
implications for planetary magnetic ¢elds
Julien Aubert Ć
, Johannes Wicht
MaxPlanckInstitute for Aeronomy, MaxPlanckStrasse 2, 37191 KatlenburgLindau, Germany
Received 3 July 2003; received in revised form 27 January 2004; accepted 29 January 2004
Abstract
We present several numerical simulations of a selfconsistent dynamo model in a rotating spherical shell. The
solutions have two different field configurations. Besides magnetic fields dominated by the axial dipole component, we
also find configurations where a dipole in the equatorial plane is the dominating component. Both types are stable in
a parameter regime of intermediate shell thickness and Rayleigh numbers close to onset of convection. Axial dipole
solutions are subcritical in all the simulations explored while the equatorial dipole cases are supercritical at low
Rayleigh numbers but become metastable at higher Rayleigh numbers. The magnetic field strength saturates at a
much lower amplitude for the equatorial dipole dynamos, and the Elsasser number is significantly smaller than in the
axial configuration. The reason is that the mainly horizontal field in the equatorial dipole solution is incompatible
with the motion of convective cyclones and anticyclones. The axial dipole field, on the other hand, is predominantly
aligned with the axis of anticyclones, only cyclones are disrupted by horizontal field lines passing through. This
configuration can therefore accomodate stronger convective flows and, consequently, is the only one remaining stable
at higher Rayleigh numbers. These arguments should pertain in all planetary dynamos that are governed by strong
rotational constraints. They offer an explanation why the Elsasser numbers inferred for Uranus and Neptune are
