The oscillatory nature of the geomagnetic field during reversals

Abstract

We investigate the phenomenology of reversals predicted by a multiscale αω dynamo model in which a hierarchical structure of length scales accounts for the nature of turbulent thermal convection at a very high Rayleigh number. Cyclones at different length scales contribute to the α-effect. During polarity chrons, a progressive population inversion invariably yields to a change of sign of the α-effect which initiates reversals. The dipole weakens and turbulent motions are able to cascade through a wide range of length scales to produce large fluctuations of the α-effect. These fluctuations (1) reinforce magnetic fields with opposite polarity, or (2) trigger large instabilities in direction and intensity of the dipole. This behaviour is reflected in the real Earth by the existence of chrons and reversals. The simulations also predict different degrees of complexity in the reversal process which depends on the spontaneous change in sign of the α-effect. The existence of similar oscillations in the detailed reversal records suggests that geomagnetic reversal would result from equivalent underlying mechanisms.