Map of the strength of the Earth’s magnetic field showing the South Atlantic Anomaly in darker colors, a spot where the magnetic field is weakening.
Finlay et al.2025/Physics of the Earth and Planetary Interiors
Using 11 years of magnetic field measurements, scientists have discovered that the weak region in Earth’s magnetic field over the South Atlantic – known as the South Atlantic Anomaly – has expanded by an area nearly half the size of continental Europe since 2014.
Back in the 19th century, the first geomagnetic surveys revealed variations in the strength of Earth’s magnetic field. One weak region is located southeast of South America and expands into the Atlantic.
Today, the South Atlantic Anomaly is of particular interest for space safety, as satellites passing over the region are faced with higher doses of incoming radiation. This can lead to malfunctions or damage to critical hardware, and even blackouts.
Swarm, an Earth Explorer mission developed under ESA’s Earth Observation FutureEO programme, comprises a constellation of three identical satellites that precisely measure changes in Earth’s magnetic field since November 2013.
A new study, based on the missions’ latest results, reveal that while the South Atlantic Anomaly expanded steadily between 2014 and 2025, a region of the Atlantic Ocean southwest of Africa has experienced an even faster weakening of Earth’s magnetic field since 2020.
“The South Atlantic Anomaly is not just a single block,” says lead author Chris Finlay, Professor of Geomagnetism at 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.”
Global magnetic field Intensity (F in Nanotesla) at Earth’s surface in the year 2014 (top) and in 2025 (bottom). During this time interval, the South Atlantic Anomaly (darker colors) is expanding eastward.
Finlay et al.2025/Physics of the Earth and Planetary Interiors
The changes are linked to how Earth’s magnetic field is generated deep within the planet. The planet’s rotation sets the liquid nickel-iron-alloy of the core in a turbulent flow, generating an electric current and a magnetic field. However, the field is not just a simple dipole, like a bar magnet, but a complex pattern of intersecting magnetic field lines.
“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. 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,” explains Finlay.
As the field weakens in some regions, it strengthens in others. Since Swarm has been in orbit, the magnetic field has expanded and strengthened over Siberia and around Canada.
Polar view showing strong magnetic fields over Siberia and around Canada, with the brightly-colored spot over Siberia even strengthening over time.
Finlay et al.2025/Physics of the Earth and Planetary Interiors
Scientists still do not fully understand why Earth’s magnetic field fluctuates in strength and occasionally even reverses polarity.
During some geological periods, Earth’s magnetic field remained stable for millions of years, whereas in others, it shifted repeatedly over intervals of just a few 1,000 to 10,000 years. This erratic behavior may explained by material flowing across the core-mantle boundary, influencing turbulence and how the magnetic field is generated in the core.
The full study, titled “Core field changes from eleven years of Swarm satellite observations,” was published in the journal Physics of the Earth and Planetary Interiors and is available online.
Additional material and interviews provided by ESA.
