Probabilistic structure of the geodynamo

One of the most intriguing features of Earth's axial magnetic dipole field, well known from the geological record, is its occasional and unpredictable reversal of polarity. Understanding the phenomenon is rendered very difficult by the highly nonlinear nature of the underlying magnetohydrodynamic problem. Numerical simulations of the liquid outer core, where regeneration occurs, are only able to model conditions that are far from Earth-like. On the analytical front, the situation is not much better; basic calculations, such as relating the average rate of reversals to various core parameters, have apparently been intractable. Here, we present a framework for solving such problems. Starting with the magnetic induction equation, we show that by considering its sources to be stochastic processes with fairly general properties, we can derive a differential equation for the joint probability distribution of the dominant toroidal and poloidal modes. This can be simplified to a Fokker-Planck equation and, with the help of an adiabatic approximation, reduced even further to an equation for the dipole amplitude alone. From these equations various quantities related to the magnetic field, including the average reversal rate, field strength, and time to complete a reversal, can be computed as functions of a small number of numerical parameters. These parameters in turn can be computed from physical considerations or constrained by paleomagnetic, numerical, and experimental data.

Figure 1

The drift coefficient ${\overline{D}}^{\left(1\right)}\left(x\right)$ in Eq. (42) fit to the PADM2M data set. Where the curve is positive, regeneration of the field is effective against its decay.

Figure 2

The diffusion coefficient, ${\overline{D}}^{\left(2\right)}\left(x\right)$ in Eq. (43), fit to the PADM2M data set.

Figure 3

Double-well potential derived from Fig. 1 (after a change of variable described in the text).