The dipole–multipole transition in planetary dynamos

Abstract

We investigate the dipole–multipole field transition in rapidly rotating dynamos in the low-inertia regime relevant to planetary cores. Here, the Rossby number is small on the planetary core depth as well as on the length scale of core convection. Attention is focused on the dynamics of slow Magnetic-Archimedean-Coriolis (MAC), or magnetostrophic, waves generated in the energy-containing scales of the dynamo. The suppression of the slow MAC waves in a strongly driven dynamo is dynamically similar to the excitation of these waves in a moderately driven dynamo evolving from a small seed magnetic field. While the former regime causes polarity reversals, the latter regime produces the axial dipole field from a multipolar state. For either polarity transition, a Rayleigh number based on the mean wavenumber of the energy-containing scales bears the same linear relationship with the peak Elsasser number measured at the transition. This self-similarity can provide an estimate of the Rayleigh number that admits polarity reversals.