Though the process can take nearly 5,000 years, the earth's magnetic field periodically reverses. According to a report published today in Nature, scientists may have detected the beginning of the field's next such reversal.

Motion of the earth's liquid core, the so-called geodynamo, generates its magnetic field. Gauthier Hulot of the Institut de Physique du Globe de Paris and his colleagues used satellite data recorded 20 years apart to track changes in this field. In two regions of the boundary between the earth's core and the overlying mantle, the researchers detected a reversed magnetic field. In a section lying beneath the southern tip of Africa, the magnetic field points toward the center of the earthopposite to the dominant outward-pointing field of the Southern Hemisphere. And a second congregation of reversed-flux patches exists near the North Pole. Having modeled the growth and movement of these inverted-flux sections, they can now account for nearly the entire decrease in the main dipole field of the earth over the past 150 years.

The new findings reveal variations in the earth's magnetic field over the shortest time scales yet but are far from straightforward. According to Peter Olson of Johns Hopkins University, the "results confirm some long-held tenets of dynamo theorybut contradict others." He cautions in a commentary accompanying the report that it remains too early to tell if the planet is in the early stage of a polarity reversal. "But the rapidly evolving reversed-flux patches suggest that an attempt at reversal may be under way," he adds. Indeed, the study authors assert that if "this asymmetric state was reached often in the past, it might account for several persistent patterns observed in the palaeomagnetic field."