Monday, January 10, 2011

Earth's magnetic field shifts

From LiveScience

The magnetic north pole is currently hovering over the North Sea and moving toward Siberia.
This means two Florida airports are renumbering their runways.

Odd as this connection may appear on the surface, the
adjustments under way at Tampa International Airport and beginning next week at Peter O'Knight Airport are the result of a natural, ongoing process.

The Earth has an iron core, and movement within its outer part is likely responsible for sustaining a
magnetic field, which constitutes much of what we measure at the Earth's surface.
As a result, the Earth resembles something of a giant magnet with two poles: magnetic north and magnetic south.
However, its field is not perfectly symmetrical and has undulations that are always moving around, according to Jeffrey Love, a research scientist with the
U.S. Geological Survey's Geomagnetism Program.

The magnetic poles don't line up with the geographic ones, and the difference between them is an angle called declination.
As if this wasn't enough of a nuisance for navigators, the
Earth's magnetic field drifts, causing the angle of declination to change over time.

In fact, it drifts about one-fifth of a degree a year at lower latitudes, such as Florida.
"So that means if you wait five years, the compass will be off by one degree," Love said.

For long-distance air travel, an error of only a couple of degrees could translate to arriving in the wrong airport, Love said.

Declination also varies depending on location.
At high latitudes, it tends to become larger, and a compass becomes increasingly unreliable.
If you were to stand over a magnetic pole, and tried to use your compass, it would not know where to point, Love said.
Longitude also factors in.

As the patterns of motion of the molten iron in the Earth's core changes, so does the shape of the magnetic field, he said.

Right now, the magnetic north pole, where the field is vertical, is located at 84.97 degrees North and 132.35 degrees West, above the North Sea and drifting generally north-northwest toward Siberia at about 55 kilometers (34.2 miles) per year, according to Love.

What's going on in Florida?

Travelers have struggled with the complexity of
navigating by compass for centuries, and modern American travelers are no exception.

Runways are designated according to the points on a compass, and the drifting magnetic north means that they, periodically, need to be renamed.

"Recently, the drift has caused our runways' orientations to be closer to the next increment on the magnetic compass," Tampa International Airport spokeswoman Brenda Geoghagan told LiveScience in an e-mail.

For example, the west parallel runway, which was named 36Left —18Right to designate compass points of 360 degrees and 180 degrees, is being renamed to 1Left — 19Right, to indicate 10 degrees and 190 degrees, since the runway designations are separated into 10-degree increments.

Adjustments to runways like this and to navigational aids are ongoing, according to Kathleen Bergen, a spokeswoman for the Federal Aviation Administration's Southern Region.
A third airport is scheduled to rename its runways in October; however, the changes aren't necessary for all airports in the Tampa Bay area, she said.

Every five years, federal agencies tabulate and publish a value called
magnetic variation, which varies by location.
Similar to declination, it correlates true direction to the magnetic compass readings needed for navigation.

These values are used to update navigational aids, such as instrument landing systems and beacons, she said.
If there is a change of more than three degrees at any given airport, then runways need to be renumbered, as is happening at the Florida airports, Bergen said.

While aviation in the United States still relies on ground-based radar, which is calibrated to the local magnetic variation, it is moving toward a satellite-based system.

"Runways are still aligned toward magnetic poles. That’s not going to change," she said.

A Much Bigger Change?

The same forces that cause the magnetic poles to drift can also cause a more
dramatic change by switching the magnetic north pole to magnetic south pole, and vice versa, Love said.

In fact, the physics behind the activity in the Earth's core could just as easily generate a polarity opposite of what exists today, he said.

poles last reversed themselves 780,000 years ago, and the reversals typically take 10,000 years or more to happen.

"In terms of what kind of havoc it could wreak or effects on humankind, we can't really say because we haven't experienced it," he said. "I don't think it will be the
end of the world."

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