The Parker–Levy reversal mechanism

Abstract

Parker and Levy proposed that geomagnetic reversals result from fluctuations in the location of cyclonic convection cells that reverse the toroidal flux in mid-latitudes, as exhibited by a mean field dynamo model with differential rotation concentrated at the outer boundary, and α at a moveable interior point. Roberts studied a more realistic model with α and ω spread over a volume, but found that it produced oscillatory rather than stationary fields without sufficient meridian circulation. This difference in behaviour is explored with a dynamo model where the concentrated quantities used by Levy have been replaced with broader functions. Stationary solutions are obtained when ω is concentrated at a greater radius than α, provided α is at sufficiently low latitudes. Toroidal flux is reversed when α is in high latitudes, as required by the Parker–Levy reversal mechanism. At very high latitudes, the dynamo oscillates, the transition between stationary and oscillatory modes suggesting a different mechanism for reversal. When ω is at a smaller radius than α, toroidal flux does not reverse, and the solutions are always steady, so in this case the reversal mechanism will not work. When α and ω overlap, stationary solutions are only possible with α at very low latitudes; otherwise, solutions are oscillatory. Sufficient meridian circulation leads to stationary solutions for all locations of α with reversed toroidal flux when α is at high latitudes. I conclude that the Parker–Levy reversal mechanism can apply provided the cyclonic convection is inside the differential rotation, or sufficient meridian circulation exists to stabilise the geodynamo when α and ω overlap. Reversal behaviour also occurs in many instances where the preferred dynamo mode becomes oscillatory.