Geodynamo Theory

The geodynamo is the magnetohydrodynamic process by which convecting, rotating liquid iron in the Earth's outer core generates electric currents that sustain the geomagnetic field against ohmic decay.

Definition

The geodynamo is a self-sustaining magnetohydrodynamic dynamo in which the motion of electrically conducting liquid metal in the Earth's outer core induces electric currents whose magnetic field continually regenerates itself, accounting for the existence, dipolar dominance, and reversals of the geomagnetic field.

Scope

This topic covers the physics of magnetic field generation in the Earth's core: the magnetohydrodynamic induction equation, the necessity of dynamo action to overcome ohmic decay, the roles of convection, rotation, and the Coriolis force, and the energy sources that power the dynamo, including thermal and compositional convection driven by inner-core growth. It treats the difficulty of analytic dynamos, the antidynamo theorems, and modern three-dimensional numerical simulations that reproduce a dipole-dominated, occasionally reversing field. The emphasis is on the generation mechanism rather than the observed field.

Core questions

• Why is dynamo action required to maintain the field rather than a permanent magnet?
• How do convection and the Coriolis force organize core flow into a working dynamo?
• What energy sources power the geodynamo over geological time?
• How do numerical simulations reproduce a dipolar, reversing field?

Key concepts

• Magnetohydrodynamic induction equation
• Self-exciting dynamo action and ohmic decay
• Convection, rotation, and the Coriolis force in the core
• Thermal and compositional convection from inner-core growth
• Numerical geodynamo simulations

Key theories

Self-exciting magnetohydrodynamic dynamo

Motion of conducting core fluid through a magnetic field induces currents that, suitably organized by convection and rotation, regenerate the field faster than ohmic diffusion destroys it, so the field is maintained without a permanent magnet, which could not survive the core's high temperature.

Numerical geodynamo simulations

Three-dimensional solutions of the coupled magnetohydrodynamic and convection equations in a rotating spherical shell, beginning with the Glatzmaier-Roberts model, spontaneously produce a dipole-dominated field and even polarity reversals, demonstrating the dynamo mechanism in silico.

Mechanisms

Heat loss and the freezing of the inner core drive thermal and compositional convection in the liquid outer core; the rapidly rotating, electrically conducting flow stretches and twists magnetic field lines, and through the combined induction of differential rotation and helical convection regenerates both toroidal and poloidal field components, balancing ohmic dissipation to maintain a statistically steady, occasionally reversing field.

Clinical relevance

Geodynamo theory explains why the Earth has a protective magnetic field that shields the surface from solar wind, accounts for polarity reversals used to date the seafloor, and links the field to the thermal and compositional evolution of the core.

History

Elsasser and Bullard developed dynamo theory in the 1940s and 1950s as the only viable explanation for a long-lived field, antidynamo theorems clarified what flows cannot work, and the 1995 Glatzmaier-Roberts simulation marked the start of the modern era of self-consistent numerical geodynamos.

Key figures

• Walter Elsasser
• Edward Bullard
• Paul Roberts
• Gary Glatzmaier

Seminal works

• merrill1996
• glatzmaier1995
• robertsking2013

Frequently asked questions

Why can't the Earth's field be due to a giant permanent magnet?

The interior is far hotter than the Curie temperature at which materials lose permanent magnetism, so no buried magnet could survive; instead the field must be actively generated by electric currents driven by fluid motion in the core, which is the geodynamo.

What powers the geodynamo?

It is driven by convection in the liquid outer core, fed by heat escaping from the core and by the release of light elements and latent heat as the inner core slowly freezes and grows, with the Earth's rotation shaping the flow into an efficient field generator.