According to recent study of Davies and colleagues, changes in the direction of Earth’s internally generated magnetic field may take place 10 times faster than previously thought..
Figure: Earth’s magnetic field lines. Image credit: NASA’s Goddard Space Flight Center.
The magnetic field of Earth is generated and maintained by a convective flow of molten metal that forms our planet’s outer core.
Motion of the liquid iron creates the electric currents that power the field, which not only helps guide navigational systems but also helps shield us from harmful extra terrestrial radiation and hold our atmosphere in place.
The geomagnetic field is constantly changing. To capture the evolution of the field back through geological time, geoscientists 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.
Dr. Davies and his colleague, Professor Catherine Constable from the Scripps Institution of Oceanography, combined computer simulations of the geomagnetic field generation process with a reconstruction of time variations in the field spanning the last 100,000 years
Their results show that changes in the direction of the geomagnetic field reached rates that are up to 10 times larger than the fastest currently reported variations of up to one degree per year.
They 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 geomagnetic excursions when the dipole axis — corresponding to field lines that emerge from one magnetic pole and converge at the other — moves far from the locations of the North and South geographic poles.
The clearest example of this is a sharp change in the geomagnetic field direction of roughly 2.5 degrees per year 39,000 years ago.
This shift was associated with locally weak field strength, in a confined spatial region just off the west coast of Central America, and followed the global Laschamp excursion, a short reversal of the Earth’s magnetic field roughly 41,000 years ago.
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.
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.
References: C.J. Davies & C.G. Constable. 2020. Rapid geomagnetic changes inferred from Earth observations and numerical simulations. Nat Commun 11, 3371; doi: 10.1038/s41467-020-16888-0..