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updated 4/7/2004 8:30:25 PM ET 2004-04-08T00:30:25

Next time Earth's magnetic field flips, compass needles will point South instead of North. But scientists can't say when it will occur, and until now they've disagreed on how long the transitions take.

A new study pins down how long it took for the last four reversals to play out. It also finds that the dramatic turnarounds occur more quickly nearer the equator than at higher latitudes closer to the poles.

That means folks living during the next reversal -- which some scientists speculate might be underway -- will see compasses change and behave differently in different locations, study leader Brad Clement, of Florida International University, told SPACE.com.

Giant magnet
Earth's magnetic field is thought to be generated deep inside the planet. An inner core of solid iron is surrounded by an outer core of molten iron. They rotate at different rates, and the interaction between the regions creates what scientists call a "hydromagnetic dynamo." It's something like an electric motor, and it generates a magnetic field akin to a giant bar magnet.

The process is not completely understood. In fact, one study suggests the planet's mantle, which surrounds the core, also plays a role.

However it works, the setup has been in place for at least 3 billion of Earth's 4.6 billion years, scientists figure. But the field is shifty, periodically growing stronger and weaker, moving around, and even flipping its polarity entirely.

In the past 15 million years, there have been four reversals every 1 million years, or about one shift each 250,000 years, Clement explained. The last one, however, was 790,000 years ago. That might suggest we're overdue for a big change. Not necessarily so, Clement says. The flips are not periodic, meaning they don't adhere to a schedule of even intervals.

Yet the intensity of the magnetic field has been dropping for the last 2,000 years, and "it has dropped significantly" during the past two decades, Clement said. One recent study shows the decline in strength amounts to 10 percent over the last 150 years.

Flip in progress?
Some scientists speculate a reversal is underway. Clement said that's like forecasting that the bottom will drop out of the stock market because it's gone down the past few days. "We just don't know," he said.

Researchers also have not known how long it takes for the magnetic field to make a transition. Studies have suggested anywhere from 1,000 to 28,000 years are required to initiate and complete a reversal.

"It is generally accepted that during a reversal, the geomagnetic field decreases to about 10 percent of its full polarity value," Clement said. "After the field has weakened, the directions undergo a nearly 180 degree change, and then the field strengthens in the opposite polarity direction. A major uncertainty, however, has remained regarding how long this process takes."

Clement examined sediment cores gathered from deep-ocean sites in a National Science Foundation (NSF) program. The cores provided readings at multiple sites for the past four flips. He found that each took about 7,000 years.

Interestingly, however, there is significant variation depending on latitude.

Which way is up?
It takes less time -- around 5,000 years -- for the reversal to occur at lower latitudes. And it takes longer -- about 10,000 years, for the flip to play out nearer the poles. So not only would compasses gradually do a somersault in readings, but Arctic dwellers would see changes that wouldn't match what tropical observers would note across the generations.

Nobody understands how the shift occurs. Perhaps, Clement says, the magnetic field shrinks to essentially nothing, leaving several "mini-poles" at the surface before the main poles rebuild on opposite sides of the world.

Scientists have plenty of reasons to seek a better understanding. For one, the magnetic field lines extend out beyond Earth's atmosphere and provide the first line of defense against strong solar storms. And Clement wonders how the reversals might affect navigation by migrating birds and other animals that key in on the magnetic field to find their way.

"But 7,000 years is probably enough for them to adapt," he said.

"Clement has demonstrated that magnetic field reversal events occur within certain time-frames, regardless of the polarity of the reversal," said Carolyn Ruppel, program director in NSF's division of ocean sciences.

The study is detailed in the April 8 issue of the journal Nature.

"The work is an important contribution to our understanding of magnetic field reversal on our planet," said Ronald Merrill, a professor of geophysics at the University of Washington, in an analysis written for the journal.

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