Insight into a magnetic mystery
Earth's magnetic field has long confounded scientists. New data give
Thousands of years ago, humans first noted evidence of Earth's magnetic force when iron ore rocks seemed to move of their own accord. In this century, man discovered that Earth's magnetic field protects life by warding off damaging cosmic rays and solar radiation.
But a deeper understanding of the origin of Earth's magnetic field - a conundrum that Albert Einstein classified as one of the great unsolved problems of physics - has persistently eluded scientists. Today, with the help of supercomputers and an 800,000-year historical record of Earth's magnetic field released this week, researchers are closer than ever to unraveling the mystery.
With this data might come clues to climate variations, extinction patterns, fluctuations in the strength of the magnetosphere, and other events that could link the activity in Earth's core with conditions on the surface and in the atmosphere.
"We are getting computer simulations that create magnetic fields that are fairly similar to that of the earth," says Gary Glatzmaier, a professor of earth sciences at the University of California at Santa Cruz. "It has taken many years to come up with a numerical method to solve the equations that describe the magnetism, but we are close."
Tracing Earth's past
Earth's magnetic field can be traced back at least 3 billion years. Its source is Earth's iron-rich superheated core - a moon-size solid inner core and a fluid outer core. Yet several things about the magnetic field have confounded scientists:
*Because of the core's high temperature and its electrical conductivity, scientists had calculated that the magnetism of Earth should have dissipated after only 20,000 years.
*Older models of Earth's magnetic field could not account for why the planet's magnetic field flip-flops from pole to pole every 200,000 years on average.
*Scientists could not understand why the planet's magnetic field is concentrated at the poles and not more evenly distributed.
These discrepancies led to the conclusion that there must be some mechanism inside Earth that regenerates and redirects the magnetic force and causes the polar reversals. Scientists theorized that an interior convection system was driving a "magnetic dynamo" in Earth's core.
In short, heat transfer from the inner core to the outer core - and changes in the chemical composition of the outer core - cause the liquid portion to swirl and move.
"You are converting kinetic energy from fluid energy into magnetic energy," says Dr. Glatzmaier. "But it's not obvious why the earth's magnetic field is as intense as it is, or why it looks so dipolar, or why it undergoes field reversals occasionally."
During the past four years, computer models have begun to unravel some of these mysteries. Until this decade, the computational power needed to create such models was not widely available. Then, in 1995, Glatzmaier and Paul Roberts, a professor at the University of California at Los Angeles, used supercomputers to create the first physical models of Earth's geodynamo that were able to mathematically sustain a magnetic field.
According to the models, the inner core rotates faster than the outer core and the mantle. This helps explain the strength of the magnetic field - it would act like a dynamo on an electric motor that spins to create electrical charge.
The Glatzmaier-Roberts models also showed that the magnetic field tended to form a cylindrical pattern about Earth's north-south axis with opposing charges at either end, explaining the dipolar magnetic field of Earth.
Finally, the models indicated that the fluid motion in the outer core actually twists and shears the magnetic field, generating a new magnetic field to replace the parts of the field that diffuse.
While Glatzmaier and others work with computers, geologists have been collecting samples of sediment from deep inside the planet in an attempt to answer the same questions about Earth's magnetic field.
Iron-rich minerals in sediments indicate the polar direction and the magnetic intensity of the field through their north-south alignment, like the needle of a compass. By determining how old the sediments are and matching that to the magnetic direction and strength of the field, scientists can re-create a history of Earth's magnetic field.
This week in the journal Nature, a team of researchers unveiled a history of Earth's magnetic field stretching back 800,000 years. Pieced together from 33 sediment reports, it goes back far enough to cover the last polar reversal 780,000 years ago.
Weak magnetic field
The history indicates that Earth's magnetic field declines dramatically over several thousand years preceding a polar reversal. "During reversals at the lowest part of the curve, the magnetic field is so low that some people believe that the earth is not protected by the magnetosphere during this time," says Jean Pierre Valet, a geologist and one of the authors of the paper.
Yet no correlation has been established between reductions in the magnetosphere and mass extinctions as a result of exposure to the solar wind, Dr. Valet says. Nor have any climate variations like the onset of ice ages been conclusively connected to changes in the magnetic field.
But drastic changes in the field would certainly affect our physical surroundings. In fact, they may already be changing. Scientists have noted that Earth's magnetic field has been declining for the past 150 years. This has led some to speculate that a field reversal is under way.
"While the earth's magnetic field is reversing, the magnetosphere and ionosphere [the electrically conducting level of the atmosphere] would be different from what they are today," says Glatzmaier.