Facts of the Matter Richard Brill

Earth's magnetic field constantly shifts

Gilbert sculpted a piece of lodestone (a naturally occurring magnetic variety of the mineral magnetite) into a sphere and showed that its magnetism resembled that of a bar magnet.

Today, we know that Earth's magnetic field near the surface resembles the force field that surrounds a bar magnet. Lines of magnetic force represent the direction of the force that the field would exert on the north pole of a magnet.

Magnetism is a strange phenomenon related to electricity. Like gravity, we understand how to describe it, but do not know what it is (for a more detailed description of magnetism, see my column from Feb. 1, 2004, at starbulletin.com/2004/ 02/01/business/brill.html).

An idealized magnetic picture of Earth near the surface would show the lines of force entering the north magnetic pole, passing through the earth and emerging from the south magnetic pole in continuous concentric loops completely surrounding the planet.

Although the field resembles that of a bar magnet near the earth, its actual shape is much more complex due to the way it is generated and the sun's effect on it.

The solar wind, which consists of charged particles expelled by the sun, causes the field to distort into a teardrop shape the farther out it is from Earth's surface. The shape and the extent of deformation change with the changing intensity of the solar wind.

The magnetic poles and the geographical poles are not in the same place.

The north magnetic pole (NMP) is in the Canadian arctic, 430 miles from the geographical North Pole, and the south magnetic pole is off the coast of Antarctica south of Australia.

Unlike the geographic poles, the magnetic poles are constantly moving.

The NMP is now moving northwest at about 25 miles per year having accelerated by a factor of four in the past century.

Since magnetic compasses point to the NMP and not to true north (toward the geographic pole), it is important to keep track of the NMP's location and how it is changing.

The way that the magnetic field changes is not constant, making it impossible to predict where the NMP will be at any point in the distant future.

By constantly measuring the strength and direction of the field and the location of the NMP, we can create a mathematical model to make predictions that will be reasonably accurate for a few years into the future. This model must be continually revised and updated, based on new data as it is collected.

Finding the NMP is no easy task because it might not be at only one point, but could range over an area, or there might be more than one location within that area. It even moves significantly on a daily basis.

Knowing the location of the NMP was a major objective of the British Royal Navy in the 19th century because navigation depended on it. The Navy included magnetic observers on many of its Arctic expeditions and offered great rewards to anyone who found its exact location.

When James Clark Ross first found it on June 1, 1831, it was on the west coast of Boothia Peninsula (70 degrees north, 97 degrees west), to the west of Baffin Island. It is currently located at 83 degrees north, 115 degrees west, in the Arctic Ocean north of Queen Elizabeth Islands, 800 miles from where Ross found it.

EARTH'S MAGNETIC FIELD is changing in other ways, too. Compass needles in various parts of the world drift as much as one degree per decade, and the magnetic field globally has weakened by 10 percent in the past century.

This should not cause a "Chicken Little" response. This amount of change is mild compared with the changes it has undergone in the past.

Magnetic stripes around midocean ridges and the magnetism of volcanic rocks on land reveal the history of Earth's magnetic field over millions of years.

Sometimes the field completely flips with the north and the south poles swapping places. These reversals have been irregular and are unpredictable for the past several hundred million years at least. The last one was 780,000 years ago.

The current decline does not mean a reversal is imminent. The field increases and decreases all the time. Earth's present-day magnetic field is much stronger than normal, twice the average of the past 1 million years.

Earth's magnetic field is produced in the core of the planet, which is a solid iron ball about as hot as the surface of the sun. It is a world within a world, 70 percent the diameter of the moon, and it spins at a rate as much as two degrees of longitude per year faster than the earth above it.

The solid inner core has its own ocean. It is surrounded by a thick layer of liquid iron, which is the source of the magnetic field.

The liquid outer core seethes and roils. It has weather. Whirlpools like hurricanes and frontal systems in the atmosphere are powered by heat and by Earth's rotation. These complex motions generate our planet's magnetism through a process called the dynamo effect.

The changes and flips that we observe and which have taken place throughout history are also the result of the swirling turbulence of the outer core.

Gary Glatzmaier and Paul Roberts at the University of California have used the equations of magnetohydrodynamics to create a supercomputer model of Earth's interior.

Their software heats the inner core and stirs the metallic ocean above it, then calculates the magnetic field that results. They run it for hundreds of thousands of simulated years and watch the magnetic field wax, wane, drift and flip, just like the real Earth.

The model also shows what happens during a magnetic reversal. Contrary to former ideas, the magnetic field does not vanish; it just gets more complicated.

Magnetic lines of force near Earth's surface become twisted and tangled, and magnetic poles pop up in unusual places: a south magnetic pole appears over Africa; a north pole appears over Tahiti; multiple poles appear in different places.

It is weird and unexpected, but it still protects the planet from space radiation and solar storms, causing many geologists and biologists to rethink theories that excessive cosmic ray bombardment during times of reversal might have caused mutations or mass extinctions.

The magnetic field protects us by channeling charge particles from cosmic rays and solar storms along the lines of force to the magnetic poles. One noticeable result is the interaction of the shower of ions with the rarefied upper atmosphere to produce the spectacular light show known as the aurora.