Abstract
The natural simplifying assumptions often put forward in the theoretical investigations of the magnetohydrodynamic turbulence are that the turbulent flow is statistically isotropic, homogeneous and stationary. Of course, the natural turbulence in the planetary interiors, such as the liquid core of the Earth is neither, which has important consequences for the dynamics of the planetary magnetic fields generated via the hydromagnetic dynamo mechanism operating in the interiors of the planets. Here we concentrate on the relaxation of the assumption of statistical stationarity of the turbulent flow and study the effect of turbulent wave fields in the Earth’s core, which induces non-stationarity, on the turbulent resistivity in the non-reflectionally symmetric flow and the geodynamo effect. It is shown that the electromotive force, including the so-called α-effect and the turbulent magnetic diffusivity η¯, induced by non-stationary turbulence, evolves slowly in time. However, the turbulent α¯ coefficient, responsible for the dynamo action and η¯ evolve differently in time, thus creating periods of enhanced and suppressed turbulent diffusion and dynamo action somewhat independently. In particular, periods of enhanced α¯ may coincide with periods of suppressed diffusion, leading to a stable and strong field period. On the other hand, it is shown that when enhanced diffusion occurs simultaneously with suppression of the α-effect, this leads to a sharp drop in the intensity of the large-scale field, corresponding to a geomagnetic excursion.
Highlights
The terrestrial magnetic field is generated by the hydromagnetic dynamo action in theEarth’s liquid core driven thermally and compositionally [1,2,3]
Obviously no decisive conclusion about predictions of geomagnetic excursions can be made from the presented analysis, the paper points to the non-stationarity of the turbulent transport coefficients ᾱ and η, which are likely to slowly evolve in time due to the non-stationarity of the entire background turbulence in the core
Enhancement of the α-effect may coincide with suppression of diffusion and vice versa, enhancement of diffusion may coincide with suppression of the dynamo effect
Summary
The terrestrial magnetic field is generated by the hydromagnetic dynamo action in theEarth’s liquid core driven thermally and compositionally [1,2,3]. Non-stationarity has been recently shown by [12,13,14,15], as an effective mechanism of generation of the large-scale electromotive force through interactions of waves with distinct but close frequencies of oscillations (the beating effect). Such an EMF slowly evolves on timescales comparable or larger than the typical time scales of variation of the large-scale field. It is shown here that statistically non-stationary, helical turbulence induces a slowly varying in time α-effect and turbulent magnetic diffusion, and their variations are out of phase
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