A Dynamo Mechanism for Rapid Decrease of the Geomagnetic Dipole Moment

The dipole moment of the geomagnetic field is decreasing at 6% per century, nearly one order of magnitude faster than its theoretical ohmic decay rate in the core. This change is significant because a sustained decline in dipole moment may indicate instability of the geodynamo. For example, according to the paleomagnetic record, polarity reversals typically begin with a large dipole moment decrease. Maps of geomagnetic secular variation on the core-mantle boundary show that much of the recent decrease in the dipole moment is associated with two phenomena: (i) emergence, growth, and poleward drift of reversed magnetic flux, and (ii) weakening of some normal polarity, high-intensity magnetic flux patches at located high latitudes. We have used numerical dynamo models to identify mechanisms for rapid decrease in dipole moment. Dynamo models indicate that rapid fluctuations in the dipole moment occur during bifurcations in the pattern of convection in the core. Bifurcations in the convection pattern alter the number, distribution, and intensity of the normal polarity flux patches, and trigger outbursts of reversed flux on the core-mantle boundary, all of which affect the dipole moment. We have analyzed a chaotic dynamo model that exhibits sudden decrease in dipole moment, similar to the recent geomagnetic dipole decrease, during bifurcation from an m=4 to an m=3 convection pattern. In this model, most of the reversed flux on the core-mantle boundary consists of poloidal magnetic field expelled from the core by convective plumes. During the bifurcation, reversed magnetic flux is expelled at low latitudes by transient plumes concentrated near the equatorial plane, transported poleward by meridional flow, and mixed with normal polarity field at higher latitudes, reducing the dipole moment.