Geoscientist Online

Geoscientist 17.5 May 2007

French sci-fi pioneer Jules Verne published Journey to the Centre of the Earth in 1864. Now, 142 years later, French scientists have succeeded in bringing the core into a laboratory, and generated a self-sustaining magnetic field… Amarendra Swarup reports.

It is one of the great ironies of science that for all of our knowledge about the inner workings of countless stars, black holes and distant galaxies, we still know relatively little about what goes on within the core of the Earth itself - and in particular, about the origins of our planet’s magnetic field. Now, a bold new experiment by a team of French physicists has successfully mimicked how turbulent flows of molten metal within the Earth's core create our magnetic environment. The results have sent scientists scurrying to their theoretical drawing boards and could give new insights into how planets and stars acquire such large and long-lived magnetic fields.

First conjectured by the noted physicist Joseph Larmor in 1919 as an explanation for sunspots, scientists now believe that all cosmic magnetic fields, including Earth’s, are generated by the dynamo effect – a self-sustaining feedback mechanism by which mechanical energy is converted into magnetic energy. The key to this effect lies in the small random magnetic fields in molten iron. As atoms of molten metals swirl choppily around each other within the core, these intermittent magnetic fields induce tiny fluctuating electric currents, similar to those produced in electricity generators. These currents in turn generate other magnetic fields. If the flow is rapid and turbulent enough, the generated magnetic fields reinforce the original ones, forming a self-sustaining loop that rapidly amplifies to cosmic proportions.

However, experimental proof of the theory has been thin on the ground. In 2000, two groups – based in Riga, Latvia, and Karlsruhe, Germany, respectively – obtained the first ever self-generating magnetic field through the movement of a liquid metal. However, the flow of the molten metal, sodium in this case, was carefully controlled through a complex array of pipes – a far cry from the more turbulent environment in the Earth’s core. As a result, the magnetic field generated lacked the unique features observed in cosmic magnetic fields - such as variability, and spontaneous reversal. Worse still, subsequent numerical simulations shed doubt on the possibility of obtaining a self-generating magnetic field in more natural conditions, where turbulent motions could significantly modify the flow properties.

Now the French VKS collaboration has reopened the debate by creating a magnetic field within a large tank filled with free-flowing liquid sodium – a far more realistic simulation of Earth’s core. “Our experiment is much closer to cosmic objects than anything studied so far and shows that even with strong turbulent motions, self-generating magnetic fields are possible” says team member Bérengère Dubrulle of the Atomic Energy Commission (CEA) in Saclay, France. “Moreover, the field displays a rich variety of dynamics, such as periodic or erratic reversals and variability, that are reminiscent of cosmic magnetic fields.”

Core values

For their experiment, the researchers filled a half-metre-long cylindrical vessel with 150 litres of molten sodium. Two iron propellers then rotated in opposite directions at varying speeds of up to 26 revolutions per second to stir the liquid and create turbulent flows. Above a certain speed, the team found spontaneous magnetic fields began to appear. Intriguingly, this happened at much lower than expected average flow speeds, contrary to assumptions that turbulent flows would require much higher speeds to create the dynamo effect.

The race is now on to understand why. “There is no theory for dynamo generation in a turbulent environment” explains Dubrulle. “Numerical simulations can provide hints for a theory, but they are very far from cosmic objects.”

According to Dubrulle, their experiment now gives them a working system to play with and study the inner dynamics of cosmic objects in detail. The system has three features that distinguish it from previous experiments and contributed to the spontaneous magnetic field observed: a layer of sodium at rest, in order to obtain the most ideal conditions for magnetic field growth prior to turbulent motion; a thin ring in the middle to stabilise large scale turbulent motions; and propellers made of iron rather than stainless steel. The problem is no one yet knows which one - or ones - caused the effect.

“When we can identify which modifications led to the dynamo action, a great step towards understanding the generation of magnetic fields in a turbulent environment will be achieved,” says Dubrulle. “There has already been an international meeting in Paris where our results were discussed but no one really agreed about which point was the most important!”

Ref: Generation of a Magnetic Field by Dynamo Action in a Turbulent Flow of Liquid Sodium, R Monchaux, M Berhanu, M Bourgoin, M Moulin, Ph Odier, J-F Pinton, R Volk, S Fauve, N Mordant, F Pétrélis, A Chiffaudel, F Daviaud, B Dubrulle, C Gasquet, L Marié, and F Ravelet, Physical Review Letters 98, 044502 (2007).