Three thousand kilometres beneath our feet, the liquid iron of the outer core spins and churns, forging a magnetic field that envelopes the entire globe. Yet, above Brazil and the South Atlantic, this invisible shield weakens so markedly that satellite operators follow a well-oiled procedure: cut the sensitivity instruments before each pass. This zone, the magnetically weakest region on Earth, allows solar energetic particles to descend far closer to the surface, to the point of regularly disturbing onboard electronics.
- The Earth’s magnetic field anomalously weakens over the South Atlantic, allowing solar energetic particles to come closer to the surface.
- Satellites must deactivate their sensitive instruments on each crossing to avoid malfunctions caused by this heightened radiation.
- Since 2013, the anomaly has expanded, drifted westward, and since 2020 shows signs of splitting into two distinct cells.
A field born in the core, but never perfectly round
The Earth’s magnetic field is not a fixed, symmetric halo. It arises from the core, at the boundary between the liquid outer core and the solid mantle, and extends beyond the surface to act as a shield that repels and traps charged particles arriving from the Sun. This mechanism, known as the geodynamo, generates a field whose strength varies widely from one region to another across the globe. The result is that some areas are strongly protected, while others are far more exposed, with little relation to latitude or proximity to geographic poles.
The South Atlantic anomaly is not a new phenomenon. First described in 1958, the South Atlantic anomaly is a region with a weak magnetic field, whose underlying mechanisms still baffle scientists. Researchers know that the local weakness of the field is tied to deep irregularities, even if all the exact causes remain unclear.
What it concretely changes for satellites
Over South America and the South Atlantic, this unusual weak spot in the field permits particles to descend much closer to the surface, and the radiation they carry can disable onboard computers and disrupt data collection on satellites that cross the area. On the International Space Station, the GEDI instrument, which maps forest ecosystems, experiences this almost monthly. The ISS regularly crosses the anomaly, and while astronauts are shielded, the exterior instruments undergo increased exposure: the GEDI instrument lead reports occasional malfunctions and reboots, resulting in a limited loss of data each month.
Space telescopes are not spared either. Some lose several minutes of observing time per orbit when they pass over the zone, as the most delicate sensors are put into standby as a precaution. Space agencies’ engineers have no choice but to adapt to this wobble of the magnetic field by turning off instruments on satellites as they pass through the area and accepting a loss of certain data. Other missions, such as the ICON ionospheric explorer, adjust their operations at each crossing.
Astronauts, for their part, recount something even more strange. With eyelids closed, some people report perceiving flashes of light as their vessel traverses the region. This testimony has circulated for decades in accounts of human spaceflight.
A fault that grows, drifts west, and splits
Since the European Swarm constellation launched in 2013, the European Space Agency has finally gained continuous, precise monitoring of this zone. From 1970 to 2020, the minimum field strength in this region declined from about 24,000 nanoteslas to 22,000 nT, and the anomaly area moved westward at roughly 20 kilometres per year. Between 2014 and 2025, considering only the surface where the field drops below 26,000 nT, the area containing the weakest field expanded by 0.9% of Earth’s total surface, while the minimum intensity fell by 336 nanoteslas, from 22,430 nT to 22,094 nT. To give that figure some scale, this expansion is nearly half the area of Europe.
Another, more recent change: the zone no longer forms a single bloc. In the last five years, a second center of minimum intensity emerged southwest of Africa, suggesting the anomaly could split into two distinct cells. Geophysicist Chris Finlay of the Technical University of Denmark sums up this uneven dynamic: “The field changes differently toward Africa than near South America. Something special is happening in that region that weakens the field more intensely.” This splitting complicates the models used to plan the orbits of future satellites.
What this phenomenon does not mean
The local weakening of the field regularly fuels a dramatic hypothesis: an imminent reversal of the magnetic poles. Scientists are adamant on this point. Although the behavior shares some features with patterns that precede a full geomagnetic reversal, there is nothing to indicate an imminent flip. The anomaly remains a case study used to refine models of the Earth’s field, not a confirmed precursor signal.
No health effects have been demonstrated on the ground for people living in the affected region. The anomaly does not produce any noticeable impact on daily life at the surface, and its magnetic weakening stays within what scientists consider normal variation. The phenomenon primarily affects electronics in orbit, where charged particles strike components directly, far from the atmosphere that shields the ground.
Sources: x.com | futura-sciences.com
