School of Mathematics, Statistics and Physics

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Wide vortices of molten iron swirl inside the Earth’s core

Work led by a researcher from the School of Mathematics, Statistics and Physics establishes that the flow of molten iron in the deep interior of our planet is organised in kilometre-wide vortices.

The rotation of our planet influences the motions of its atmosphere and oceans, and leads to the formation of vortices, either cyclones or anticyclones. Interestingly, this phenomenon also occurs in the liquid iron core located deep inside our planet. In the core, the vortices generate the Earth’s magnetic field by a dynamo process. Up to now, scientists thought that the fluid viscosity controlled the size of the vortices. Liquid iron is weakly viscous like water, so this idea implied that the core was populated by vortices each about 10 m across, because the viscous forces can only affect vortices of this small size.

Dr Céline Guervilly and her colleagues from Grenoble study the dynamics of vortices in a turbulent rotating fluid using state-of-the-art computer models. Their work, published in the leading academic journal Nature, shows that the vortex size is directly related to the flow speed.  In the Earth’s core, in the absence of magnetic field, the turbulent vortices would be about 30 km wide, which is about 1% of the width of the liquid core.

These findings imply that the core contains a few thousand turbulent vortices, rather than billions of viscous vortices as previously believed.  This is a very promising result because thousands of vortices (unlike billions!) can be modelled using modern supercomputers.  Surprisingly perhaps, the turbulence, which is usually a bugbear for physicists, takes over from the fluid viscosity in the Earth’s core, and opens the door to realistic computer simulations of planetary interiors to study the generation of magnetic fields.

This research is funded by the UK Natural Environment Research Council (https://nerc.ukri.org) and the French Agence Nationale de la Recherche (https://anr.fr/en/).

This work “Turbulent convective length scale in planetary cores” appears in Nature (https://doi.org/10.1038/s41586-019-1301-5).

Flow in the Earth’s liquid core where the yellow vortices are cyclones and the blue vortices are anti-cyclones.
Flow in the Earth’s liquid core where the yellow vortices are cyclones and the blue vortices are anti-cyclones. Credits: N. Schaeffer/CNRS.
Flow in the Earth’s liquid core where the yellow vortices are cyclones and the blue vortices are anti-cyclones. Credits: N. Scha

published on: 23 July 2019