A pronounced weak spot in the Earth’s magnetic field typically centered over the South American country of Paraguay has not only been recorded as weakening over the centuries, but has more recently been found to be expanding eastward across the South Atlantic toward South Africa, a situation that could spell trouble for orbiting satellites that usually rely on the planet’s magnetic field for protection against damaging cosmic radiation.

This expansion of the South Atlantic Anomaly (SAA), the area where the Earth’s magnetic field is at its weakest, was charted by the European Space Agency’s trio of geomagnetic-mapping Swarm satellites, with the data showing the clear expansion of the weakest part of the anomaly via a lobe that is extending across the Atlantic Ocean toward the western tip of South Africa, a situation that has doubled the size of the magnetic gap over the past 11 years.

“The South Atlantic Anomaly is not just a single block,” according to lead study author Chris Finlay, a Professor of Geomagnetism at Denmark’s Technical University. “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.”

First discovered in 1958, the South Atlantic Magnetic Anomaly is an area where the Earth’s magnetic field is at its weakest. Typically centered on the landlocked country of Paraguay, the weakest portion of the SAA straddles the breadth of the lower portion of the South American continent. This lowered field intensity allows the inner Van Allen radiation belt, a series of doughnut-shaped zones in the Earth’s magnetosphere that trap energetic charged particles emanating from the Sun, to dip to within 200 kilometers (120 miles) of the planet’s surface.

The SAA itself is caused by a particular asymmetry in the planet’s structure that causes the center of Earth’s magnetic field to be located in a different place than the planet’s physical center; instead, it is located somewhat north of the Earth’s physical center and slightly toward Singapore, resulting in the outer edge of the field dipping down toward the South Atlantic.

And as the extent of the area of the SAA broadens, so does its detrimental influence on satellites that pass through the danger zone, heightening the vulnerability of both spacecraft and astronauts to incoming cosmic radiation, increasing the likelihood of onboard equipment malfunctioning and negative health effects on human spacefarers.

The reason for the lack of uniformity of the planet’s magnetic field is due to it being generated not by a single source, as we would see with a regular bar magnet, but is instead an amalgamation of numerous regional zones in the outer core that act like massive electrical generators that, in turn, generate a series of localized magnetic fields. While these individual fields form the larger effect we see as the Earth’s magnetic field, differences in the flow and density of material within individual regions—the previously-mentioned asymmetry—cause the field to vary in strength at different points at the Earth’s surface.

“When you’re trying to understand Earth’s magnetic field, it’s important to remember that it’s not just a simple dipole, like a bar magnet,” Finlay explains. “It’s only by having satellites like Swarm that we can fully map this structure and see it changing.”