From taylor state to model-Z?

A geodynamo model is studied of so-called “intermediate type”, in which the dynamical effects of the Lorentz force in generating meridional circulation, magnetic wind, and geostrophic flow are included, but in which the α-effect and the thermal wind are regarded as given. The core is supposed to be coupled weakly to the mantle through viscosity. The resulting nonlinear αω-dynamo was integrated for a range of different α-strengths, with a fixed thermal wind and for two different strengths of core-mantle coupling. Three regimes of behavior were isolated. For large α-effect, the geostrophic motion dominates the core flow, leading to solutions of model-Z form (Braginsky, 1975), For small α, the thermal wind is large compared with the magnetic wind and the geostrophic flow; the meridional motions are small. The dynamo here conforms more to Taylor's (1963) concept, according to which core-mantle coupling is unimportant; the solution for the two strengths of viscous coupling are almost the same. In this regime the dynamo operates subcritically i.e. at α-strengths smaller than that at which an infinitesimal field can be maintained according to kinematic theory. Between these two regimes lies a third regime in which a local optimal dynamo activity occurs, and in which the dipole moment and magnetic energy have local maxima.