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How Does Earth’s Magnetic Field Reverse?

Por: | 07 de abril de 2014

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By François Petrelis, Ecole Normale Supérieure

Earth’s magnetic field has been used for a long time to find north using a compass. As a good approximation, Earth’s magnetic field can be visualised as a magnetic dipole, with the South Magnetic Pole presently located near Earth’s geographic North Pole, and the North Magnetic Pole near the Earth’s geographic South Pole. This magnetic field is generated by the flow of liquid metal in the earth’s liquid core. Since this liquid conducts electricity (it is, in part, molten iron), it is able to generate currents and thus a magnetic field at the expense of some of the energy of its motion — this is the dynamo effect.

Since the early works of Bernard Brunhes, a French researcher at the beginning of the 20th century, scientists have know that Earth’s magnetic field reverses at irregular intervals, ranging from tens of thousands to millions of years. This has been determined, for instance, by the study of volcanic basalt that records the Earth's magnetic field at the time the rock freezes, a record that acts much like a tape recorder. With this data, scientists estimate that the last magnetic field reversal occurred about 780,000 years ago.

A few detailed properties of reversals are also known. A reversal occurs in two phases. A first phase, during which the dipole amplitude decreases slowly, seems to last around 50,000 years. The second phase is faster — 10,000 years are required for the dipole to recover with the opposite polarity. Geophysicists have also identified events during which the magnetic field first decreases as for a reversal, but it does not reverse — rather it increases again with the same polarity. This is an aborted reversal and is sometimes called an excursion.

Although volcanic basalt records when reversals occurred, it is much more difficult to find out why or how Earth’s magnetic field reverses. Numerical simulations are of little help because existing computational power is still far too limited to be able to simulate Earth’s liquid core in a realistic parameter range.

In a recent study, performed with colleagues form the Ecole Normale Supérieure and the Institut de Physique du Globe, both in Paris, we have proposed a general mechanism that provides a simple explanation for field reversals.

We have shown that if the dipolar field of the Earth is coupled to another magnetic mode (a quadrupolar field, for instance), this coupling provides a path for the dipole to flip to its opposite. If this coupling is strong enough, the magnetic field will spontaneously oscillate between the two modes and their opposite polarities. We will then observe periodic reversals of the magnetic field. We believe that a similar process is involved in the case of the solar magnetic field that oscillates with a period of 22 years. In the case of Earth, the coupling is not strong enough and oscillations are not observed. Velocity fluctuations in the liquid core are then needed to trigger a reversal.

Some properties observed in the paleomagnetic data are simple consequences of this mechanism. For instance: the reversals are predicted to occur in two phases. The first one is slow in contrast to the second one. At the end of the first phase, the mechanism predicts that the magnetic field can reverse, but it can also increase in strength with the same polarity — an excursion.

Other predictions can be made. Fluctuations in the flow of the liquid core do not switch off the magnetic field and then regenerate it with the opposite polarity. Rather, the dipolar field continuously changes shape during a reversal because the amplitude of the other mode (the quadrupole, for instance) continuously increases, whereas the dipole decreases. When the dipolar component vanishes, it starts to increase again with the opposite polarity, whereas the amplitude of the other mode decreases. If such a detailed behaviour could be identified in paleomagnetic data, that would be of great interest in particular to identify the spatial structure of the second mode.

It is worth pointing out that the total magnetic field does not vanish during reversals, only its geometry is modified. This implies that Earth remains shielded against cosmic rays during reversals, which helps to explain why life has survived so many magnetic field reversals.

François Petrelis
Ecole Normale Supérieure
www.atomiumculture.eu

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