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The planet's wandering magnetic poles

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Daedalus
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« on: December 27, 2007, 08:10:34 pm »

Wandering poles

Draw a line between the north and south geographic poles, and it runs smack through the center of the planet. Earth's rotation around this axis once each 24 hours produces the familiar cycle of day and night. Unlike the geographic poles, however, our planet's north and south magnetic poles aren't located directly opposite one another, says Nils Olsen, a geophysicist at University of Copenhagen.

Earth's geographic poles are fairly stable, wobbling back and forth across the landscape only a few meters every year or so (SN: 8/12/00, p. 111). The north and south magnetic poles are far more mobile, and they move independently of one another, says Olsen. Now located in the Arctic Ocean just north of Canada, the north magnetic pole is moving northwest toward Siberia by about 50 km each year. The south magnetic pole, just off the Antarctic coast south of Australia, is also—for now—heading northwest, but only at around 5 km/yr, Olsen and Mioara Mandea, a geophysicist at the National Research Center for Geosciences in Potsdam, Germany, report in the July 17 Eos.

Such wanderings stem from irregularities in the process that generates the magnetic field, says Olsen. Although Earth's inner core is solid and primarily composed of iron, its outer core is a molten mix of iron and lighter metals that is constantly on the move. The flow of that material, which carries charged particles and conducts electricity, produces the magnetic field, says Olsen. Long-lived eddies and swirling currents in the fluid, which moves at an average speed of about 20 km/yr and is no more viscous than water, make the magnetic field deep within Earth much more complex than it is at the planet's surface. "It's a highly chaotic system," says Olsen.

In particular, he notes, that turbulence can create "reversed-flux patches," regions on the surface of the outer core where magnetic field lines point opposite to those predominant at the Earth's surface. Variations in the size and strength of these patches significantly affect the location and the motion of the magnetic poles. For instance, the growth and movement of a reversed-flux patch beneath northern Canada is causing the north magnetic pole to surge toward Siberia.

At its current rate, the north magnetic pole will pass within 400 km of the north geographic pole in 2018, Olsen and Mandea report. Because of the chaotic nature of the field-generating processes in the outer core, predicting the pole's location more than a decade into the future is tricky, says Olsen. Nevertheless, the pole has been moving toward the northwest, although with varying speed, for more than a century.

In the past few decades, the strengthening of reversed-flux patches—especially ones beneath Canada and the South Atlantic Ocean—has weakened Earth's magnetic field, says Olsen. If the field's overall strength keeps dropping at today's rate, it will reach zero in a few hundred years. However, he notes, it's not clear whether recent fluctuations in field strength are routine variations or the prelude to a full-blown reversal of Earth's magnetic field—something that happens, on average, every quarter-million years or so.

Although the strength of the Earth's magnetic field is now dropping, it is 50 percent stronger than the estimated average for the past 60 million years, says Lisa Tauxe, a paleomagnetist at the Scripps Institution of Oceanography in La Jolla, Calif. At its most recent peak, about 2,000 years ago, the magnetic field was about twice as strong as it is now. "Data is spotty, but we have a crude idea of what's going on [with the magnetic field]," she notes. The data also suggest that "the field can change rapidly over a shorter time than [scientists] had thought."

« Last Edit: December 27, 2007, 08:13:49 pm by Daedalus » Report Spam   Logged


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