THE PARKER-LEVY REVERSAL MECHANISM

Parker and Levy proposed that geomagnetic reversals result from fluctu ations in the location of cyclonic convection cells that reverse the t oroidal flux in mid-latitudes, as exhibited by a mean field dynamo mod el with differential rotation concentrated at the outer boundary, and alpha at a moveable interior point. Roberts studied a more realistic m odel with alpha and omega spread over a volume, but found that it prod uced oscillatory rather than stationary fields without sufficient meri dian circulation. This difference in behaviour is explored with a dyna mo model where the concentrated quantities used by Levy have been repl aced with broader functions. Stationary solutions are obtained when om ega is concentrated at a greater radius than alpha, provided alpha is at sufficiently low latitudes. Toroidal flux is reversed when alpha 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 omega is at a smaller radius than alpha, toroidal flux does not reverse, and the solutions are always steady, so in this cas e the reversal mechanism will not work. When alpha and omega overlap, stationary solutions are only possible with alpha at very low latitude s; otherwise, solutions are oscillatory. Sufficient meridian circulati on leads to stationary solutions for all locations of alpha with rever sed toroidal flux when alpha is at high latitudes. I conclude that the Parker-Levy reversal mechanism can apply provided the cyclonic convec tion is inside the differential rotation, or sufficient meridian circu lation exists to stabilise the geodynamo when alpha and omega overlap. Reversal behaviour also occurs in many instances where the preferred dynamo mode becomes oscillatory. (C) 1998 Elsevier Science B.V.