Modélisation des inversions magnétiques

Earth's magnetic field reversals mimicked in the lab

Philip Ball

Poles apart: our planet's field flips every now and then - though no one knows why. NASA

Every few hundred thousand years or so, the Earth's north and south magnetic poles switch places. No one knows what triggers these geomagnetic field reversals, but a team in France has now reproduced them in the lab.

Michaël Berhanu of the École Normale Supérieure in Paris and his co-workers have spun enough molten sodium to fill a small bathtub in a copper cylinder at many revolutions per second. This provided a rough simulation of the Earth's spinning core of molten iron¹.

Electrical currents that are produced spontaneously in these swirling liquid metals set up a magnetic field in a process known as dynamo action. The same principles are used in industrial dynamos to create electricity, by moving metal wires through a magnetic field. Although it has been known for some time that turbulent motions in the Earth's core create the geomagnetic field this way, the details of how that happens still aren't clear.

In particular, geomagnetic reversals remain puzzling. Our magnetic poles have, in the past, faded away and then re-emerged upside down. These events are recorded in magnetic sedimentary rocks, which reveal the strength and direction of the prevailing field when it was formed. But no one knows why the reversals occurred when they did.

http://www.nature.com/news/2007/070305/full/070305-14.html