Why does the South Atlantic region have an unusually weak geomagnetic field?

The South Atlantic region experiences an unusually weak geomagnetic field because Earth's magnetic field is not a perfect, centered dipole. While the dipole model accounts for more than 90 percent of the field, the remaining portion—known as the non-dipole field or regional geomagnetic anomaly—creates significant variations in field strength across the planet's surface.1

The specific weakness in the South Atlantic is a prominent example of this regional anomaly. This phenomenon can be partially explained by the fact that the best-fit dipole is not perfectly centered within Earth; if it were, the field strength would be nearly constant along the equator. Instead, the offset of this dipole, combined with other complex internal sources, results in areas where the observed field deviates from the expected dipolar strength. These anomalies are significant because they influence the behavior of particles trapped in the outer field and affect compass readings in affected regions.1