Puzzling Point in Earth's Magnetic Field Explained After Almost 100 Years

An unusual point in Earth's magnetic field that has puzzled scientists for almost a century may have been explained almost a century after first being noticed. Using computer simulations, a team of researchers has reconstructed the goings on deep beneath Earth's surface to show how the liquid outer core near the core-mantle boundary and an enormous gyre appear to control the Pacific region.

The magnetic field in the Pacific region varies very little over time, compared with the rest of the planet. "This is something that has been a puzzle since the 1930s, when it was first noticed," Mathieu Dumberry, from the University of Alberta, Canada, told Newsweek in an email.

Dumberry is lead author on a study looking at the anomaly published in Nature Geoscience.

Earth's magnetic field is generated by movements within the planet's liquid core. The field extends far out into space. It protects the planet from the harmful radiation coming from the sun and helps maintain our atmosphere. The field is not uniform across the planet and exactly how it is generated is not entirely understood.

Over the Pacific, the magnetic field has shown little variation over time. It is known this is linked to the pattern of flows in the liquid outer core, which is a good electrical conductor. "Just like winds in the atmosphere or currents in the ocean, there are fluid motions in the liquid core," Dumberry said. "It is these core flows that are generating and maintaining the Earth's magnetic field, and this also the main reason why the field is changing with time. Magnetic field lines are carried by core flows, so we can use the observed changes in the magnetic field as tracers to reconstruct the pattern of core flows.

"The core flows are weaker under the Pacific and also feature a planetary scale current (or gyre) 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."

Dumberry and co-author Colin More, also from the University of Alberta, created computer simulations to look at the dynamics of Earth's core in a bid to explain the anomaly. They included variations in the electrical conductivity conditions from the bottle of the mantle that is in contact with core flows. The base of the mantle is a good electrical conductor, he said, meaning the magnetic field lines coming out of the core are slightly more strongly anchored to the mantle. This provides friction that slows down core flows.

"We show in our study that when the conductance—the electrical conductivity multiplied by the layer thickness—of the lowermost mantle is higher under the Pacific than elsewhere, this larger magnetic friction weakens the local core flows. It also deflects the main planetary gyre away from the Pacific region, as the gyre is reorganized to avoid the region of higher conductance."

As well as protecting the planet, Earth's magnetic field is also used for our navigation systems. Understanding how it changes over time is important to keeping these systems up to date. Over the last year, for example, the magnetic north pole moved unexpectedly, leading scientists to issue an unscheduled update to the World Magnetic Model, on which satellite navigation systems are based.

Dumberry said that as well as helping to explain the Pacific anomaly, their findings highlight the diversity of the core-magnetic boundary. "The magnetic field that we observe at the Earth's surface captures then a signature of the structures, composition and evolution of the lowermost mantle and the core-mantle boundary region," he said. "It thus provides an additional tool to interrogate and better understand the deep structures on our planet. 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."