Probing the Geodynamo [Preview]

Scientists have wondered why the polarity of Earth's magnetic field occasionally reverses. Recent studies of fer intriguing clues about how the next reversal may begin

Symptoms of a possible reversal inducing change came to light when another group analyzed the Magsat and Oersted satellite maps. Gauthier Hulot and his colleagues at the Geophysical Institute in Paris noticed that sustained variations of the geomagnetic field come from places on the core-mantle boundary where the direction of the flux is opposite of what is normal for that hemisphere. The largest of these so-called reversed flux patches stretches from under the southern tip of Africa westward to the southern tip of South America. In this patch, the magnetic flux is inward, toward the core, whereas most of the flux in the Southern Hemisphere is outward.

Patch Production
ONE OF THE MOST significant conclusions that investigators drew by comparing the recent Oersted magnetic measurements with those from 1980 was that new reversed flux patches continue to form on the core-mantle boundary, under the east coast of North America and the Arctic, for example. What is more, the older patches have grown and moved slightly toward the poles. In the late 1980s David Gubbins of the University of Leeds in England--using cruder, older maps of the magnetic field--noticed that the proliferation, growth and poleward migration of these reversed flux patches account for the historical decline of the dipole.

Such observations can be explained physically by using the concept of magnetic lines of force (in actuality, the field is continuous in space). We can think of these lines of force as being "frozen" in the fluid iron core so that they tend to follow its motion, like a filament of dye swirling in a glass of water when stirred. In Earth's core, because of the Coriolis effect, eddies and vortices in the fluid twist magnetic lines of force into bundles that look somewhat like piles of spaghetti. Each twist packs more lines of force into the core, thereby increasing the energy in the magnetic field. (If this process were to go on unchecked, the magnetic field would grow stronger indefinitely. But electrical resistance tends to diffuse and smooth out the twists in the magnetic field lines enough to suppress runaway growth of the magnetic field without killing the dynamo.)

Patches of intense magnetic flux, both normal and reversed, form on the core-mantle boundary when eddies and vortices interact with east-west-directed magnetic fields, described as toroidal, that are submerged within the core. These turbulent fluid motions can bend and twist the toroidal field lines into loops called poloidal fields, which have a north-south orientation. Sometimes the bending is caused by the rising fluid in an upwelling. If the upwelling is strong enough, the top of the poloidal loop is expelled from the core [see box below]. This expulsion creates a pair of flux patches where the ends of the loop cross the core-mantle boundary. One of these patches has normally directed flux (in the same direction as the overall dipole field in that hemisphere); the other has the opposite, or reversed, flux.

When the twist causes the reversed flux patch to lie closer to the geographic pole than the normal flux patch, the result is a weakening of the dipole, which is most sensitive to changes near its poles. Indeed, this describes the current situation with the reversed flux patch below the southern tip of Africa. For an actual planetwide polarity reversal to occur, such a reversed flux patch would grow and engulf the entire polar region; at the same time, a similar change in overall regional magnetic polarity would take place near the other geographic pole.

Supercomputer Simulations
TO FURTHER INVESTIGATE how reversed flux patches develop and how they may signal the onset of the next polarity reversal, researchers simulate the geodynamo on supercomputers and in laboratories. The modern era of computer dynamo simulations began in 1995, when three groups--Akira Kageyama, now of JAMSTEC, and his co-workers; Paul H. Roberts of UCLA and one of us (Glatzmaier); and Christopher A. Jones of the University of Exeter in England and his colleagues--independently developed numerical simulations that generated magnetic fields resembling the magnetic field at Earth's surface. Since then, simulations representing hundreds of thousands of years have demonstrated how convection can indeed produce patches of reversed magnetic flux on the core-mantle boundary--just like those seen in the satellite images. These patches come and go in computer simulations, but sometimes they lead to spontaneous magnetic dipole reversals.

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