North By North-Where?
The (Uncertain) Science Of Geomagnetism, And The Search For Earth's Past
Magnetic North is off and running. Again. And Joseph Stoner doesn't know when it might return. But then, no one else does either.
Stoner is a paleomagnetist at Oregon State University. Last November at the American Geophysical Union annual meeting, he and his colleagues reported that the Earth's Magnetic North Pole is on the go. Following 400 years of relative stability in northern Canada, it has moved almost 1,100 kilometers out into the Arctic Ocean over the last century, accelerating to more than 40 kilometers per year over the last few decades.
At this pace, Magnetic North will be in Siberia within fifty years. And when that happens, Alaska's brilliant northern lights may decrease as they become more visible across much of northern Europe and Asia.
But for the doomsday scenarios that a wobbly magnetic pole sounds like it might portend, geologists still aren't sure what to make of the phenomenon. Nor are they entirely certain what causes it. Identifying Earth's north pole seems like it should be a relatively simple exercise, after all. The Earth is a (roughly) spherical object. Point to the top of that sphere, and there it is: the North Pole.
Well, maybe. Pinpointing the pole's actual location is actually far more complicated. "We have a whole confusing literature on poles, says Ron Merrill, a professor of geophysics at the University of Washington. This is because there are several North Poles: Geographic North, Geomagnetic North, and Magnetic North.
But there are a few constants. Geographic North is the easiest to identify--the uppermost point of the sphere. Geomagnetic North, from Earth's geomagnetic field, is trickier.
Most of the Earth's the magnetic field, called the main field, is of internal origin and originates primarily from electric currents flowing in Earth's iron-rich core. About 75 percent of the main field is called the dipole field. Although it is too hot for there to be any permanently magnetized material in the core, the dipole field can be pictured as coming from a bar magnet at Earth's center that is tilted with respect to Earth's rotation axis by about 11 degrees. A line drawn through the two poles of this magnet intersects the Earth's surface at two points referred to as the geomagnetic north and south poles. The remaining 25 percent of the main field is referred to as the nondipole field. It is a complex field that exhibits several ups and downs over the surface of Earth.
The magnetic poles are the two places on Earth where the main magnetic field is vertical. The geomagnetic poles and magnetic poles would coincide if there were no nondipole field. The nondipole field changes more rapidly than the dipole field, resulting in a relatively rapid change in the positions of
the magnetic poles.
Stoner has been tracking the changes in time of the north magnetic pole by using a recording of the magnetic field stored in sediments. He doesn't think that this latest magnetic pole movement prefigures a wholesale reversal of the Earth's magnetic field, an event that occurs every 500,000 years or so. Rather, he believes the shift is part of the normal process. "This may be a natural oscillation, he says. "[The pole] may migrate back to Canada again–there's a lot of variability and the movement may just be a jerk.' We're not sure.
Indeed, these are early days in the science of magnetic pole measurement, and Stoner is happy to have detected such movement at all. "A lot of things we only figure out once they happen to us, Stoner says.
Not that a fugitive Magnetic North Pole was a previously unknown phenomenon. People have kept an interested eye on it for a long time. Arctic explorer Sir John Ross began observing Magnetic North in 1831 from the Boothia Peninsula in Canada. Prior to that, sailors recorded Magnetic North for almost 300 years, marking variations in their ship's logs to keep compasses as accurate as possible.
All these observations provided a sort of field model, and Stoner used the records to track the pole's magnetic position, in a sense ground-truthing it with his own measurements from the geological record.
But once he reached the end of the mariners' logs, he had to dig even deeper to uncover Earth's magnetic past.
Stoner and his collaborators with the Canadian Geological Survey went to the Canadian Arctic, where they took core samples from several lakes. The lakes, between 40 and 80 meters deep and under two to three meters of ice, carry Earth's history in the layers of sediment that make up their bottom.
Each core was close to five meters long, and contained some 2,500 to 4,000 years of the Earth's history in the form of magnetite, a ferromagnetic (iron-based) material that aligns with Earth's magnetic field. By taking long cores, Stoner was able to trace the progress of the North Magnetic Pole by looking at the orientation of the magnetite. Changes in orientation meant that the pole had shifted.
Like tree rings, the yearly layers within the core were sometimes no more than two or three millimeters thick; usually they were one or less. "The lake conditions gave us nice age control, Stoner says. "Before that, it was hard linking changes in the magnetic field to time. We didn't have a good time constraint. Hopefully that will get better.
Stoner hopes to focus his chronology to a decadal scale or finer. To do that, he says he'll need to go to more lakes and get more cores. "The more samples we get, the more confidence we have in our records, he says. "One spot tells us the strength of the field and direction it's moving, but three would let us triangulate the signal. That would be great.
In the meantime, he will continue to follow the Magnetic North Pole as it wobbles its way to Siberia. (And for those still concerned, Magnetic South is a much more obedient child–it doesn't run off nearly as often as Magnetic North does.)
Eric Wagner is a graduate student in biology at the University of Washington.
Image at top:
The North Magnetic Pole helps form the Northern Lights, which form when radiation bounces across the magnetic field in the upper atmosphere. But as Magnetic North drifts, so too do the Northern Lights. Images like this from Alaska might become more rare. Photo: Mark Urwiller, courtesy NASA