Earth's Magnetic Field Is Weakening — The South Atlantic Anomaly Is Growing

There is a dent in Earth's invisible armor — and it has been growing for decades. A region of weakened magnetic field stretching from South America to southern Africa is now large enough to disrupt satellites, spike radiation doses on the International Space Station, and force mission controllers to switch off sensitive instruments every time their spacecraft crosses through it. The South Atlantic Anomaly (SAA) is not a new phenomenon, but the latest data from orbiting satellites shows it is expanding faster than before — and it is beginning to split in two.

Background

Earth's magnetic field is generated deep inside the planet, roughly 1,800 miles (2,900 km) below the surface, where swirling currents of molten iron in the outer core act like a massive, constantly shifting dynamo. This invisible shield deflects most of the charged particles streaming from the Sun and from deep space, without which Earth's atmosphere would slowly be stripped away — a fate that befell Mars billions of years ago.

The field, however, is not perfectly uniform. It has two structural quirks that produce the South Atlantic Anomaly: the magnetic axis is tilted relative to the planet's rotational axis, and the outer core hosts an unusual patch of reversed magnetic flux — where field lines dive back into the core rather than emerging from it — anchored beneath the African continent. Together, these features create a region over South America and the South Atlantic where the magnetic field is dramatically weaker than average. At its center, the field strength dips to around 22,000 nanotesla, compared to roughly 60,000 nanotesla near the poles.

Direct measurements of the global field's strength have been possible since 1840. Over that roughly 180-year span, the field has weakened by approximately nine percent on a global average, with the decline rate sitting at around 5–6 percent per century. The SAA is both a symptom and an amplifier of that broader trend.

A Dent That Is Splitting in Two

For most of modern history, the South Atlantic Anomaly was mapped as a single elongated blob of weakened field. That changed in 2020. Data from ESA's Swarm satellite constellation — three spacecraft dedicated to measuring Earth's geomagnetic field with extraordinary precision — revealed that the anomaly has begun dividing into two distinct lobes. One lobe remains centered over South America. A second, younger lobe is intensifying southwest of Africa, where the field has been weakening at an accelerated rate since 2020.

The total footprint tells a striking story. Since 2014, the weak region has expanded by an area nearly half the size of continental Europe. Over the same period, a zone of strong magnetic field over Canada has contracted by roughly 0.65 percent of Earth's surface area — equivalent to shrinking by approximately the area of India — while a compensating surge of field strength has grown over Siberia by roughly 0.42 percent of Earth's surface area, comparable to the size of Greenland.

Geophysicist Chris Finlay of the Technical University of Denmark, who led the Swarm analysis, described the division plainly: "The South Atlantic Anomaly is not just a single block. It's changing differently towards Africa than it is near South America. There's something special happening in this region." That "something special" is a reverse flux patch — an unusual upwelling from Earth's core — migrating steadily westward beneath southern Africa. As it moves, it drags a second center of magnetic weakness with it.

What Satellites Experience Inside the Anomaly

To understand why mission controllers treat the SAA with such caution, consider what happens inside it. Earth's inner Van Allen radiation belt — a donut-shaped zone of energetic protons trapped by the magnetic field — ordinarily sits hundreds of kilometers above most satellite orbits. Inside the SAA, the weakened field allows this belt to dip to altitudes as low as 200 kilometers, putting it squarely within the operating altitude of the International Space Station and most low-Earth orbit spacecraft.

High-energy protons in that belt can punch through spacecraft shielding and trigger what engineers call single-event upsets (SEUs): a charged particle strikes a memory cell or logic gate and flips a binary bit, potentially corrupting data, crashing software, or damaging hardware permanently. The Hubble Space Telescope accumulates the majority of its anomalous data events during SAA transits. NASA's ICON mission and instruments aboard the ISS regularly shut down non-essential systems during crossings as a protective measure.

The Globalstar satellite network provides one of the clearest historical examples of the SAA's impact: a string of satellite failures in 2007 was attributed to SAA radiation exposure, which degraded the amplifiers on board. As the anomaly expands and the second African lobe develops, the geographic corridor that satellites must manage becomes wider, increasing both transit time and cumulative radiation dose per orbit.

The Deep-Earth Source of the Anomaly

The root cause of the SAA lies in a geological feature that has no equivalent anywhere else on Earth: the African Large Low-Shear Velocity Province (LLSVP). This dense, ancient reservoir of rock, sitting atop the boundary between the lower mantle and the outer core, is thought to disrupt the normal convective flow of molten iron directly above it. The disruption generates the reverse flux patch that drives the SAA.

What makes the LLSVP remarkable is its age and size. Seismic tomography — essentially using earthquake waves as X-rays of the planet's interior — shows that this anomalous structure has existed for hundreds of millions of years, possibly since the breakup of the supercontinent Gondwana. Rock samples taken from volcanic sites in southern Africa and St. Helena Island in the South Atlantic have preserved records of locally reversed magnetic polarity dating back at least 11 million years, suggesting the SAA is not a fleeting modern phenomenon but a long-lived feature of Earth's deep-interior circulation.

This long history is both reassuring and scientifically humbling. The SAA is not a signal that something has suddenly gone wrong with Earth's core. It is a window into a slow-motion geological process that operates on timescales far beyond human civilization. Understanding it requires combining satellite measurements, rock magnetism, and core-flow simulations into a coherent picture — a challenge at the frontier of geophysics.

Should You Worry About a Pole Reversal?

The question most people ask when they read about a weakening magnetic field is a natural one: is Earth's magnetic field about to flip? Geomagnetic reversals — events where the north and south magnetic poles swap — do happen, and they have happened roughly hundreds of times over Earth's history at an average interval of around 200,000 years. The last reversal, the Brunhes-Matuyama event, occurred approximately 780,000 years ago, meaning Earth is statistically overdue.

The current evidence, however, does not support an imminent reversal. Research analyzing 9,000 years of paleomagnetic records shows that the SAA has existed in similar form for millennia, recurring and then fading without triggering a reversal. A 2022 study published in the Proceedings of the National Academy of Sciences found that the global field's dipole strength — while declining — remains far above the threshold typically associated with a reversal onset. Some models suggest the SAA may actually disappear within the next few hundred years as core-flow patterns shift, without any reversal occurring at all.

That said, reversals are not catastrophic in the way popular imagination sometimes frames them. During the transition, the field weakens significantly and becomes more complex — multiple poles may emerge temporarily — but life on Earth has survived every previous reversal in the geological record. The practical risks are elevated radiation in polar regions and disruption to technology that relies on magnetic orientation, not the apocalyptic scenarios occasionally attached to the topic.

Why It Matters

The South Atlantic Anomaly is significant on three distinct levels, each of which tends to get discussed separately but is better understood together.

For space operations, the expanding anomaly represents a concrete and growing engineering problem. Every satellite in low Earth orbit that passes over the South Atlantic — and many do, hundreds of times per day collectively — must be designed and operated with the SAA in mind. As the anomaly widens and the African lobe matures, mission planners face longer blackout windows, higher cumulative radiation budgets, and more complex orbit-design trade-offs. For commercial satellite operators launching constellations of thousands of spacecraft, the SAA is a permanent line item in risk calculations.

For climate and planetary science, the weakening magnetic field carries a subtler implication. A stronger influx of cosmic rays into the upper atmosphere during SAA transits can influence atmospheric chemistry, particularly ozone concentrations at certain altitudes. While the effect at the surface is negligible — the SAA creates no hazard for people on the ground — the atmospheric interactions are a legitimate area of ongoing research, and a substantially weakened global field over centuries could have measurable consequences for polar ozone chemistry.

For our understanding of Earth as a dynamic system, the SAA is a reminder that the planet's interior is actively changing on timescales that overlap with human civilization. The geomagnetic field is not a fixed background; it is a living output of convection in a molten iron shell thousands of kilometers below your feet. The Swarm satellites are giving scientists their first high-resolution, continuous view of that process. What they are learning is rewriting core models and, in some cases, revealing that the field's behavior is more complex and more regionally variable than any previous generation of scientists could have measured.

Frequently Asked Questions

Is the South Atlantic Anomaly dangerous to people on the ground?

No. The anomaly creates no measurable increase in radiation at Earth's surface. The weakened magnetic field allows more energetic particles to reach satellite altitudes — around 200 to 600 kilometers up — but the atmosphere provides more than enough shielding for anyone on the ground. There is no health risk to people living in South America, Africa, or anywhere else beneath the anomaly.

Is the South Atlantic Anomaly a sign that Earth's poles are about to flip?

Current evidence says no. While geomagnetic reversals do occur on geological timescales and Earth is statistically overdue for one, the SAA does not indicate an imminent reversal. Paleomagnetic records show the anomaly has persisted in similar form for thousands of years without triggering a flip. The global dipole field, though weakening, remains well above the threshold associated with reversal onset. Some models predict the SAA may fade and disappear within the next few centuries without any reversal occurring.

Why does the South Atlantic Anomaly affect satellites so much?

Inside the anomaly, the weakened field allows Earth's inner Van Allen radiation belt to dip as low as 200 kilometers altitude — the operating altitude of the ISS and most low-Earth orbit spacecraft. High-energy protons at those altitudes can penetrate spacecraft shielding and trigger single-event upsets in electronics. Satellites typically shut down non-essential systems during SAA crossings to reduce the risk of data corruption or hardware damage.

What is causing the anomaly to split into two lobes?

The split appears to be driven by a reverse flux patch beneath southern Africa — an area where molten iron flow in Earth's outer core is pushing magnetic field lines back into the core rather than outward. This patch is drifting westward and intensifying, creating a second center of weakness distinct from the original South American lobe. The geological structure responsible is the African Large Low-Shear Velocity Province, an ancient, dense region at the base of the mantle that disrupts normal core convection.

How long has the South Atlantic Anomaly existed?

Much longer than modern satellite records suggest. Volcanic rock samples from southern Africa and the South Atlantic island of St. Helena preserve records of locally reversed magnetic polarity stretching back at least 11 million years. The underlying geological structure driving the anomaly — the African LLSVP — has existed for hundreds of millions of years. The SAA is a persistent, recurring feature of Earth's deep interior, not a modern anomaly caused by human activity or any recent geological event.

The Takeaway

Earth's magnetic shield is not breaking down — but it is speaking. The South Atlantic Anomaly is an unusually clear readout from a process unfolding thousands of kilometers below the surface, one that will continue to reshape satellite operations, push geophysical models to their limits, and occasionally resurrect questions about pole reversals for decades to come. Watch the Swarm mission data: as ESA's satellites continue logging field measurements, the second African lobe will either stabilize, intensify, or eventually merge back into a single anomaly — and whatever happens will tell scientists something irreplaceable about how Earth's dynamo actually works. The planet's interior has been writing this story for millions of years; scientists finally have the instruments to read it in real time.

Sources

• NASA Researchers Track Slowly Splitting 'Dent' in Earth's Magnetic Field — NASA, 2021
• Swarm Reveals Growing Weak Spot in Earth's Magnetic Field — ESA / Swarm Mission, 2025
• Flip Flop: Why Variations in Earth's Magnetic Field Aren't Causing Today's Climate Change — NASA Science, 2024
• Are the Earth's Magnetic Poles About to Swap Places? Strange Anomaly Gives Reassuring Clue — The Conversation, 2021
• South Atlantic Anomaly: 2015 through 2025 — NASA Scientific Visualization Studio, 2025
• Weird Behavior in Earth's Magnetic Field Over South Atlantic Dates Back 11 Million Years — Space.com, 2021