Earth's Magnetic Field Is Fading

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Earth's Magnetic Field Is Fading

http://news.nationalgeographic.com/news/2004/09/0909_040909_earthmagfield.html#m\

ain

John Roach

for National Geographic News

September 9, 2004

 

Earth's magnetic field is fading. Today it is about 10 percent weaker

than it was when German mathematician Carl Friedrich Gauss started

keeping tabs on it in 1845, scientists say.

 

If the trend continues, the field may collapse altogether and then

reverse. Compasses would point south instead of north.

 

Not surprisingly, Hollywood has already seized on this new twist in

the natural-disaster genre. Last year Tinseltown released The Core, a

film in which the collapse of Earth's magnetic field leads to massive

electrical storms, blasts of solar radiation, and birds incapable of

navigation.

 

 

Top: A simulation of Earth's magnetic field structure. Bottom: An

image of what Earth's magnetic field might look like during a

reversal, something humans may have to worry about thousands of years

from now.

Funding for this Earth-systems science story was provided by the

National Science Foundation.

This special series of news stories is produced as a complement to

Pulse of the Planet, a daily sound portrait of the Earth broadcast on

radio.

Entertainment value aside, the portrayal wasn't accurate, according to

scientists who say the phenomenon of Earth's fading magnetic field is

no cause to worry.

 

"The field has reversed many times in the past, and life didn't stop,"

said Gary Glatzmaier, an earth scientist and magnetic field expert at

the University of California, Santa Cruz.

 

Glatzmaier is keeping an eye on our planet's weakening magnetic field

as he tries to learn more about how Earth's geodynamo works. The

geodynamo is the mechanism that creates our planet's magnetic field,

maintains it, and causes it to reverse.

 

Magnetic Shield

 

Earth's geodynamo creates a magnetic field that shields most of the

habited parts of our planet from charged particles that come mostly

from the sun. The field deflects the speeding particles toward Earth's

Poles.

 

Without our planet's magnetic field, Earth would be subjected to more

cosmic radiation. The increase could knock out power grids, scramble

the communications systems on spacecraft, temporarily widen

atmospheric ozone holes, and generate more aurora activity.

 

A number of Earth's creatures, including some birds, turtles, and

bees, rely on Earth's magnetic field to navigate. The field is in

constant flux, scientists say. But even without it, life on Earth will

continue, researchers say.

 

"There are small fluctuations, which lead to nothing, and large ones,

which we know from the geologic record are associated with reversals,"

said Peter Olson, a geophysicist at Johns Hopkins University in

Baltimore, Maryland.

 

When molten lava erupts onto the Earth's crust and hardens, it

preserves a snapshot of Earth's polarity, much in the way that iron

filings on a piece of cardboard align themselves to the field of a

magnet held beneath it.

 

According to Earth's geologic record, our planet's magnetic field

flips, on average, about once every 200,000 years. The time between

reversals varies widely, however. The last time Earth's magnetic field

flipped was about 780,000 years ago.

 

"We hear the magnetic field today looks like it is decreasing and

might reverse. What we don't hear is it is on a time scale of

thousands of years," Glatzmaier said. "It's nothing we'll experience

in our lifetime."

 

But several generations from now, humans just may witness a reversal.

By then, Glatzmaier said, scientists will better understand the

process and be prepared to cope with the effects.

 

Geodynamo

 

Scientists believe the magnetic field is generated deep inside the

Earth where the heat of the planet's solid inner core churns a liquid

outer core of iron and nickel.

 

The solid inner core is thought to be a mass of iron about the size of

the moon that is heated to several thousand degrees Fahrenheit. Heat

radiated by this inner core builds up at its boundary with Earth's

liquid outer core, causing the fluid there to expand.

 

"When it expands it becomes a little less dense [and more] buoyant. So

it starts to rise. That's convection," Glatzmaier said. "Hot fluid

rises, then cools off and sinks again."

 

The convection generates an electric current and, as a result, a

magnetic field.

 

Additional currents are created as Earth cools. Some of the molten

iron solidifies onto the inner core, releasing lighter material in the

process. The rotation of the Earth also generates forces that curve

the flow of fluid as it rises, twisting the magnetic field.

 

All of these currents constantly replenish the magnetic field, a

maintenance process that prevents it from decaying.

 

Typically each newly generated field lines up in the direction of the

existing magnetic field. But every now and again, some force will

cause the new field to line up in the opposite direction. This process

can lead to a net weakening of Earth's magnetic field.

 

Over time a new field can continue to grow. This further weakens the

original magnetic field. If the process continues, the two fields

would eventually cancel each other out. Earth's magnetic field would

collapse and then, maybe, flip.

 

"But more likely than not what will happen is the original [field]

will get stronger again and overwhelm the instability," Glatzmaier said.