In the 1960s Stanislav Braginsky, now at the University of California at Los Angeles, suggested that heat escaping from the upper core also causes the solid inner core to grow larger, producing two extra sources of buoyancy to drive convection. As liquid iron solidifies into crystals onto the outside of the solid inner core, latent heat is released as a by-product. This heat contributes to thermal buoyancy. In addition, less dense chemical compounds, such as iron sulfide and iron oxide, are excluded from the inner core crystals and rise through the outer core, also enhancing convection.
For a self-sustaining magnetic field to materialize from a planet, a third factor is necessary: rotation. Earth's rotation, through the Coriolis effect, deflects rising fluids inside Earths core the same way it twists ocean currents and tropical storms into the familiar spirals we see in weather satellite images. In the core, Coriolis forces deflect the upwelling fluid along corkscrewlike, or helical, paths.
That Earth has an iron-rich liquid outer core, sufficient energy to drive convection and a Coriolis force to twist the convecting fluid are primary reasons why the geodynamo has sustained itself for billions of years. But scientists need additional evidence to answer the puzzling questions about the magnetic field that emerges--and why it would change polarity over time.
Magnetic Field Maps
A MAJOR DISCOVERY unfolded over the past five years as it became possible for scientists to compare accurate maps of the geomagnetic field taken 20 years apart. A satellite called Magsat measured the geomagnetic field above Earths surface in 1980; a second satellite--Oersted--has been doing the same since 1999 [see illustration on page 33]. These satellite measurements provide an image of the magnetic field down to the level of the core-mantle boundary. But because of strong electric currents within the core, researchers cannot image the much more complicated and intense magnetic field inside the core, where the magnetic fluctuations originate. Despite the inherent limitations, several noteworthy observations came out of these efforts, including hints about the possible onset of a new polarity reversal.
Although the geodynamo produces a very intense magnetic field, only about 1 percent of the field's magnetic energy extends outside the core. When measured at the surface, the dominant structure of this field is called the dipole, which most of the time is roughly aligned with Earth's axis of rotation. Like a simple bar magnet, this fields primary magnetic flux is directed out from the core in the Southern Hemisphere and down toward the core in the Northern Hemisphere. (Compass needles point to Earth's north geographic pole because the dipoles south magnetic pole lies near it.) But the satellite missions revealed that the flux is not distributed evenly across the globe. Instead most of the dipole fields overall intensity originates beneath North America, Siberia and the coast of Antarctica.
Ulrich R. Christensen of the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany, suspects that these large patches come and go over thousands of years and stem from the ever evolving pattern of convection within the core. Might a similar phenomenon be the cause of dipole reversals? Evidence from the geologic rec-ord shows that past reversals occurred over relatively short periods, approximately 4,000 to 10,000 years. It would take the dipole about 100,000 years to disappear on its own if the geodynamo were to shut down. Such a quick transition implies that some kind of instability destroys the original polarity while generating the new polarity.
In the case of individual reversals, this mysterious instability is probably some kind of chaotic change in the structure of the flow that only occasionally succeeds in reversing the global dipole. Geoscientists who study the paleomagnetic record, such as Lisa Tauxe of the University of California at San Diego, find that dipole intensity fluctuations are common but that reversals are rare. The epochs between reversals vary in length from tens of thousands to tens of millions of years [see illustration on page 35].