The Earth's magnetic field will not flip as soon as previously predicted, according to a new research paper published in the Proceedings of the National Academy of Sciences.
Earth's magnetic poles have reversed several hundred times over the past 100 million years, most recently about 780,000 years ago when humans were present. Some scientists believe a dip in the magnetic field's intensity 41,000 years ago was also a brief reversal.
When scientists recently began noticing a decline in the magnetic field - about 10 percent over the past two centuries - it led to speculation that another polarity flip would happen soon. But the magnetic field's intensity could simply be coming down from an unusual high rather than approaching a reversal, new research suggests. That's because the magnetic field's intensity rises and falls without a clear pattern, only sometimes dipping far enough to become unstable and possibly reverse. During a reversal, geomagnetic intensity declines during a transition period that typically lasts hundreds to thousands of years, then rebuilds.
"The field may be decreasing rapidly, but we're not yet down to the long-term average. In 100 years, the field may even go back the other direction [in intensity]," said Dennis Kent, an expert in paleomagnetism at Columbia University's Lamont-Doherty Earth Observatory and co-author of the study with his former student, Huapel Wang.
As lava cools, iron-bearing minerals form inside and act like tiny magnets, aligning with the Earth's magnetic field. Scientists can analyze ancient lava to determine both the direction and the intensity of the magnetic field at the time the lava formed.
For the new study, the scientists used ancient lava flows from sites near the equator and compared the paleointensity data with what had been regarded as an anomalously low intensity obtained by others from lavas from near the South Pole.
The researchers found that the magnetic field's long-term average intensity over the past five million years was much weaker than the global database of paleointensity suggests - only about 60 percent of the field's strength today. The findings raise questions both about claims that the magnetic field may be nearing a reversal and about the database itself.
The scientists believe the difference is in how the samples are analyzed. They say the database, which catalogs paleointensity data from published papers, includes a variety of methods and doesn't clearly delineate data from two different types of magnetized mineral samples, tiny single-domain grains that come from sites that cooled quickly, like basalt glass on the outer edges of lava flows, and more common larger multi-domain grains found deeper inside lava whose magnetic behavior is more complex and require a different type of analysis.Above image: An artistic impression of how auroras could be more widespread under a geomagnetic field much weaker than today's. Credit: Huapei Wang, with source files courtesy of NASA's Earth Observatory/NOAA/DOD