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Tracking Earth's Mysterious Magnetic Field

By Robert Roy Britt, Senior Science Writer, space.com

28th Oct 1999

The force fields that surround all respectable intergalactic space ships are wimpy imposters of what Nature created long ago, namely the protective magnetic shield around our planet, fueled by mysterious processes deep within. Until recently, we've known nearly as much about the source of the force fields around fictional star ships as we've known about the natural one that envelopes Earth. But advances in computer modeling, discussed in tomorrow's issue of the journal Nature, are helping scientists grasp how the planet continually regenerates its invisible, protective magnetic shield, and the work confirms earlier suggestions that the Earth's mantle indeed plays a role in fueling the magnetism. Further, the new simulations provide fresh insight into how the magnetic north and south poles might migrate and, over time, even trade places. Earth, the Giant Magnet Magnetic energy generated in the Earth's core results in a geomagnetic field, the thing that makes compass navigation possible while at the same time deflecting and absorbing harmful solar radiation. The solid inner core, mostly iron, is surrounded by a more fluid outer core, which is mostly molten iron. The interaction of these two regions, in which material flows at different rates, creates what scientists call a "hydromagnetic dynamo," something like an electric motor that results in a magnetic field akin to a giant bar magnet. Invisible geomagnetic lines stretch from one pole, curving far out into space and back to the opposing pole. Left alone, the curved lines might appear like a wire-frame model of some giant pumpkin, but the electrically charged solar wind blows the pumpkin into a teardrop shape. As best as researchers can tell, this protective field has existed in various forms for at least 3 billion years, periodically growing stronger and weaker, shifting around, and on a few occasions even flipping its polarity entirely. Scientists examine old rocks for indications of previous reversals, which leave directional clues in solidified magnetic minerals. At times, the field is thought to weaken to practically nothing, a state that would leave earthlings exposed to high doses of solar radiation. Forces of Change How all this erratic behavior occurs is not well understood. But radical changes seem to take place in as little as a thousand year's time, after which relative stability appears to reign for another 200,000 years or so. One thing is clear, researchers say: If something did not continually regenerate the magnetic field, it would degenerate to zero and our planet would be left naked to the hideous power of solar radiation. Gary A. Glatzmaier of the University of California, Santa Cruz, has some new ideas about how this magnetism is continually reproduced. Scientists have typically assumed that the magnetism was all created in Earth's core, and in fact an earlier rendition of Glatzmaier's computer model -- which he's been working on for two decades -- helped to illustrate that process. But a new version of the simulation indicates that the next layer up, the mantle, can also play a role. Glatzmaier and his colleagues used a tremendous amount of computing power to test a relatively simple idea: Where the core and the mantle meet, varying rates of heat exchange might create changes in the magnetic field. "They note that variations in heat flow at the boundary induce a flow in the core, much like atmospheric circulation driven by differential solar heating of the Earth's surface," says Bruce Buffett of the Department of Earth and Ocean Sciences at the University of British Columbia. "The calculations are still crude approximations of the real Earth, but they lend considerable support to the argument that the mantle has an important role." Buffett, who was not involved in the research, told space.com that one of the most significant results of the work is that Glatzmaier and his colleagues were able to simulate conditions that might cause a magnetic reversal. While much about Earth's magnetism is mysterious, complete reversals of polarity -- and the slightly less dramatic "excursions," or wanderings -- are among the most intriguing of terrestrial puzzles. "The current results show how the temperature patterns on the lower mantle (above the core) can either increase or decrease the probability of magnetic reversals," Glatzmaier said. He notes that by feeding varying temperature differences into the model, the duration of the events and the intensity of the magnetic field were altered. Reversal of Fortune? Earth's magnetic field does not swap directions on any set time frame. While reversals occur every 200,000 years on average, the last one was 780,000 years ago, Glatzmaier said, adding that "we're overdue for one." Excursions of the magnetic field, in which the poles wander significantly but do not reverse, occur more frequently. Because it seems to take centuries for a reversal to occur, researchers agree that the next one is not imminent (from the perspective of a human lifetime, at least). Geologically speaking, however, these things happen in the blink of an eye, and signals currently point to a change. Earth's magnetic field is weakening as you read this, part of a long-term process that might be leading up to a complete magnetic reversal, researchers say. If and when this occurs, the further erosion of the protective magnetic shield will allow higher doses of cosmic radiation to bombard the planet. How our species (and others) might adapt is an open question. "Clearly, our earliest ancestors have survived reversals in the past," Buffett said. "The most obvious consequence is that you'd need a new compass."

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