Polar Flip-Flop
by Paul Britt, HGS Bulletin Editor
Global warming is the hot earth science issue in the news, followed by closely by the tsunami threat and lack of warning systems to populated coastal areas, then regionally by subsidence issues here in the Gulf Coast. Well down on the list of public awareness, below even the threat of an asteroid strike (a favorite of Hollywood), is magnetic polar reversal.
James Ross located the magnetic North Pole for the first time in 1831 after a lengthy arctic journey, during which his ship got stuck in the ice for four years. Roald Amundsen found the pole again in 1904 and discovered that it had moved—at least 30 miles since its discovery by Ross. The pole kept moving north at an average speed of 6 miles per year, accelerating lately to about 25 miles per year, according to Larry Newitt of the Geological Survey of Canada, whose job it is to keep track of the pole’s movement. Every few years, Newitt goes north in search of the magnetic north pole. At the moment, it is located in northern Canada, about 375 miles from the nearest town, Resolute Bay, population 300. At its current rate of movement, it could leave North America in a few decades and reach Siberia.
Globally, the Earth’s magnetic field has weakened since the 19th century by 10%. However, the dipole moment, a measure of the intensity of the magnetic field, is now 8 3 1022 amps 3 m2, twice the million-year average of 4 3 1022 amps 3 m2, according to University of California professor Gary Glatzmaier.
The Earth’s magnetic poles have reversed, or swapped places, numerous times in the past, evidenced by magnetic “stripes” found parallel to mid-ocean ridges, magnetic evidence in lava flows and other paleomagnetic evidence. The reversals come at irregular intervals spaced 5,000 to 50 million years apart, averaging about 250,000 years. The last one was about 750,000 years ago.
The Earth’s magnetic field comes from the rotation of the Earth’s core. The solid iron inner core, about 70% the size of the Moon, spins at its own rate, about 0.2% faster than the surrounding Earth. This inner core is surrounded by a molten outer core, which behaves as its own ocean, with currents and “hurricanes” much like the Earth’s surface oceans.
Glatzmaier and associates have developed a computer model based on the Earth’s core to simulate the behavior of the magnetic poles. The strength of the magnetic field waxes and wanes, the poles drift, and occasionally flip. They have also learned what happens when the poles flip. Reversals take a few thousand years to complete, and contrary to popular belief, the magnetic field does not vanish. The field gets more complicated, with poles emerging in unaccustomed places, and multiple magnetic poles. According to Glatzmaier, it is still a planetary field, protecting the Earth’s surface from radiation and solar storms.
Others who believe that the protective magnetic field will weaken sufficiently to allow additional radiation to reach the Earth’s surface contrast with Glatzmaier’s opinion on what happens during a pole reversal. And some tie the Earth’s magnetic flip-flop to the Sun’s pole reversals.
The South Atlantic Anomaly (SAA) is a region where the Earth’s inner van Allen radiation belt makes its closest approach to the planet’s surface. As a result, the radiation intensity is higher over this region. The SAA is produced by a dip in the Earth’s magnetic field, caused by the fact that the center of the magnetic field is offset from its geographic center by 280 miles. In this region, satellites passing through it receive higher doses of radiation. The International Space Station had additional shielding installed for travel through this region, and the Hubble Space Telescope does not take observations while in the SAA. Some cite the SAA as evidence of a weakening magnetic field and imminent pole reversal.
The Sun underwent a polar reversal in 2001, according to NASA reports, an event that happens every 11 years, at the peak of the sunspot cycle, or solar maximum. The Ulysses spacecraft, launched in 1990, provided a unique view—from the top down. This solar observation will likely aid us in our interpretation of the Earth’s magnetic field behavior. The next solar magnetic pole reversal is scheduled for 2012. Don’t miss it!
Of course, the pole reversal issue receives casual press, at best. Perhaps it is because we should get ample warning as the magnetic field weakens over time. Or perhaps it is because human intervention can’t be blamed for its occurrence. In any case, it is an example of the importance of studying the Earth on which we live.
Recommended reading:
http://science.nasa.gov/headlines/y2001/ast15feb_1.htm
science.nasa.gov/headlines/ y2003/29dec_magneticfield.htm
http://www.psc.edu/science/Glatzmaier/glatzmaier.html