Vast Anomaly in Earth's Magnetic Field Keeps Growing, Satellites Reveal

14 Oct. 2025, By M. STARR

https://www.sciencealert.com/vast-anomaly-in-earths-magnetic-field-keeps-growing-satellites-reveal

A map of the anomaly compiled by NASA.

  (NASA Goddard/YouTube)

A giant dent in Earth's magnetic field is continuing to expand, according to the latest data from a trio of satellites monitoring our world.

It's called the South Atlantic Anomaly, stretching across the gulf that separates Africa from South America, and the latest data suggests that it has expanded by roughly half the size of continental Europe since 2014, while its magnetic intensity weakens.

The measurements indicate that the ocean of molten iron in Earth's outer core, which generates the planetary magnetic field, isn't steady and calm but churning and complex, with behavior that can change the external field on timescales as short as years.

Earth's magnetic field is a vast web of magnetic field lines generated by the planet's core dynamo: the rotating, conductive, convecting fluid in the outer core that converts kinetic energy into magnetic energy. It stretches out into space, forming an invisible structure around our planet that helps keep the atmosphere in and cosmic rays out.

Over the eons, the magnetic field has fluctuated in intensity, even flipping entirely in complete polar reversals. These events pose no direct hazard to life on the surface, but there are other reasons to study them.

Some navigation systems rely on Earth's magnetic field, for example. The magnetic field also deflects charged particles; a weaker magnetic field makes satellites more vulnerable to hazardous charge buildup.

What's more, the magnetic field deflects solar and cosmic radiation, so astronauts and people flying at high altitudes are exposed to higher radiation doses where the magnetic field is weaker.

Understanding changes in the magnetic field can reveal what's happening deep inside our planet, which, in turn, may help scientists build better predictive models of future behavior to mitigate these problems.

The size and strength of the anomaly in 2014 (top) and 2025 (bottom). 

(ESA)

The South Atlantic Anomaly, or SAA, has been known since at least the 1960s, but there were no detailed, continuous studies on it until the launch of ESA's Swarm mission in 2013 – three satellites designed to work together to map the geomagnetic field.

The latest results of the Swarm mission represent the longest continuous monitoring of Earth's magnetic field to date, revealing new complexities to the SAA.

"The South Atlantic Anomaly is not just a single block," says geophysicist Chris Finlay of the Technical University of Denmark. "It's changing differently towards Africa than it is near South America. There's something special happening in this region that is causing the field to weaken in a more intense way."

Scientists don't know exactly what's causing the anomaly, but they do know that the magnetic field inside the planet below that region is not behaving as expected. Earth's magnetic field is roughly dipolar; the north magnetic pole is where magnetic field lines dive into the planet, and the south magnetic pole is where they emerge.

This is a very simplified version; the magnetic field as a whole is rather more complicated, but by and large, this model describes how the field is expected to behave. At the SAA, some of the magnetic flux beneath Earth's surface is curiously reversed.

"Normally we'd expect to see magnetic field lines coming out of the core in the southern hemisphere. But beneath the South Atlantic Anomaly we see unexpected areas where the magnetic field, instead of coming out of the core, goes back into the core," Finlay explains.

"Thanks to the Swarm data we can see one of these areas moving westward over Africa, which contributes to the weakening of the South Atlantic Anomaly in this region."

This magnetic flux reversal could be linked to a large, mysterious blob of super-hot material outside Earth's core known as the African Large Low-Shear-Velocity Province (LLSVP) under the SAA.

https://www.youtube.com/watch?v=cQX1vumsJms&t=2s

That blob could disrupt convection from the core, which in turn would change the behavior of the magnetic field above it. It's thought that this is a normal Earth behavior; we just didn't have the tools to study it until recently.

Other changes Swarm has observed in Earth's magnetic field include a slight weakening over Canada and a slight strengthening over Siberia, linked to a shifting magnetic structure underneath North America.

"It's really wonderful to see the big picture of our dynamic Earth thanks to Swarm's extended timeseries," says Anja Stromme, Swarm mission manager at ESA. "The satellites are all healthy and providing excellent data, so we can hopefully extend that record beyond 2030, when the solar minimum will allow more unprecedented insights into our planet."

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