Earth's magnetic field could be gearing up for a flip, sending magnetic north to new digs in Antarctica, a study suggests. But don't throw out your compass just yet--the change, if it happens at all, will probably take a few thousand years.
Although such a reversal has never been recorded by humans, switches have occurred many times in Earth's past. Little is known about why this happens. But researchers have suspected for years that currents of molten iron circulating in Earth's outer core (creeping at about 1 meter per hour) set up the opposite magnetic poles at the antipodes. And computer models hint that vortices in the molten flow that swirl in a direction that weakens the magnetic field might begin the pole-flipping process.
Now, satellite observations have turned up evidence that these subterranean vortices do in fact exist. By comparing the strength and orientation of the magnetic field measured by two satellites in 1980 and 2000, researchers at the Physics of the Globe Institute of Paris (IPGP) and the Danish Space Research Institute in Copenhagen were able to plot the currents of molten iron that create the magnetic dipole. In an 11 April letter to Nature, the team describes large whorls off the southern tip of Africa and near the poles--areas where the magnetic field has already flipped.
Physicist Gauthier Hulot, a team member at IPGP, suggests that the vortices in the molten iron rotate in a direction that reinforces a reverse magnetic field. As these vortices grow and proliferate, they weaken the dominant field; that could account for observations that the dipole has waned over the last 150 years, Hulot says. If the trend continues--which isn't certain--the current situation could represent the first steps toward a new polarity, he says.
Geophysicists are gratified to see the predictions of their computer models borne out, says geophysicist Peter Olson of Johns Hopkins University in Baltimore, Maryland. But he cautions that the magnetic field has been fickle in the past, and the trend toward reversal could reverse itself at any time.