Why dynamos are prone to reversals

Abstract

In a recent paper (Phys. Rev. Lett. 94 (2005), 184506; physics/0411050) it was shown that a simple mean-field dynamo model with a spherically symmetric helical turbulence parameter alpha can exhibit a number of features which are typical for Earth's magnetic field reversals. In particular, the model produces asymmetric reversals, a positive correlation of field strength and interval length, and a bimodal field distribution. All these features are attributable to the magnetic field dynamics in the vicinity of an exceptional point of the spectrum of the non-selfadjoint dynamo operator. The negative slope of the growth rate curve between the nearby local maximum and the exceptional point makes the system unstable and drives it to the exceptional point and beyond into the oscillatory branch where the sign change happens. A weakness of this reversal model is the apparent necessity to fine-tune the magnetic Reynolds number and/or the radial profile of alpha. In the present paper, it is shown that this fine-tuning is not necessary in the case of higher supercriticality of the dynamo. Numerical examples and physical arguments are compiled to show that, with increasing magnetic Reynolds number, there is strong tendency for the exceptional point and the associated local maximum to move close to the zero growth rate line. Although exemplified again by the spherically symmetric alpha^2 dynamo model, the main idea of this ''self-tuning'' mechanism of saturated dynamos into a reversal-prone state seems well transferable to other dynamos. As a consequence, reversing dynamos might be much more typical and may occur much more frequently in nature than what could be expected from a purely kinematic perspective.

11 pages, 10 figures

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• Determining role of Krein signature for 3D Arnold tongues of oscillatory dynamos
• On the spectrum of the magnetohydrodynamic mean-field alpha^2-dynamo operator
• Perturbation of multiparameter non-self-adjoint boundary eigenvalue problems for operator matrices
• Cylindrical anisotropic ${\alpha }^2$ dynamos
• Inferring basic parameters of the geodynamo from sequences of polarity reversals
• Coexisting stochastic and coherence resonance in a mean-field dynamo model for Earth's magnetic field reversals
• Dynamics of charged fluids and 1/L perturbation expansions
• A paradigmatic model of Earth's magnetic field reversals
• Magnetic field dynamos and magnetically triggered flow instabilities
• Parametric instability in periodically perturbed dynamos
• Forward and inverse problems in fundamental and applied magnetohydrodynamics
• Laboratory experiments and numerical simulations on magnetic instabilities