The South Atlantic Anomaly

The South Atlantic Anomaly is a large region over the South Atlantic and South America where Earth's magnetic field is unusually weak — the dark area in the map above, where total field strength drops to around 22,000 nanotesla compared with over 60,000 nT near the poles. Because the field is weaker here, more radiation from space reaches lower altitudes, which is why the region matters for satellites and high-altitude systems.

The anomaly is not fixed. Long-running satellite measurements show it has grown by an area nearly half the size of Europe since 2014, and part of it is weakening faster than the rest. Tracking a changing field is exactly what ground-based readings are for — every measurement submitted to OpenMag adds a real data point to the picture, especially from locations in or near the anomaly.