Magnetic Flip-Flops
Considering that ships, planes and Boy Scouts steer by it, Earth's magnetic field is less reliable than you'd think. Rocks in an ancient lava flow in Oregon suggest that for a brief erratic span about 16 million years ago magnetic north shifted as much as 6 degrees per day. After little more than a week, a compass needle would have pointed toward Mexico City.
The lava catches Earth's magnetic field in the act of reversing itself. Magnetic north heads south, and -- over about 1,000 years -- the field does a complete flip-flop. While the Oregon data is controversial, Earth scientists agree that the geological evidence as a whole -- the "paleomagnetic" record -- proves such reversals happened many times over the past billion years.
"Some reversals occurred within a few 10,000 years of each other," says Los Alamos scientist Gary Glatzmaier, "and there are other periods where no reversals occurred for tens of millions of years." How do these flip-flops happen, and why at such irregular intervals? The geological data, invaluable to show what happened, registers only a mute shrug when it comes to the deeper questions.
For that matter, why is it that instead of quietly fading away, as magnetic fields do when left to their own devices, Earth's magnetic field is still going strong after billions of years? Einstein is said to have considered it one of the most important unsolved problems in physics. With a year of computing on Pittsburgh's CRAY C90, 2,000 hours of processing, Glatzmaier and collaborator Paul Roberts of UCLA took a big step toward some answers. Their numerical model of the electromagnetic, fluid dynamical processes of Earth's interior reproduced key features of the magnetic field over more than 40,000 years of simulated time. To top it off, the computer-generated field reversed itself.
"We weren't expecting it," says Roberts, "and were delighted. This gives us confidence we've built a credible bridge between theory and the paleomagnetic data." Their surprising results, reported as a cover story in Nature (Sept. 21, 1995), provide an inner-Earth view of geomagnetic phenomena that have not been observed or anticipated by theory. Furthermore, the Glatzmaier-Roberts model offers, for the first time, a coherent explanation of magnetic field reversal.
Journey to the Center of the Earth
Roughly speaking, Earth is like a chocolate-covered cherry -- layered, with liquid beneath the surface and a solid inner core. Beneath the planet's relatively thin crust is a thick, solid layer called the mantle. Between the mantle and the inner core is a fluid layer, the outer core. According to generally accepted theory -- the dynamo theory -- interactions between the churning, twisting flow of molten material in the outer core and the magnetic field generate electrical current that, in turn, creates new magnetic energy that sustains the field. "The typical lifetime of a magnetic field like Earth's," says Glatzmaier, "is several tens of thousands of years. The fact that it's existed for billions of years means something must be regenerating it all the time."
How do we know if the dynamo theory is right? To the consternation of our desire to understand what's happening inside the planet we live on, Jules Verne's Journey to the Center of the Earth is still fiction. There's no way to penetrate 4,000 miles to Earth's center, nor to monitor fluid motions or magnetism in the outer core.
The Glatzmaier-Roberts computational model may be the next best thing to a guided tour of inner Earth. While other models have given good clues that theory is on track, they have been limited by a two-dimensional approach that required simplifying assumptions. Roberts and Glatzmaier set out to implement a fully three-dimensional model, based on a computer program Glatzmaier developed over many years, that would allow the complex feedbacks between fluid motion and the magnetic field to evolve on their own -- in other words, to be solved "self consistently."
