Dipole Field Collapse in a Reversing Dynamo Model

We analyze the collapse of the external dipole magnetic field in a three-dimensional numerical dynamo. A dynamo model driven by thermal convection in a rotating spherical fluid shell with an electrically conducting solid inner core exhibits collapse of its external dipole field prior to polarity reversal at Rayleigh number 300, Ekman number 0.01, Prandtl number 1, and Roberts number 20. Helical motion in rising plumes lifts and twists the toroidal magnetic field, producing loops of poloidal magnetic field. Expulsion of these poloidal field loops results in patches of reversed mangetic flux on the core-mantle boundary. The external dipole collapses when the reversed flux patches are transported to the pole by meridional flow. Seen from the surface, polarity change occurs when reversed flux is transported by meridional flow over the whole core-mantle boundary. In this model, reversed magnetic flux on the core-mantle boundary above rising plumes is a precursor to dipole collapse and polarity reversal. Collapse of the external dipole does not, however, indicate a dynamo blackout. Although the external dipole collapses prior to polarity change, both the total magnetic energy and the dipole energy within the core actually increase during a reversal.