Analysis of the variability of the axial dipole moment of a numerical geodynamo model

J. Kuipers , P. Hoyng , J. Wicht , G.T. Barkema

We have analysed the time evolution of the Axial Dipole Moments (ADMs) from three numerical geodynamo models by relating it to the Fokker-Planck equation governing the systematic and random ADM motion. We have determined the effective growth rate of the ADM and the diffusion coefficient characterising its random uctuations. We find that the numerical ADM data exhibit a nonlinear quenching that is not significantly di_erent from that of the Sint-2000 data. The quenching is only partly due to a reduction of the r.m.s. convective ow speed with increasing ADM. Our results suggest that in these numerical models similar mechanisms may be at work as in the earth's core, and that the results of Brendel et al. (2007) are unlikely to be an artifact caused by the restricted length of the dataset. They also suggest that the dynamics of the ADM is that of a Brownian particle (i.e. driven by additive noise) in a bistable potential, and we illustrate some consequences of this idea.

Mots clés: Geodynamo; Hydromagnetic dynamos; Reversals; Secular variation; Turbulent convection; Stochastic processes

J. Kuipers, P. Hoyng, J. Wicht, G.T. Barkema. Analysis of the variability of the axial dipole moment of a numerical geodynamo model. Physics of the Earth and Planetary Interiors, 2009, 173 (3-4), pp.228. ⟨10.1016/j.pepi.2008.12.001⟩. ⟨hal-00521159⟩