The South Atlantic Anomaly (SAA) is a region over South America and the southern Atlantic Ocean where Earth’s magnetic field is significantly weaker than anywhere else on the planet. This weak spot allows energetic charged particles from space to dip much closer to Earth’s surface than normal, creating a zone of increased radiation that affects satellites and spacecraft but has no measurable impact on people living below it.
Why the Magnetic Field Is Weak There
Earth’s magnetic field is generated by the churning motion of liquid iron in the planet’s outer core, roughly 1,800 miles beneath the surface. This process, called the geodynamo, doesn’t produce a perfectly uniform shield. The magnetic field’s axis is tilted relative to Earth’s rotational axis, and the center of the magnetic field is offset from the geometric center of the planet. These asymmetries mean the protective field is naturally stronger in some places and weaker in others.
Over South America and the southern Atlantic, these factors combine to create the thinnest part of the magnetic shield. Globally, surface magnetic field intensity ranges from about 27,300 to 82,500 nanoteslas. The SAA sits at the lowest end of that range, meaning the field there is roughly a third as strong as it is at its peak near the poles. The slow, ongoing changes in the flow of liquid iron in the outer core are also responsible for the SAA’s observed westward drift over time.
What It Does to Satellites and Spacecraft
Earth’s magnetic field normally traps high-energy particles from the sun in doughnut-shaped zones called the Van Allen radiation belts, keeping them thousands of miles above the surface. In the SAA, the weakened field lets those particles sink to much lower altitudes, right into the orbital paths of satellites and the International Space Station.
When these particles strike onboard electronics, they can flip bits in computer memory, a phenomenon called a single-event upset. Satellites passing through the SAA routinely experience more of these glitches than anywhere else in their orbit. Some missions shut down sensitive instruments or switch to backup systems while transiting the region. The Hubble Space Telescope, for example, cannot take observations during roughly 15% of its orbital time because it passes through the SAA. Astronauts aboard the ISS also receive a higher dose of radiation during these transits, though spacecraft shielding and mission planning keep exposure within safe limits.
No Danger at Ground Level
Despite occasional claims that the SAA increases radiation for people on the ground, careful measurements show this isn’t the case. Any slightly elevated radiation readings at the surface in parts of South America have been traced to naturally occurring radioactive elements in the local rock and soil, not to particles leaking through the weakened magnetic field. The atmosphere itself provides a thick additional layer of shielding that stops these particles long before they reach the ground.
The same holds true for commercial aviation. A study published in Scientific Reports specifically investigated whether flights through the SAA at cruising altitude experienced increased radiation exposure. The researchers found no indication of elevated radiation levels compared to flights at similar altitudes outside the anomaly. The idea that the SAA poses a risk to airline passengers or crew is, as the study’s authors put it, an urban legend.
The Anomaly Is Splitting in Two
The SAA isn’t static. Observational data collected between 2015 and 2020 revealed that the region of minimum magnetic field strength has started splitting from a single valley into two distinct lobes. Models projecting out to 2025 show this split continuing, with the anomaly also expanding westward and continuing to weaken in overall intensity.
This splitting creates new headaches for satellite operators. Instead of one concentrated weak zone that spacecraft pass through on a predictable schedule, two separate lobes mean more frequent encounters with elevated radiation across a broader swath of orbit. Mission planners have to account for these shifts when designing shielding, scheduling sensitive observations, and programming automated safeguards for onboard electronics.
Is It a Sign of a Pole Reversal?
The weakening and splitting of the SAA has fueled speculation that Earth’s magnetic poles might be gearing up to flip, a process that has happened hundreds of times over geologic history, most recently about 780,000 years ago. During a reversal, the global magnetic field weakens substantially before the north and south magnetic poles swap positions.
While the SAA’s behavior is consistent with the kind of field changes that precede a reversal, it’s also consistent with the normal fluctuations that happen all the time without leading to a flip. The geomagnetic field has always had regional weak spots that grow, shrink, drift, and disappear over centuries. Scientists can’t yet distinguish between a reversal in progress and ordinary variation, so the SAA alone doesn’t confirm one is coming. Even if a reversal were underway, the process takes thousands of years to complete, not a human lifetime.
NASA continues to monitor the SAA using ground-based observatories and satellite measurements. The practical concern remains focused on protecting the growing fleet of satellites in low Earth orbit, not on any risk to people on the surface.