Position and Shape
The Van Allen radiation belts are symmetric about the Earth's magnetic axis, which is tilted with respect to the Earth's rotational axis by an angle of ~11 degrees. The intersection between the magnetic and rotation axes of the Earth is located not at the Earth's centre, but some 500 kilometres (300 mi) further North. Because of this asymmetry, the inner Van Allen belt is closest to the Earth's surface over the south Atlantic ocean where it dips down to 200km (124mi) altitude, and farthest from the Earth's surface over the north Pacific ocean.
One reason for the variation is that if we represent the Earth's magnetism by a bar magnet of small size but strong intensity ("dipole"), the best description is obtained by placing that magnet not at the center of Earth but some distance away from it, more or less in the direction of Singapore. As a result, over South America and the south atlantic, near Singapores antipodal point the magnetic field is relatively weak, resulting in a lower repulsion to trapped particles of the radiation belts here, and as a result these paricles reach deeper into the upper atmosphere than they otherwise would.
The shape of the SAA changes over time. Since its initial discovery in 1958, the southern limits of the SAA have remained roughly constant while a long-term expansion has been measured to the northwest, the north, the northeast, and the east. Additionally, the shape and particle density of the SAA varies on a diurnal basis, with greatest particle density corresponding roughly to local noon. At an altitude of approximately 500 km (300 mi), the SAA spans from -50° to 0° geographic latitude and from -90° to +40° longitude. The highest intensity portion of the SAA drifts to the west at a speed of about 0.3 degrees per year, and is noticeable in the references listed below. The drift rate of the SAA is very close to the rotation differential between the Earth's core and its surface, estimated to be between 0.3 and 0.5 degrees per year.
Current literature suggests that a slow weakening of the geomagnetic field is one of several causes for the changes in the borders of the SAA since its discovery. As the geomagnetic field continues to weaken, the inner Van Allen belt gets closer to the Earth, with a commensurate enlargement of the SAA at given altitudes.
Read more about this topic: South Atlantic Anomaly
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