Our planet's magnetic field is in a constant
state of change, say researchers who are beginning to understand
how it behaves and why.
by Dr Tony Phillips
Every few years, scientist
Larry Newitt of the Geological Survey of Canada goes hunting. He
grabs his gloves, parka, a fancy compass, hops on a plane and flies
out over the Canadian arctic. Not much stirs among the scattered
islands and sea ice, but Newitt's prey is there - always moving,
His quarry is Earth's
north magnetic pole.
At the moment it's
located in northern Canada, about 600 km from the nearest town:
Resolute Bay, population 300, where a popular T-shirt reads "Resolute
Bay isn't the end of the world, but you can see it from here." Newitt
stops there for snacks and supplies - and refuge when the weather
gets bad. "Which is often," he says.
Scientists have long
known that the magnetic pole moves. James Ross located the pole
for the first time in 1831 after an exhausting arctic journey during
which his ship got stuck in the ice for four years. No one returned
until the next century. In 1904, Roald Amundsen found the pole again
and discovered that it had moved - at least 50 km since the days
The pole kept going
during the 20th century, north at an average speed of 10 km per
year, lately accelerating "to 40 km per year," says Newitt. At this
rate it will exit North America and reach Siberia in a few decades.
Keeping track of the
north magnetic pole is Newitt's job. "We usually go out and check
its location once every few years," he says. "We'll have to make
more trips now that it is moving so quickly."
Credit: Geological Survey of Canada - more
The movement of Earth's north magnetic pole across the Canadian
arctic, 1831 - 2001.
Earth's magnetic field
is changing in other ways, too: Compass needles in Africa, for instance,
are drifting about 1 degree per decade. And globally the magnetic
field has weakened 10% since the 19th century. When this was mentioned
by researchers at a recent meeting of the American Geophysical Union,
many newspapers carried the story. A typical headline: "Is Earth's
magnetic field collapsing?"
Probably not. As remarkable
as these changes sound, "they're mild compared to what Earth's magnetic
field has done in the past," says University of California professor
Sometimes the field
completely flips. The north and the south poles swap places. Such
reversals, recorded in the magnetism of ancient rocks, are unpredictable.
They come at irregular intervals averaging about 300,000 years;
the last one was 780,000 years ago. Are we overdue for another?
No one knows.
According to Glatzmaier,
the ongoing 10% decline doesn't mean that a reversal is imminent.
"The field is increasing or decreasing all the time," he says. "We
know this from studies of the paleomagnetic record." Earth's present-day
magnetic field is, in fact, much stronger than normal. The dipole
moment, a measure of the intensity of the magnetic field, is now
8 Ã— 1022 amps Ã— m2. That's twice the million-year
average of 4Ã— 1022 amps Ã— m2.
To understand what's
happening, says Glatzmaier, we have to take a trip ... to the centre
of the Earth where the magnetic field is produced.
At the heart of our
planet lies a solid iron ball, about as hot as the surface of the
sun. Researchers call it "the inner core." It's really a world within
a world. The inner core is 70% as wide as the moon. It spins at
its own rate, as much as 0.2Â° of longitude per year faster than
the Earth above it, and it has its own ocean: a very deep layer
of liquid iron known as "the outer core."
Earth's magnetic field
comes from this ocean of iron, which is an electrically conducting
fluid in constant motion. Sitting atop the hot inner core, the liquid
outer core seethes and roils like water in a pan on a hot stove.
The outer core also has "hurricanes" - whirlpools powered by the
Coriolis forces of Earth's rotation. These complex motions generate
our planet's magnetism through a process called the dynamo
Image credit: USGS - more
stripes around mid-ocean ridges reveal the history of
Earth's magnetic field for millions of years. The study
of Earth's past magnetism is called paleomagnetism.
Using the equations
of magneto hydrodynamics, a branch of physics dealing with conducting
fluids and magnetic fields, Glatzmaier and colleague Paul Roberts
have created a supercomputer model of Earth's interior. Their software
heats the inner core, stirs the metallic ocean above it, then calculates
the resulting magnetic field. They run their code for hundreds of
thousands of simulated years and watch what happens.
What they see mimics
the real Earth: The magnetic field waxes and wanes, poles drift
and, occasionally, flip. Change is normal, they've learned. And
no wonder. The source of the field, the outer core, is itself seething,
swirling, turbulent. "It's chaotic down there," notes Glatzmaier.
The changes we detect on our planet's surface are a sign of that
They've also learned
what happens during a magnetic flip. Reversals take a few thousand
years to complete, and during that time - contrary to popular belief
- the magnetic field does not vanish. "It just gets more complicated,"
says Glatzmaier. Magnetic lines of force near Earth's surface become
twisted and tangled, and magnetic poles pop up in unaccustomed places.
A south magnetic pole might emerge over Africa, for instance, or
a north pole over Tahiti. Weird. But it's still a planetary magnetic
field, and it still protects us from space radiation and solar storms.
And, as a bonus, Tahiti
could be a great place to see the Northern Lights. In such a time,
Larry Newitt's job would be different. Instead of shivering in Resolute
Bay, he could enjoy the warm South Pacific, hopping from island
to island, hunting for magnetic poles while auroras danced overhead.
Supercomputer models of Earth's magnetic field. On the left
is a normal dipolar magnetic field, typical of the long
years between polarity reversals. On the right is the
sort of complicated magnetic field Earth has during the
upheaval of a reversal.
Sometimes, maybe, a
little change can be a good thing.