Who hasn’t played at explorers by trying to orient themselves with a compass? Since the XVand century, the orientation property of these small magnetic needles is well known to navigators. It reflects the existence of an earth’s magnetic field, which, in addition to providing a reliable means of orientation, protects the Earth and its biosphere from solar radiation. However, the Earth’s magnetic field is far from stable. The magnetic poles are actually not fixed and their position evolves over the years, until they are reversed.
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These episodes of magnetic field reversal, when the North Pole becomes South Pole and vice versa, have occurred hundreds of times throughout Earth’s history. But why does the magnetic field occasionally reverse? What is the origin and the engine of these inversions?
Earth’s magnetic field, what is it?
Imagine a big magnet dipole placed at the center of the Earth. The magnetic lines of force thus created represent what is called the earth’s magnetic field. These lines of force intersect the surface of the Earth at an angle which varies according to the latitude. They “enter” vertically at a point called the “magnetic north pole” and exit in the same way at the magnetic south pole. At the level ofequator magnetic, the field lines are horizontal. Currently, the magnetic axis is offset from the Earth’s rotation axis, which means that the magnetic and geographic poles do not match. The Magnetic North Pole is currently located in Siberia and the South Pole in Adélie Land, off theAntarctic.
If the image of a large dipole magnet in the center of the Earth allows in first approximation the understanding of the magnetic field, its origin is much more complex. Everything happens at the level of earth corewhich would work as a dynamo self-sustaining. Because of temperature variations with depth, Earth’s rotation and Coriolis forcesthe iron liquid forming the outer core is traversed by currents of convection important, structured in swirling columns parallel to the axis of rotation of the Earth. It is this powerful convective motion, creating a dynamo effectwhich would give rise to the magnetic field.
Magnetic field reversals over time
In the early 1900s, scientists discovered that rocks of volcanic origin have varying directions of magnetization. Some measurements indicated a magnetization in the same direction as the current magnetic field, others indicated a totally opposite magnetization, creating ” anomalies magnetic”. From these observations was born the idea that the Earth’s magnetic field can be reversed episodically. This hypothesis has been supported over time and today there are magnetic polarity scales listing and dating the various reversals that have taken place during the history of the Earth.
These scales define periods of “normal” polarity when the measured field is identical to the current one, and periods of “reverse” polarity when the measured field is in the opposite direction. It is clear that the inversions are neither regular nor of equal duration. The time intervals corresponding to a stable polarity (normal or reverse) are called chrons and are numbered starting from the current one (C0). The duration of each chron is extremely variable, ranging from less than a million years to several tens of millions of years (we then speak of a superchron). Their frequency is also very chaotic. The magnetic field has thus reversed around 300 times over the past 200 million years. The last reversal took place 773,000 years ago.
The origin of reversals in the Earth’s magnetic field
Thanks to the measurements, we observe that the poles are not immobile and move. Over the past twenty years, the North Pole has thus migrated several hundred kilometers and is currently moving about 55 km/year. On the other hand, the South Pole moves only 10 km/year. These variabilities, which mean that the two magnetic poles are not necessarily located at antipodes, are linked to the complex dynamics of the magnetic field, which is not only dipolar, but has multipolar components. This multipolarity is linked to the movements of liquid iron within the Earth’s core. Disturbances in the core can modify the convective structures and cause secondary magnetic loops to appear which are added to the main dipole. This multipole component would be able to temporarily weaken the Earth’s dipole, leading either to a complete reversal of the magnetic field, or to a reestablishment of the poles to their original position after a period of “excursion”.
Just before a reversal, the magnetic poles therefore seem to follow complex and sinuous trajectories on the surface of the Earth, in association with a drastic reduction in the intensity of the magnetic field, but without this disappearing completely. However, the precise mechanisms associated with the reversals of the magnetic field are still poorly understood, and in particular the origin of the disturbances in the Earth’s core. It would seem in all cases that a reversal takes place over a relatively short time interval, from 1,000 to 20,000 years at the most.
Is the current acceleration of the movement of the magnetic North Pole therefore synonymous with a short-term reversal? Nothing is less certain, the South Pole remaining relatively stable. The current state of our knowledge does not allow us in any case to predict when the next excursion or inversion will take place.
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