Inconstant Field

By Carl Zimmer|Tuesday, February 01, 1994
Swear not by magnetic north--it drifts, takes long excursions, and sometimes even heads south. Earth’s patchy core may be to blame.

Although it has long served as a fixed reference for navigators, Earth’s magnetic field is anything but static. Over the course of decades and centuries, in what is called secular variation, the pattern of the field drifts randomly, such that at a given geographic location the direction a compass needle points may change by tens of degrees. Every 30,000 years or so things get more extreme: the magnetic poles suddenly begin shifting toward the equator, only to snap back into place. And changes even more radical than these geomagnetic excursions happen every few hundred thousand years, when the field flips completely upside down-- north becoming south and vice versa. Some geophysicists think we may be in for such a polar reversal soon; over the past 2,000 years the whole magnetic field seems to have been weakening steadily, which may be the prelude to a flip.

In the past, researchers have often tried to explain these various changes separately--and with limited success. Now, however, a California geophysicist has put forward a theory that makes these events different faces of the same phenomenon: the waxing and waning of patches of magnetism on the surface of Earth’s core.

Everyone agrees that the spinning core is the dynamo that generates the geomagnetic field. While the center of Earth’s core is solid, the outer 1,500 miles are made of liquid metals, mostly iron. Hot globs of metal rise to the core’s outer surface (its boundary with the rocky mantle), where they cool and sink back toward the center of the planet. The movement of the metal creates an electric current. As Earth rotates, the movement of the current generates a magnetic field shaped roughly like that of a bar magnet: lines of force sprout out of the planet near the South Pole and loop back toward the North Pole.

The magnetic field on the surface of Earth is thus a product of the field on the surface of the core. Working backward from measurements of the field at the surface of the planet, geophysicists in the 1980s were able to map out these buried patterns. In most of the Southern Hemisphere the field lines flow out of the core, as one would expect, while in the Northern Hemisphere they flow in. But the geophysicists also uncovered several huge patches on the surface of the core with the wrong magnetic polarity--areas in the Southern Hemisphere, say, where the field lines pointed inward. Using magnetic field measurements from the past 400 years, the researchers then showed that core patches change: they are born, grow, combine, split, and die.

When the patches change, the magnetic field at the surface changes as well. For example, it now seems clear that the reason the field has been weakening is that several patches with the wrong orientation have been growing. In the process they have been canceling out a progressively greater amount of the overall magnetic field.

But Kenneth Verosub, a geophysicist at the University of California at Davis, thinks the patches are responsible for the other types of field changes as well. The continual birth and death of relatively small patches, he says, could produce the fairly small and short-term fluctuations of secular variation. And every so often, Verosub says, you could get one patch that grows to be very large, or several coalescing into a big one--and that would be a geomagnetic excursion. Then the patch simply might disperse or shrink, and you’d go back to the same polarity. This way you can explain excursions as large-scale secular variations.

Moreover, an excursion-creating patch might not always die. A big patch in one hemisphere would tend to be balanced by a patch of opposite polarity in the other hemisphere. If these patches continued to grow rather than shrink--if they became the patches that ate Chicago, as Verosub calls them--then they could consume their respective hemispheres. At that point north would become south: the field would flip. Secular variations, excursions, and reversals are all on one continuum, says Verosub.

Support for this idea comes from a computer model he has developed with mathematicians Gerry Puckett and Igor Aleinov, also at the University of California at Davis. The three researchers divided the surface of the core into 20,000 triangles and gave each triangle the magnetic orientation needed to reproduce the patches as they now exist. Then they let the triangles evolve according to a few simple rules--rules derived in part from basic physical laws about magnetism but mostly from observations of how real core patches behave.

At each time interval, each triangle had a chance of flipping directions. That probability was determined by the triangle’s neighbors--a north triangle surrounded by south patches would be likely to flip to south. But since the historical record shows patches emerging in the middle of broad expanses of opposite magnetic flux, Verosub and his colleagues included a rule stating that there was always a small chance that a triangle would flip no matter what its surroundings. To find the direction and strength of Earth’s field at a given time, the computer simply adds up the effects of all the patches.

Simple as the model is, its results resemble the real thing. The magnetic poles wiggle gently most of the time as patches halfheartedly form and die. Occasionally a big patch is born, creating a geomagnetic excursion, but most often it too vanishes. Once in a blue moon, however, a truly enormous patch establishes itself, and the whole field flips with amazing speed. All of a sudden it avalanches into a reversed state, Verosub says. Most compelling to him is the fact that the reversals, excursions, and variations all happen with a frequency that matches Earth’s actual statistics.

If patches create the magnetic field, then what creates the patches and causes them to behave the way they do? Neither Verosub nor any other geophysicist really knows--in part because the conditions of the core, with its tremendous heat and pressure, are all but impossible to reproduce in a lab. Verosub’s model is based more on a statistical description of the core than on physical understanding. The problem now is to put some more physics into what is a simple model, he admits. Our argument is, you use this kind of model where you don’t understand the physics, and the model helps you figure it out.
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