Reversals of Earth's Magnetic Field Explained by Small Core Fluctuations

[Joanne98]

Although volcanic basalt reveals when reversals occurred, it’s much more difficult to find evidence for why or how the Earth’s magnetic field reverses. In a recent study, scientists from the Ecole Normale Supérieure and the Institut de Physique du Globe de Paris, both in Paris, have proposed a general mechanism that provides a simple explanation for field reversals. In their model, small fluctuations in convective flow in Earth’s core can push the planet’s sensitive magnetic system away from one pole toward an intermediate state, where the system becomes attracted to the opposite pole.

“We have found a mechanism that gives simple explanations of many features of the reversals of Earth’s magnetic field,” François Pétrélis of Ecole Normale Supérieure told PhysOrg.com. “In particular, it explains the existence and the shape (slow phase followed by fast phase) of reversals, the existence and the shape of aborted reversals (‘excursions’), the statistical properties of reversals, and the possibility for very long durations without reversals (‘superchrons’).”

At present times, the Earth’s magnetic field can be described as a magnetic dipole, with the magnetic south pole currently located near the Earth’s geographic north pole, and the magnetic north pole near the geographic south pole (both magnetic poles are misaligned along the Earth’s rotational axis by about 11.3 degrees). The existence of such a long-lived magnetic field can be explained by dynamo theory, which describes how a convective, electrically conducting fluid that rotates can maintain a magnetic field.

As the scientists suggest, the reversal mechanism relies on the existence of a second magnetic mode, in addition to the dipolar field. The presence of a second mode, such as a quadrupolar field, can have significant effects on how the magnetic system reacts to changes in equatorial symmetry. As the researchers explain, the equator can be thought of as a plane of symmetry, and the convective flow in the Earth’s outer core is usually north-south symmetric. Previous studies on paleomagnetic data have proposed that reversals involve an interaction between the dipolar and quadrupolar modes, which would correlate with changes in equatorial symmetry. In support of this idea, some recent numerical simulations have shown that reversals do not occur when the convective flow remains equatorially symmetric.

http://www.physorg.com/news159704651.html