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Science and Technology

Which Way Is Up?

Earth's magnetic realm
Earth's magnetic realm (image: NASA)

Important variations in the magnetic field, observed by satellites in certain areas of the globe, could herald an inversion of the poles. This phenomenon has already come about several times in the history of the planet. Is the Earth losing its compass? This is what a study by the Department of Geomagnetism and Paleomagnetism of the Paris Geophysical Institute (l’Institut de physique du globe de Paris [IPG]) would have us believe.

We are all contained by the Earth’s magnetic field without really noticing it. We become aware of it only when we use a compass to find our way. This is the most obvious manifestation of the Earth’s magnetism, which has existed for 3 billion years and is generated 3,000 kilo-meters under our feet by the stirring of our planet’s liquid iron core. This liquid iron core causes the Earth to act like a giant magnet; the magnetic lines are organized on a bipolar basis, more or less in alignment with the Earth’s rotational axis.

This bipolar configuration, however, is not permanent. It varies with the movement of the Earth’s liquid core, and in the past, the positions of the magnetic poles have been known to switch entirely. These phenomena were verified by paleomagnetic studies on ancient volcanic basalts. The latter contain magnetic grains that kept both the orientation and strength of the Earth’s magnetic field when they became solid.

Before an inversion takes place, it is announced by a series of intermediary stages, manifested as anomalies in the magnetic field. These could be the events recently uncovered by the IPG team led by Gauthier Hulot. The team’s study concludes that “the mechanism at work in the variations we found could be similar to that of magnetic inversions.” Nevertheless, they could not confirm with absolute certainty that such a phenomenon is about to occur.

Researchers have gathered a few clues. They traced the evolution of the Earth’s magnetic field in the past 20 years from the measurements taken by the American satellite Magsat in 1980 and the Danish satellite Oersted in 2000. The Earth’s magnetism can be measured by satellites or observatories around the globe. Such observation is possible because the layer surrounding the Earth’s liquid core contains silicates—a weak electrical conductor—making the core magnetically “transparent.”

Scientists have noticed variations in the magnetic field in four main areas. The first, under the Pacific Ocean in the Northern Hemisphere, is subject to relatively minor change. But the three other regions are sites of far more significant changes. One of them is in the Southern Hemisphere, underneath the African continent. Here, the field is weaker and less bipolar. The other two are situated at the poles.

These variations are directly linked to the circular motions, in both directions, that stir the liquid iron at the surface of the Earth’s core. Because direct observation is impossible, scientists have relied on numerical simulations, which reconstituted an Earth and a magnetic field from models. These simulations, however, do not show the disparity we know exists between the Earth’s two hemispheres in terms of the recent variations in the magnetic field.

How can we explain this asymmetrical structure? According to the measurements of magnetic observatories on the planet—some of them three centuries old—the significant variation in the African zone seems a relatively recent occurrence in the Earth’s magnetic field. It is tied, “mathematically, to the fact that the Earth’s bipolar force has decreased in the last 2,000 years,” notes Hulot.

Indeed, the bipolar structure of the Earth’s magnetic field is weakening. If this process continues at the present pace, researchers predict, the entire bipolar structure might actually disappear. We would then find on our planet “many north poles and many south poles scattered everywhere.”

Do these facts really suggest an imminent  inversion of the Earth’s magnetic field? Scientists are cautious. No inversion has occurred in 780,000 years, while paleomagnetic studies indicate that in the past 10 million years, inversions have happened about every 200,000 years. In a way, “Our current era is unusually stable, and an inversion would not be strange,” says Hulot.

But other arguments point in the opposite direction. The strength of the Earth’s magnetic field, even if it is declining rapidly, is still high compared to previous eras, and it could suddenly turn upward again. Moreover, the Earth’s magnetic field has been known to take “vacations.” These are a kind of missed magnetic inversion, which never come to pass. Much more observation and study will be necessary before we know for certain whether or not the Earth is losing its bearings.