Dipole and octapole field reversals in a rotating spherical shell: Magnetohydrodynamic dynamo simulation

Abstract

Computer simulation results of a compressible magnetohydrodynamic (MHD) dynamo in a rotating spherical shell are presented. The reversal of either the dipole or the octapole field polarity is observed accompanying flip–flop magnetic energy transitions. Three energy transitions are observed. The dipole reversal occurs in the first transition. In the initial stage of the dipole reversal, the magnetic energy is about five times larger than the kinetic energy, and the convection pattern consists essentially of six straight cyclonic and anticyclonic vortex column pairs aligned to the rotation axis. In the intermediate stage, the magnetic energy decreases and the field is observed to change direction in some regions. This is accompanied by reorganization of the vortex columns and a transition to a new energy level. In the final stage, the field is completely reversed, the number of convection columns is increased and the magnetic energy is lower than in the second stage. In the other two magnetic energy transitions, the octapole component reverses polarity.