A new study by scientists at Scripps Institution of Oceanography at the University of California San Diego and the University of Leeds in England reveals that changes in the direction of the Earth’s magnetic field may take place 10 times faster than previously thought.
The study gives new insight into the flow of iron 2,800 kilometers (1,730 miles) below the planet’s surface and how it has influenced the movement of the magnetic field during the past 100,000 years.
Earth’s magnetic field is generated and maintained by a convective flow of molten metal that forms the Earth’s outer core. Motion of the liquid iron creates the electric currents that power the field, which provides guidance for navigational systems, helps shield us from harmful extra-terrestrial radiation, and holds our atmosphere in place.
The magnetic field is constantly changing. Satellites provide modern means to measure and track its current shifts but the field existed long before the invention of human-made recording devices. To capture the evolution of the magnetic field back through geological time, scientists analyze the magnetic fields recorded by sediments, lava flows and human-made artifacts. Accurately tracking the signal from Earth’s core field is extremely challenging and so the rates of field change estimated by these types of analysis are still debated.
Now, geophysicist Catherine Constable from Scripps Oceanography and Chris Davies at Leeds University have taken a different approach. They combined computer simulations of the field generation process with a recently published reconstruction of time variations in Earth's magnetic field spanning the last 100,000 years.
Their study in Nature Communications, nearly a decade in the making, shows that changes in the direction of Earth’s magnetic field reached rates that are up to 10 times larger than the fastest currently reported variations of up to one degree per year.
A longstanding question has been how well numerical simulations are able to reproduce the details of real field behavior, and whether they reflect the underlying physics sufficiently accurately to enhance understanding about processes in Earth’s core.
The simulations demonstrate that these rapid changes are associated with local weakening of the magnetic field. This means these changes have generally occurred around times when the field has reversed polarity or during events such as one scientists refer to as the Laschamp geomagnetic excursion 41,000 years ago when, in contrast to times of strong stable polarity field, the weakened field produces highly variable magnetic field directions.
“Understanding whether computer simulations of the magnetic field accurately reflect the physical behavior of the geomagnetic field as inferred from geological records can be very challenging,” said Constable, “but in this case, we have been able to show excellent agreement in both the rates of change and general location of the most extreme events across a range of computer simulations. Further study of these simulations could help document how such rapid changes occur and whether they are also found during times of stable magnetic polarity like what we are experiencing today.”
The clearest example of this in their study is a rapid change in the geomagnetic field direction of roughly 2.5 degrees per year 39,000 years ago. This shift was associated with a locally weak field strength, in a confined spatial region just off the west coast of Central America, and followed the global Laschamp excursion. Similar events are identified in computer simulations of the field which can reveal many more details of their physical origin than the limited paleomagnetic reconstruction.
"We have very incomplete knowledge of our magnetic field prior to 400 years ago,” said Davies. “Since these rapid changes represent some of the more extreme behavior of the liquid core they could give important information about the behaviour of Earth’s deep interior.”
Their detailed analysis indicates that the fastest directional changes are associated with movement of reversed flux patches across the surface of the liquid core. These patches are more prevalent at lower latitudes, suggesting that future searches for rapid changes in direction should focus on these areas.
This research was supported by the National Science Foundation and a Natural Environment Research Council fellowship.
University of Leeds press officer Anna Harrison, email@example.com
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