MAGNETIC FIELD

A shield generator

Without it there would be no life on the planet. The Earth's magnetic field - the first line of defence against solar attacks - is the fortunate consequence of the swirling currents at its core. Researchers are seeking to arrive at a better understanding of this phenomenon, with the hope of developing a revolutionary new electricity generator.

La convection dans le noyau est à l’origine de la génération du champ magnétique de la Terre par effet dynamo. Les mouvements de fluide conducteur (structures tourbillonnaires en bleu) étirent les lignes du champ magnétique, leur donnant de l’énergie. La persistance de la dynamo terrestre durant les derniers 3.5 milliards d’années apporte des contraintes sur le scénario d’évolution thermique de la planète. © Julien Aubert, CNRS-IPGP
Convection in the core is at the origin of the generation of the Earth’s magnetic field due to the dynamo effect. Movements of the conducting fluid (the swirling structures shown in blue) stretch the magnetic field lines, giving them energy. The persistence of the Earth’s dynamo over the past 3.5 billion years brings constraints to the scenario of the planet’s thermal development. ©Julien Aubert, CNRS-IPGP

Every second the Earth is bombarded with intense flows of particles emitted at the time of solar eruptions.Fortunately, the magnetosphere protects us against these ionising rays by deflecting their trajectory by means of electromagnetic repulsion. Satellites, travelling high in the atmosphere, are not so fortunate, with some being destroyed at the time of the last major magnetic storms, in 2003. To protect the on-board electronics from such attacks, meteorologists are trying to forecast magnetic storms by analysing variations in solar light. There is a reasonable timeframe in which to act as, while light rays reach us in eight minutes, it takes 1-2 days for harmful particles to travel the same route. Of limited efficiency, these forecasting models must still be improved and a better knowledge of our own magnetic field would certainly be of assistance.

The geodynamo

A genuine dynamo: this is what our planet can be compared to. The reason is the liquid nature of its outer core that surrounds the hard inner core, with various local differences in temperature and pressure. That is all it takes to engender swirling currents within this liquid core. Consisting principally of molten metals (iron and nickel), these animated fluids are moving electric charges. Add to that the multitude of magnetic solar and galactic fields in which the system is bathed and you have all the ingredients for a self-perpetuating dynamo.

The external magnetic fields cause fluctuations in the electric current produced by the nuclear swirls, with this movement of electrons creating an internal magnetic field that replaces the initial field. The phenomenon is therefore self-perpetuating. Produced by the core, the intense magnetic field is at the origin of the magnetosphere that protects us.

A source of energy in the making

Over the past 20 years, scientists have been trying to provide the facts to support the theory. Electromagnetic interactions are well known but the "self-perpetuating" aspect remains hypothetical. Philippe Cardin of the Internal Geophysics Laboratory at the Grenoble Observatory (FR) explains that "the experimental command of the phenomenon would bring attractive applications. We can already engender a magnetic field by imposing vigorous rotation on a metal sphere containing liquid sodium that is itself subject to an external magnetic field. If the self-perpetuating conditions are re-created, electricity will be generated through electromagnetic interactions. Copying our planet would thus enable us to develop a revolutionary electricity generator."

Marie-Françoise Lefèvre


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Enough to disorientate you...

"You don't have to be a scout to know that a compass points north!" Yet contrary to this commonly held belief, the compass only very rarely points to the "Septentrion". The present 8° difference between the polar axis and the magnetic axis is the primary cause.

But the magnetic field also frequently varies, in space and in time. A compass in Chicago points directly North while in New York it is oriented 15° West. And if the situation is analysed over a million years, it is probable that this same compass sometimes points South. This is because the magnetic poles reverse about every 250 000 years, with major variations in time. Geophysicists believe that the next inversion will be coming "soon" as the phenomenon last occurred 780 000 years ago.

A final detail to really lose you: historically, the "North Pole" on compasses was the name given to the side that pointed to our geographical North, or at least approximately so. More precise measurements subsequently caused us to identify, alongside the geographical North, a terrestrial "magnetic" North. It is to this that magnets point. Yet when it comes to magnets, it is opposites that attract. The reason our terrestrial magnetic North attracts the North of magnets is because it is in reality a magnetic South Pole...



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