Multipolar dance could flip Earth’s magnetic field
Apr 24, 2009 10 comments
Researchers in France have developed a new model of Earth’s magnetic field that includes a simple explanation for why it has flipped direction many times throughout Earth history.
Most geophysicists agree that the main component of Earth’s magnetic field is generated by convection currents in the molten iron of the planet’s core. This dipole field — which defines the Earth’s magnetic poles — has reversed polarity tens of thousand of times in the past. We know this because ancient field configurations are “frozen” into the rocks, as magnetic particles align with field lines.
Various theories have been put forward to explain the reversal mechanism but they are usually highly complex, where random variations in the flow of the liquid core are the main triggers.
A meeting of magnets
Now, François Pétrélis and his colleagues of the École Normale Supérieure and Institut de Physique du Globe have simplified the problem by reducing the Earth’s field to a set of basic equations, which show strong agreement with the predictions of more complicated models. They did this by focussing on the interplay between the dipole and and quadrupole components of the field.
“We are proposing that reversals result from the competition between the dipolar mode and a second, unstable, dynamo mode,” said Petrelis.
A geomagnetic field reversal takes approximately 10, 000 years — a very short period on a geological timescale — during which time, the field drops to approximately 10 percent of its normal intensity. In previous models, fluctuations in the flow of molten iron “switch off” the main dipole component and then regenerate it with the opposite polarity.
A deep-Earth salsa dance
In this new proposal, it is not the turbulent flow but a second field known as a “quadrupole” that drives the reversal. Leading up to the flipping of the poles, the dipole reduces in intensity as the quadrupole grows in strength. Once the dipole component has vanished it can start to grow again, as the quadrupole now drops in strength.
“This is a study — like a number of others — in the spirit of replacing a very complicated physical system by a highly simplified low-dimensional set of equations,” said Ulrich Christensen, a geophysicist at the Max Plank Institute for solar System Research in Germany.
Petrelis told physicsworld.com that direct measurements of the Earth’s magnetic field suggest that we are not due a reversal any time soon.
This research is published in Physical Review Letters.
About the author
James Dacey is a reporter for physicsworld.com