Geomagnetic pole reversals have long provided fodder for late-night-radio doomsayers and Sony Pictures. But these magnetic flip-flops are nothing new. Our magnetic poles have flipped continually; perhaps some 10,000 times over Earth’s storied 4.5 billion-year history.
In truth, our magnetic fields are so dynamic, they change from year to year in a kind of self-contained chaos. Next year, the European Space Agency plans to take advantage of new technology to get a better handle on the chaos, and its effects on such global concerns as climate, weather and space radiation.
During a magnetic field reversal, the orientation of the current magnetic field flips from one pole to the other, with magnetic South becoming magnetic North.
During the process of such a flip, Stanford University geophysicist Norm Sleep says, auroras would blossom at the equator instead of the poles. Compasses would cease to be reliable. Radio transmissions would be adversely affected, and cosmic rays would pierce Earth’s atmosphere more readily.
Still, life would persevere. And for all the predictions of calamity, it’s easy to forget that our planet’s geomagnetism shields us from a lion’s share of the cosmos’ flux of hazardous radiation.
The geomagnetic dynamo, at the heart of Earth’s magnetic field, is driven by electrically conductive molten nickel-iron circulating our planet’s central solid iron core. This thermal movement of the liquid core is ultimately influenced by Earth’s rotation. The resulting north-south magnetic pole is roughly aligned with Earth’s axis of rotation. At the surface, we can only observe less than 1 percent of the magnetic field’s total energy. But even within this observed 1 percent, geomagnetic surface anomalies abound.
The most pronounced example of such deep core magnetic field disturbances straddles the South Atlantic Ocean and is known as the South Atlantic Anomaly.
Convection patterns in the core, driven by gigantic volumes of liquid nickel-iron, says Gary Glatzmaier, a geodynamicist and geophysicist at the University of California, Santa Cruz, manifest themselves at the surface as reduced field intensities. In the case of the SAA, its diameter covers some 5,000 surface kilometers (3,100 miles) — from South Africa to Chile, from the Carribean to the Falkland Islands.
But is this local anomaly a harbinger of a forthcoming global magnetic field reversal (say in, umm, 2012)? While that’s not out of the realm of possibilities, it’s not thought to be likely.
“The SAA could be caused by a reverse flux patch in the top of the liquid core,” said Glatzmaier. “This liquid iron is trying to get heat out of the center. It does that by rising and dumping the heat at the top and sinking again.”
Normally, geomagnetic field lines flow out of the Southern Hemisphere, envelop the Earth and then head back down toward the surface in the Northern Hemisphere.
But a significant percentage of the Earth’s magnetic field lines are unusually returning to the surface within the same hemisphere, in the area of the SAA.
This, in turn, leads to a diminished geomagnetic field in the area around the SAA. The SAA’s diminished magnetic field, in turn, has contributed to a decrease in the entire Earth’s magnetic field over the last century. This trend is thought to be part of a continual decrease dating back thousands of years.
However, Juergen Matzka, a geophysicist at the Danish Meteorological Institute in Copenhagen, says that while the SAA is not likely a harbinger of a global magnetic reversal, such a reversal would usually come after the magnetic field completely collapses — what is known as a “geomagnetic excursion.”
The last major reversal was 780,000 years ago. The length of time between reversals can range from tens of thousands to millions of years, but the average time between reversals is roughly 250,000 years.
Geophysicists have little or no information about the fluid dynamics inside Earth’s fluid core. Thus, measurements of the strength, direction and variation of our planet’s core magnetic field are about the only way researchers can learn more.
In the past, studies of the SAA have been hampered by a lack of ground stations in the South Atlantic. Late last year, with Matzka as principal investigator, the Danish Meteorological Institute, together with the Danish National Space Center, installed a geomagnetic monitoring station on the island of Tristan da Cunha, the most isolated inhabited island on earth.
Two more of these ground-based magnetometers are to be set up on Tristan in September. That’s well in advance of the European Space Agency’s Swarm mission, due for launch in late 2010. Swarm’s three geomagnetic measuring satellites will provide the best ever multi-year survey of Earth’s magnetic field.
“All we can do is measure the surface magnetic field structure and how it changes over time,” said Glatzmaier. “Earth’s core is probably a lot more turbulent than we can simulate on a computer. In a hundred years, with many more paleomagnetic measurements and much faster computers, we may be able to predict these reversals.”
If our distant progeny are still around during the onset of the next global magnetic reversal, they will not only live through it, but should be able to duly record it for future generations.
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