Over the geological ages, the Earth has undergone several erratic reversals of its magnetic field. The sun’s magnetic field is reversed regularly every 22 years according to its cycle. These magnetic dynamics, which are still shrouded in mystery, play a role in our planet’s exposure to cosmic rays.

The joint VKS experiment¹  (CEA², CNRS^3,4, the Ecole Normale Supérieure in Lyon³ and the Ecole Normale Supérieure in Paris⁴) has, for the very first time, been able to observe magnetic field reversals in laboratory conditions thanks to a highly turbulent flow of liquid sodium. The experiments should help scientists to understand more about cosmic magnetic field reversals. The results are published in Europhysics Letters, Volume 77, March 2007.

 The Earth’s magnetic field is created by highly disordered movements that churn up the liquid iron core at the center of the Earth: this is known as the “dynamo” effect. One of its most astonishing characteristics, revealed thanks to paleomagnetic research, is that reversal of the magnetic poles is totally random. They remain close to the Earth’s geographic poles and flip between north and south about once every 100,000 years or so, although longer periods have been found between reversals. On average, these reversals last for a few thousand years.

 The cause and timescale of such reversals, together with the geometry of the magnetic field during a reversal remain the subject of much debate. The consequences may be considerable: during a reversal, the magnetosphere that protects the Earth from solar and cosmic radiation is significantly weakened. Life on Earth, and human life in particular, has survived this kind of situation in the past (the last reversal occurred 700,000 years ago), but it would severely interfere with our modern communications systems (satellites and networks, etc.).