Earth's magnetic field protects the surface from charged particles streaming from the Sun. Except where it doesn't. Over the South Atlantic, the field is anomalously weak — roughly a third of the strength found at equivalent latitudes elsewhere. Satellites passing through this region experience elevated radiation doses, causing hardware malfunctions, data corruption, and occasional blackouts. The Hubble Space Telescope cannot take observations while transiting it. The International Space Station's crew quarters are shielded against it.
ESA's Swarm satellite constellation, providing the longest continuous record of magnetic field measurements from space, shows the anomaly has expanded by an area nearly the size of continental Europe since 2014 (ESA, 2026). It is also splitting. One lobe has migrated toward South America. A second has developed southwest of Africa, where the field has weakened fastest over the last five years. Beneath both lobes, reversed-polarity flux patches — regions where magnetic field lines point back into the core instead of outward — are growing and drifting westward.
The expansion rate is not constant. The African lobe is accelerating. The dynamics below the surface — convective churning in the liquid iron outer core — are driving field geometry changes at the surface that no current model fully predicts. The anomaly may be a precursor to a geomagnetic reversal, a fluctuation that will stabilize, or a feature of the normal field that happens to be in a growth phase. The uncertainty is genuine: the observational record is too short to distinguish these scenarios.
The general principle: a system can develop a growing vulnerability in a region that functions normally everywhere else, and the growth can be observed without being explained. The measurement is precise — the field strength is known to nanotesla accuracy — but precision in the observation does not translate to certainty in the cause. Watching a hole grow and understanding why it grows are different activities with different data requirements.