A vast and growing region of magnetic weakness in Earth’s protective shield is developing over the South Atlantic, according to more than a decade of precise measurements from space. Data from the European Space Agency’s (ESA) Swarm satellite trio reveals this area, known as the South Atlantic Anomaly, has expanded significantly and its intensity has continued to drop, increasing the risk of damage to orbiting satellites from high-energy solar particles.
These findings, based on an 11-year dataset from 2014 to 2025, provide the most detailed picture yet of this curious feature of our planet’s invisible armor. The anomaly is not a new discovery, having been first charted in the 19th century, but its recent evolution has commanded special attention. The consistent, high-fidelity data from the Swarm mission allows scientists to track its development with unprecedented accuracy, revealing a complex and dynamic process driven by the restless ocean of molten iron churning deep within the planet’s core.
A Shield in Flux
Earth’s magnetic field is a fundamental force, essential for protecting life from the harsh environment of space. It deflects the bulk of cosmic radiation and charged particles from the sun, preventing them from stripping away our atmosphere. This shield, however, is not static. Generated by the turbulent motion of liquid iron in the planet’s outer core, approximately 3,000 kilometers beneath the surface, the field is constantly shifting, strengthening in some areas while weakening in others. It is more akin to a living, breathing entity than a simple bar magnet.
This geodynamic engine creates a complex field with irregularities that wander over time. The most prominent of these is the South Atlantic Anomaly (SAA), a large area stretching from the southeastern coast of South America to the southwestern coast of Africa where the magnetic field strength is markedly lower than anywhere else on the globe. This dip in magnetic intensity allows the Van Allen radiation belts, zones of trapped energetic particles, to come closer to the planet’s surface.
The Swarm Constellation’s Vigilant Eye
Launched in November 2013, the ESA’s Swarm mission is a trio of identical satellites designed to disentangle the various sources of Earth’s magnetism. Flying in carefully coordinated orbits, the satellites measure the strength and direction of the magnetic field with extreme precision. This allows researchers to separate the dominant field generated by the core from weaker magnetic signals originating in the mantle, crust, oceans, and the electrically charged layers of the upper atmosphere known as the ionosphere and magnetosphere.
By providing a continuous and high-resolution record of the magnetic field, Swarm has built the longest and most detailed dataset of its kind from space. This long-term monitoring is crucial for understanding not only the present state of the magnetic shield but also for forecasting its future evolution. The mission’s data is vital for scientific research, space weather prediction, and improving the accuracy of navigation systems that rely on magnetic field models.
Inside the South Atlantic Anomaly
The latest results from the Swarm mission, published in the journal Physics of the Earth and Planetary Interiors, paint a detailed picture of the SAA’s recent changes. The anomaly is a persistent feature, but its character is far from stable.
Growth and Evolution
Between 2014 and 2025, the overall area of the South Atlantic Anomaly has expanded steadily, now covering a region nearly half the size of continental Europe. The minimum field strength within this zone has also decreased, dropping from approximately 22,430 nanoteslas to 22,094 nanoteslas over the 11-year period. Furthermore, the evolution is not uniform. According to the study’s lead author, Chris Finlay from the Technical University of Denmark, the anomaly is changing differently near Africa than it is near South America, with an accelerated rate of weakening observed in a region southwest of Africa since 2020.
Risks to Space-Based Technology
The primary practical concern related to the SAA is its effect on satellites in low-Earth orbit. As spacecraft pass through this region, the reduced magnetic protection exposes them to a higher flux of energetic particles. This increased radiation can disrupt sensitive electronic systems, causing temporary glitches, memory corruption, or even permanent hardware damage. Satellite operators often put their instruments into a safe mode or power down non-essential components when their orbits traverse the anomaly to mitigate these risks. The continued weakening and expansion of the SAA mean that more orbital paths are affected for longer durations, posing a growing challenge for space agencies and commercial satellite operators.
Journey to the Earth’s Core
The driver of the SAA’s behavior lies at the core-mantle boundary, nearly 3,000 kilometers below the surface. The swirling liquid iron of the outer core generates the magnetic field, and subtle changes in its flow can produce significant effects at the surface. Swarm’s data has allowed scientists to peer into this hidden world with greater clarity.
The analysis reveals unusual patterns in the magnetic field lines beneath the SAA. In most of the Southern Hemisphere, field lines are expected to emerge from the core. However, under southern Africa and the South Atlantic, Swarm has identified “reversed flux patches,” where the field lines unexpectedly flow back into the core. The movement of these patches appears to be directly linked to the weakening of the field at the surface. One such patch is observed moving westward under Africa, contributing to the intense weakening in that part of the anomaly.
A Global and Asymmetrical Field
The changes observed in the South Atlantic are part of a larger, global pattern of magnetic field fluctuation. The Swarm data shows that while the field is weakening in some areas, it is strengthening in others. For instance, over the same 11-year period that the SAA has weakened, a region of strong magnetic field over Siberia has grown by an area comparable to the size of Greenland. Conversely, another strong magnetic patch located over Canada has shrunk by an area nearly the size of India. These simultaneous changes highlight the complex and interconnected nature of the global geodynamo. They also underscore that the overall weakening trend is not uniform across the planet’s surface.
Future Outlook and Ongoing Research
While the weakening of the magnetic field in the SAA is a significant event, scientists emphasize that it is not necessarily a sign of an impending complete magnetic pole reversal. Such reversals have occurred many times in Earth’s history but unfold over thousands of years. The current changes are more likely part of normal, long-term fluctuations in the magnetic field’s intensity and structure. Some researchers suggest the pattern may be part of an oscillatory cycle within the outer core rather than a precursor to a planet-wide transition.
Continued monitoring by the Swarm constellation and future missions will be essential for understanding the long-term trajectory of the SAA and the global magnetic field. By refining models of the Earth’s core, scientists can improve forecasts of space weather and better understand the fundamental processes that sustain our planet’s vital protective shield. As technology becomes increasingly reliant on space-based infrastructure, tracking the flux of this invisible force is more critical than ever.
