Geodynamo theory and simulations

Abstract

80 years ago, Joseph Larmor planted the seed that grew into today's imposing body of knowledge about how the Earth's magnetic field is created. His simple idea, that the geomagnetic field is the result of dynamo action in the Earth's electrically conducting, fluid core, encountered many difficulties, but these have by now been largely overcome, while alternative proposals have been found to be untenable. The development of the theory and its current status are reviewed below. The basic electrodynamics are summarized, but the main focus is on dynamical questions. A special study is made of the energy and entropy requirements of the dynamo and in particular of how efficient it is, considered as a heat engine. Particular attention is paid to modeling core magnetohydrodynamics in a way that is tractable but nevertheless incorporates the dynamical effects of core turbulence in an approximate way. This theory has been tested by numerical integrations, some results from which are presented. The success of these simulations seems to be considerable, when measured against the known geomagnetic facts summarized here. Obstacles that still remain to be overcome are discussed, and some other future challenges are described. CONTENTS

Chaotic Dynamos Generated by a Turbulent Flow of Liquid Sodium

“…Reversals occur nearly periodically in the Sun, roughly every 11 years. Direct numerical simulations have successfully displayed reversals of the magnetic field [2], but they cannot be performed in a realistic parameter range. Although laboratory experiments cannot reach the kinetic Reynolds numbers Re of the Sun or even Earth, they provide useful information about the effect of turbulence on the dynamo process.…”

Supermodulation of the Sun's magnetic activity: the effects of symmetry changes

“…Field reversals and associated symmetry changes arise also in several related systems, notably the geodynamo (e.g. Roberts and Glatzmaier 2000;Pétrélis et al 2009;Pétrélis & Fauve 2010;Gallet et al 2012) and the VKS experiment, where dynamo action is driven by ferromagnetic rotors in a tank of molten sodium (Ravelet et al 2008;Monchaux et al 2009;Berhanu et al 2010).…”

Effective diffusivity of passive scalars in rotating turbulence

“…Turbulent transport in anisotropic flows plays an important role in a wide variety of astrophysical and geophysical processes (see, e.g., [1][2][3] for examples of transport in astrophysics, [4] for an example in the context of the geodynamo, [5] for an example in atmospheric flows, and [6] for a study of vertical transport and diffusion in the ocean). In many of these examples, the resulting mixing is often studied and modeled by means of anisotropic transport coefficients [5,7,8].…”