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Tackles one of the world’s toughest problems: the generation of Earth’s magnetic field. The field arises from the planet’s fluid outer core. There, according to the prevailing picture, convection and rotation drive a dynamo that generates and sustains the field. Because the underlying equations are nonlinear and complex, modeling any magnetohydrody-namical system is tough. Earth’s dynamo poses an additional challenge: Many of the features that geophysicists want to understand, such as the field’s sporadic reversals of polarity, occur over tens of millennia, with the fluid’s viscosity close to that of water. To work at all, computer models of Earth’s dynamo must artificially raise the viscosity. The first comprehensive model, published 10 years ago, relied on a boost factor of a million and yielded one tentative field reversal (see Physics Today, January 1996, page 17). Now, using Japan’s powerful Earth Simulator computer (shown in the photo), Futoshi Takahashi of...