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See Inside Our Ever Changing Earth

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

Earths core has a large magnetic Reynolds number, probably around 1,000, primarily because it has a large linear dimension (the radius of the core is about 3,485 kilometers). Simply put, it is very difficult to create a large magnetic Reynolds number in small volumes of fluid unless you can move the fluid at extremely high velocities.

The decades-old dream of generating a spontaneous magnetic field in a laboratory fluid dynamo was first realized in 2000, when two groups--one led by Agris Gailitis of the University of Latvia and one by Robert Stieglitz and Ulrich Müller of the Karlsruhe Research Center and Fritz Busse of the University of Bayreuth, both in Germany--independently achieved self-generation in large volumes of liquid sodium. (Liquid sodium was used because of its high electrical conductivity and low melting point.) Both groups found ways to achieve high-speed fluid flow in a system of one- to two-meter-long helical pipes, resulting in the critical magnetic Reynolds number of about 10.

These experimental results support the theory, which gives us a measure of confidence when we apply our theoretical ideas about dynamos to Earth and other planets. In labs across the world--at the University of Grenoble in France, the University of Maryland, the University of WisconsinMadison and the New Mexico Institute of Mining and Technology--scientists are developing the next generation of lab dynamos. To better simulate Earth-like geometry, these experiments will stir the liquid sodium inside massive spherical chambers--the largest nearly three meters in diameter.

Besides the ongoing plans for more realistic laboratory dynamos and 3-D computer simulations, the international satellite CHAMP (short for Challenging Minisatellite Payload) is charting the geomagnetic field with enough precision to directly measure its changes at the core-mantle boundary in real time. Investigators anticipate this satellite will provide a continuous image of the geomagnetic field over its five-year mission, allowing them to watch for continued growth of the reversed flux patches as well as other clues about how the dipole field is waning.

We expect that a synthesis of these three new approaches--satellite observations, computer simulations and lab experiments--will occur in the next decade or two. With a more complete picture of the extraordinary geodynamo, we will learn whether our current ideas about the magnetic field and its reversals are on the right track.

THE AUTHOR
GARY A. GLATZMAIER and PETER OLSON develop computer models to study the structure and dynamics of the interiors of planets and stars. In the mid-1990s Glatzmaier, then at the Institute of Geophysics and Planetary Physics (IGPP) at Los Alamos National Laboratory, created (together with Paul H. Roberts of the University of California, Los Angeles) the first geodynamo simulation that produced a spontaneous magnetic dipole reversal. Glatzmaier has been a professor in the department of earth sciences and IGPP at the University of California, Santa Cruz, since 1998. Olson is particularly interested in how Earths core and mantle interact to produce geomagnetic fields, plate tectonics and deep mantle plumes. He joined the department of earth and planetary sciences at Johns Hopkins University in 1978, where he has introduced geophysics to more than 1,000 students.

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