Dipole collapse and reversal precursors in a numerical dynamo

Precursors to extreme geomagnetic field changes are examined in a numerical dynamo with a reversing dipolar magnetic field. A dynamo model with compositional convection in a rotating spherical shell produces a strongly dipolar external magnetic field over 6 Myr of simulated paleomagnetic time, with stable polarity epochs and occasional dipole collapses, some of which result in polarity reversals or dipole axis excursions. We analyze the model behavior during two dipole collapses, one that leads to a polarity reversal and one that does not, focusing on observable precursors. Reversed magnetic field induced in the dynamo interior by intermittent convective variability is the primary cause of dipole collapse. Spots of reversed magnetic flux emerge on the outer boundary at an early stage, then re-emerge with greater intensity at the height of the collapse. The energy in the external field cascades to higher harmonics as these reversed patches appear. Butterfly diagrams showing the reversed and normal flux contributions to the axial dipole reveal poleward migration of the patches during dipole collapse. Axial dipole reduction by precursory reversed flux is several times larger in the reversing case, compared to the non-reversing case. A butterfly diagram of the geomagnetic field since 1840 shows high latitude reversed flux emerging on the core-mantle boundary. Although the reversed geomagnetic flux is presently too weak to be labeled a reversal precursor, it is consistent with early stage dipole collapse in the dynamo model. (c) 2008 Elsevier B.V. All rights reserved.