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Planetary science

Molten sodium mimics Earth’s magnetic-field flipping

09 Mar 2007

It has long been known that the Earth's magnetic field flips direction every quarter of a million years or so resulting in the north and south poles changing places. Now physicists in France have witnessed this phenomenon of magnetic-field reversal for the first time in the laboratory, by monitoring a 160-litre vat of swirling molten sodium. Their observations could eventually lead to a better understanding of "dynamo action", which is responsible for celestial bodies like the Earth being able to generate their own magnetic field (Europhys. Lett. 77 59001).

Swirling sodium

Dynamo action involves the flow of an electrically conducting fluid converting mechanical energy into magnetic energy. In the Earth, the fluid is the liquid iron of the outer core, which is in constant turbulent motion because of convection and the Earth’s overall rotation. But such dynamos are difficult to study – particularly when one wants to understand its effects on the scale of the Earth. Experiments require massive amounts of energy to sustain large volumes of swirling molten metal, and it is almost impossible to model the Earth’s inner dynamics computationally because of the huge number of variables involved.

Nicolas Mordant and colleagues from the École Normale Supérieure in Paris and other French institutions, however, have witnessed one of the peculiar effects of dynamo action for the first time. The team heated 160 litres of molten sodium to over 110 °C in a chamber, keeping the metal in highly turbulent motion using two counter-rotating propellers. Then they monitored the size and direction of the magnetic field and noticed a strange effect when they happened to set the speed of one propeller to 16 Hz and the other to 22 Hz – the magnetic field of the entire sample began to flip direction at irregular intervals ranging from 10 s to 180 s.

Similar “magnetic reversals” are thought to have occurred in the Earth’s history, but are not clearly understood. The last event is believed to have occurred some 780,000 years ago, but prior events could have been separated by anything from tens of thousands to many millions of years. Mordant’s team thinks that his experiment demonstrates this same “hierarchy” of time intervals, albeit on a much reduced scale.

However, Mordant warns that their observations should not necessarily be used to draw insights into large-scale systems. “We may have just been lucky [to see this effect],” he told Physics Web. “We should be cautious as to whether the physical origin of the flipping in the Earth is the same as in the experiment.”

Mordant’s experiment builds upon work performed in 1999 at the Riga Sodium Facility in Latvia, where a team led by Agris Gailitis first managed to create a self-excited magnetic field by keeping molten sodium in turbulent flow. (See related story: “Mini Earth created in the lab”.)

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