The stochastic excitation of reversals in simple dynamos

Abstract

Several variations of the αω disc dynamo models are known to exhibit chaotic magnetic field behaviour typical of non-linear, dissipative systems. Although these models demonstrate that geomagnetic reversals can be generated by simplified dynamo equations, the behaviour of the magnetic field itself is generally too simple, showing especially an absence of long polarity epochs in most of the models. We show that the addition of three varieties of stochastic processes (Gaussian, flicker and brown noise) enriches the field evolution and can lead to realistic palaeomagnetic behaviour. We argue that noise processes must be present in the actual fluid core and suggest, from a physical point of view, a flicker noise stimulation of the dynamo. We find two features of the palaeomagnetic record that would favour the presence of noise in the dynamo process, namely the absence of a linear oscillation in field intensity between reversals or, even if present, the absence of an increase in amplitude of this oscillation prior to a reversal. We also consider the addition of a random component to the helicity driving function of the α 2 dynamo process and show that various types of reversal can occur. Unfortunately, realistic field behaviour cannot be maintained over long time periods due to the tendency for the magnitudes of the poloidal and toroidal fields to equilibrate during a polarity epoch.