Mathieu Dumberry, associate professor in the Department of Physics. Photo credit: John Ulan
An almost century-old mystery of Earth’s magnetic field has a breakthrough new solution, according to a new study by University of Alberta physicists. The researchers provide an explanation for why changes to the planet’s magnetic field over time—that gives us the Northern Lights and shields us from charged particles from space—are weaker in the Pacific.
“This is something that has been a puzzle since the 1930s when it was first noticed,” said Mathieu Dumberry, associate professor in the Department of Physics and lead author of the study. “Core flows are weaker under the Pacific and also feature a planetary scale current that hangs close to the equator in the Atlantic region, but then is deflected to higher latitude in the Pacific region. But why is that? That is the part that was not understood.”
Just like winds in the atmosphere or currents in the ocean, there are fluid motions in the liquid core of the Earth, Dumberry explained. It is these core flows that generate and maintain the Earth’s magnetic field—which scientists model for use in a variety of applications, including determining orientation when looking at a map on your smartphone. Looking at the field can give new insight into the core flows that create it.
“Our model provides an answer to this puzzle. Our explanation involves the electrical conductivity of the lowermost mantle,” said Dumberry. “We show that if the conductance of the lowermost mantle is higher under the Pacific than elsewhere on the planet, and this larger ‘magnetic friction’ weakens the local core flows. It also deflects the main planetary current flow away from the Pacific region as it avoids the region of higher conductance, leading to smaller changes in the Earth’s magnetic field in the region.”
The model poses new questions about the makeup of the core-mantle boundary region and what it can tell us about other regions on Earth.
“Our study highlights that the core-mantle boundary region is quite heterogeneous. The conductance of the lowermost mantle is most probably not uniform around the globe,” said Dumberry. “The results demonstrate the impact that this may have on core flows and the magnetic field. We hope that our results will motivate geophysicists to further investigate the possible differences between the Pacific region and elsewhere on the core-mantle boundary.”
The paper, “Weak magnetic field changes over the Pacific due to high conductance in lowermost mantle,”was published in Nature Geoscience (doi: 10.1038/s41561-020-0589-y).