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

Computer-generated polarity reversals provided researchers with the first rudimentary glimpse of how such switches may originate and progress [see box on page 34]. One three-dimensional simulation--which had to run for 12 hours a day every day for more than a year to simulate 300,000 years--depicted the onset of a reversal as a decrease in the intensity of the dipole field. Several patches of reversed magnetic flux, such as those now forming on the core-mantle boundary, then began to appear. But rather than extinguishing the magnetic field completely, the reversed flux patches created a weak field with a complex mix of polarities during the transition.

Viewed at the surface of the model Earth, the reversal of the dipole occurred when the reversed flux patches begin to dominate the original polarity on the core-mantle boundary. In total, it took about 9,000 years for the old polarity to dissipate and for the new polarity to take hold throughout the core.

What Might Be Missing
BASED IN PART on these successes, computer dynamo models are proliferating rapidly. At last count, more than a dozen groups worldwide were using them to help understand magnetic fields that occur in objects throughout the solar system and beyond. But how well do the geodynamo models capture the dynamo as it actually exists in Earth? The truth is that no one knows for certain.

No computer dynamo model has yet simulated the broad spectrum of turbulence that exists in a planetary interior, primarily because massively parallel supercomputers are not yet fast enough to accurately simulate magnetic turbulence with realistic physical parameters in three dimensions. The smallest turbulent eddies and vortices in Earth's core that twist the magnetic field probably occur on a scale of meters to tens of meters, much less than what can be resolved with the current global geodynamo models on the current supercomputers. That means that all 3-D computer models of the geodynamo so far have simulated the simple, large-scale flow of laminar convection, akin to the hot mineral oil rising through a lava lamp.

To simulate the effects of turbulence in laminar models, investigators have used unrealistically large values for the fluid viscosity. To achieve realistic turbulence in a computer model, researchers must resort to a two-dimensional view. The trade-off is that 2-D flow cannot sustain a dynamo. These models do, however, suggest that the laminar flows seen in current geodynamo simulations are much smoother and simpler than the turbulent flows that most likely exist in Earths core.

Probably the most significant difference is in the paths the fluid follows as it rises through the core. In simple laminar convection simulations, large plumes stretch all the way from the bottom of the core to the top. In the turbulent 2-D models, on the other hand, convection is marked by multiple small-scale plumes and vortices that detach near the upper and lower boundaries of the core and then interact within the main part of the convection zone.

Such differences in the patterns of fluid flow could have a huge influence on the structure of Earth's magnetic field and the time it takes various changes to occur. That is why investigators are diligently pursuing the next generation of 3-D models. Someday, maybe a decade from now, advances in computer processing speeds will make it possible to produce strongly turbulent dynamo simulations. Until then, we hope to learn more from laboratory dynamo experiments now under way.

Laboratory Dynamos
A GOOD way to improve understanding of the geodynamo would be to compare computer dynamos (which lack turbulence) with laboratory dynamos (which lack convection). Scientists had first demonstrated the feasibility of lab-scale dynamos in the 1960s, but the road to success was long. The vast difference in size between a laboratory apparatus and the actual core of a planet was a vital factor. A self-sustaining fluid dynamo requires that a certain dimensionless parameter, called the magnetic Reynolds number, exceed a minimum numerical value, roughly 10.

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