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A journey to the centre of the Earth
Feb 1st 2007
From The Economist print edition

Recreating the planet's magnetic field in a laboratory

HOW does the Earth's magnetic field originate? Sir Joseph Larmor, an Irish mathematician, proposed in 1919 that it was generated spontaneously by the swirling of molten metal inside the planet. His idea, though it later found favour, was ignored at the time. One problem was that it is extremely difficult to recreate the effect in the laboratory. But, almost 90 years after Larmor's paper, Stéphan Fauve of the Ecole Normale Superiéure in Paris and his colleagues have managed to do so. They have published their method in this week's issue of Physical Review Letters.

Creating a magnetic field using electricity is easy: an electric current flowing in a wire generates a magnetic field around that wire. Similarly, moving a magnet creates an electric current in a neighbouring wire. This is the basis of electric motors and dynamos respectively. But when it comes to using the turbulent flow of liquid metal rather than well-behaved cables and wires, the experiment becomes rather harder.

Dr Fauve's model of the Earth's core was a cylindrical tank some 50cm long filled with liquid sodium. To recreate the effect of heat flowing through the planet's interior, the researchers stirred this brew with iron paddles attached to iron discs placed at each end of the tank.

The discs whirred in opposite directions some 26 times a second. At this speed, the flow of the molten metal through the random, small magnetic fields that exist in all magnetic materials was such that it created an electric current that, in turn, generated a self-sustaining magnetic field. For such self-generation to occur, the flow of liquid sodium had to be both complex and rapid. This is because the metal has to be mixed both longitudinally and latitudinally, so that it tangles up the magnetic fields faster than they can untangle. That suggests a lot of rapid convection in the Earth's interior (though not as rapid as in Dr Fauve's simulation, since things are happening on a grander scale, and in liquid iron, not liquid sodium).

This result certainly clears up one mystery. But it leaves a second unresolved. Why does the field spontaneously reverse itself every few hundred thousand years? If Dr Fauve could mimic that in his sodium bath, it really would be a “eureka” moment.

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