Creation of a magnetic field in a turbulent fluid

March 10, 2007

Understanding the origin and behavior of the magnetic fields of planets and stars is the goal of research being carried out by many teams from all over the world. The VKS collaboration (CEA, CNRS, Ecole normale supérieure in Lyon, Ecole normale supérieure in Paris) has succeeded in creating in the laboratory a magnetic field in a highly turbulent flow of liquid sodium.

Although the extreme conditions specific to astrophysical and geophysical environments cannot all be reproduced in the laboratory, the magnetic field observed shows remarkable similarities with magnetic fields observed in the cosmos. The findings represent a significant advance in the understanding of the mechanisms at work in the formation of natural magnetic fields. They are published in Physical Review Letters dated 26 January 2007.

Most of the astrophysical objects which surround us (planets, stars and galaxies) have a magnetic field, whose origin is poorly understood. Such magnetic fields can play a major role in the evolution of various structures throughout the Universe. The Earth's magnetic field, which is very probably caused by the movement of liquid iron in the core, not only makes compass needles point north, but also protects us from the harmful effects of cosmic rays and the solar wind.

As early as 1919, Larmor put forward the hypothesis that the Sun's magnetic field is generated by a "dynamo" effect, in other words by the movement of a fluid that conducts electricity. Because of their highly chaotic (turbulent) nature, the analysis of geophysical and astrophysical flows is beyond the current capacities of numerical simulations, and, until now, has thwarted all attempts at a theoretical approach.

It is only through experimental work that it is possible to reproduce the dynamo phenomenon with parameters that are similar to those that occur naturally. Following experiments carried out in 2000 by teams in Riga and Karlsruhe, the challenge facing the physicists was to show that the fully turbulent motion of a conducting liquid could spontaneously generate a magnetic field.

Since 1998, the VKS collaboration has been studying a highly turbulent flow produced by the movement of two turbines revolving in opposite directions in a cylinder filled with liquid sodium. Liquid sodium is an excellent conductor of electricity, while having a density similar to that of water, unlike many other metals which are much denser. In September 2006, the VKS experiment showed that, when the turbines revolve faster than a critical speed (1020 rpm), the flow spontaneously generates a magnetic field. This is the first time that such results have been observed in a highly turbulent medium.

The result proves that fluid dynamos continue to operate in the presence of strong turbulence of the kinds that occur under natural conditions. The achievement of the dynamo experiment under laboratory conditions opens up many new prospects. In particular, it will make it possible to study the energy balance involved in the production of a magnetic field as well as its dynamics. It may therefore be possible to understand the origin of the pseudoperiodic oscillations in the solar cycle or the irregular reversals of the Earth's magnetic field.

Source: CNRS

Explore further: Rosetta witnesses birth of baby bow shock around comet

Related Stories

Rosetta witnesses birth of baby bow shock around comet

December 12, 2018

A new study reveals that, contrary to first impressions, Rosetta did detect signs of an infant bow shock at the comet it explored for two years – the first ever seen forming anywhere in the solar system.

Growing magnetic fields in deep space: Just wiggle the plasma

November 5, 2018

Contrary to what many people believe, outer space is not empty. In addition to an electrically charged soup of ions and electrons known as plasma, space is permeated by magnetic fields with a wide range of strengths. Astrophysicists ...

Fast-flowing electrons may mimic astrophysical dynamos

October 29, 2018

A powerful engine roils deep beneath our feet, converting energy in the Earth's core into magnetic fields that shield us from the solar wind. Similar engines drive the magnetic activity of the sun, other stars and even other ...

Magnetic pumping pushes plasma particles to high energies

November 5, 2018

As you walk away from a campfire on a cool autumn night, you quickly feel colder. The same thing happens in outer space. As it spins, the sun continuously flings hot material into space, out to the furthest reaches of our ...

Recommended for you

Magic number colloidal clusters

December 14, 2018

Complexity in nature often results from self-assembly, and is considered particularly robust. Compact clusters of elemental particles can be shown to be of practical relevance, and are found in atomic nuclei, nanoparticles ...

Tangled magnetic fields power cosmic particle accelerators

December 13, 2018

Magnetic field lines tangled like spaghetti in a bowl might be behind the most powerful particle accelerators in the universe. That's the result of a new computational study by researchers from the Department of Energy's ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.