Earth is not heading toward a doomsday reversal of its magnetic field, new research assures.
The planet’s magnetic field is about 10 percent wimpier today than when physicists began keeping tabs on it in the 1800s. In the geologic past, such weakening preceded geomagnetic reversals —swaps of the north and south magnetic poles. Such reversals temporarily make the planet more vulnerable to charged particles blasted off the sun that can disrupt power grids and disable satellites.
But that’s not what’s happening now, a new study suggests. While weakening, Earth’s magnetic field is still strong by historical standards. Retracing the strength of Earth’s magnetic field over the last 5 million years, geophysicists have discovered that the field has been much weaker in the past than previously thought. That means that the average strength of Earth’s magnetic field over that longer time period is about 60 percent of its present-day value, the researchers report online November 23 in the Proceedings of the National Academy of Sciences.
Earth’s magnetic field “is just returning back to its long-term average,” not weakening toward a reversal, says study coauthor Dennis Kent, a paleomagnetist at Rutgers University in New Brunswick, N.J. A magnetic flip “is well down the list of things to worry about.”
Scientists track Earth’s magnetic field through time using lava rocks. Grains of magnetic minerals inside freshly spewed lavas become magnetized by the planet’s magnetism. Once the lavas cool, the grains become permanent record keepers of the magnetic field strength at the time of the eruption.
Decoding that magnetic record can be tricky, though. The grains are often large enough that the magnetization of one part of a grain can alter the magnetization of the rest of the grain. The unique internal structure of each grain alters how it changes under a magnetic field, so grains can record different magnetic signals when exposed to the same magnetic field.
Kent and colleagues counteracted this problem by heating lava grains from the Galápagos Islands in the presence of a known magnetic field, mimicking the formation of the lava rocks millions of years ago. Since the researchers knew the intensity of the new field, they could deduce how each grain records magnetism. With that information in hand, the team could use the original magnetizations of the grains to accurately calculate the strength of the ancient magnetic field. Experiments without this critical process yielded deceptively strong measurements of ancient fields, the researchers discovered.
A relatively stronger present-day field helps explain why Earth hasn’t had a geomagnetic reversal for 780,000 years, says geophysicist Peter Driscoll of the Carnegie Institution for Science in Washington, D.C. Magnetic flips historically happen every 250,000 years or so, but stronger fields are less prone to reversals, he explains.