Reversal sequence in a multiple scale dynamo mechanism

We investigate the temporal evolution of the magnetic dipole field intensit y of the Earth through a multiscale dynamo mechanism. On a large range of s patio-temporal scales, the helical motions of the fluid flow are given by a schematic model of a fully developed turbulence. The system construction i s symmetric with respect to left-handed and right-handed cyclones. The mult iscale cyclonic turbulence coupled with a differential rotation (schematic alpha omega dynamo) or alone(schematic a dynamo) is the ingredient of the l oop through which poloidal and toroidal fields are built from one another. Two kinds of reaction of the magnetic field on the flow are considered: the presence of a magnetic field first favours a larger-scale organization of the flow, and, second, impedes this flow by the effect of growing Lorentz f orces. We obtain the general features of the geomagnetic field intensity ob served over geological times and describe a general mechanism for reversals , excursions and secular variation. The mechanism happens to keep a memory during the chrons and loose it during the events (excursions and inversions ). (C) 2000 Elsevier Science B.V. All rights reserved.