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The South Atlantic Anomaly (SAA) is an area where the Earth's inner Van Allen radiation belt comes closest to the Earth's surface dipping down to an altitude of 200 km (124 mi). This leads to an increased flux of energetic particles in this region and exposes orbiting satellites to higher-than-usual levels of radiation. The effect is caused by the non-concentricity of the Earth and its magnetic dipole, and the SAA is the near-Earth region where the Earth's magnetic field is weakest relative to an idealized Earth-centered dipole field.
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 approximately 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 200 km (124 mi) 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 Singapore's 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 particles 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.
The South Atlantic Anomaly is of great significance to astronomical satellites and other spacecraft that orbit the Earth at several hundred kilometers altitude; these orbits take satellites through the anomaly periodically, exposing them to several minutes of strong radiation, caused by the trapped protons in the inner Van Allen belt. The International Space Station, orbiting with an inclination of 51.6°, requires extra shielding to deal with this problem. The Hubble Space Telescope does not take observations while passing through the SAA. Astronauts are also affected by this region which is said to be the cause of peculiar 'shooting stars' (phosphenes) seen in the visual field of astronauts. Passing through the South Atlantic Anomaly is thought to be the reason for the early failures of the Globalstar network's satellites.
The PAMELA experiment, while passing through the SAA, detected antiproton levels that were orders of magnitude higher than those expected from normal particle decay. This suggests the Van Allen belt confines antiparticles produced by the interaction of the Earth's upper atmosphere with cosmic rays.
NASA has reported that modern laptops have crashed when space shuttle flights passed through the anomaly. In October 2012, the SpaceX CRS-1 Dragon spacecraft attached to the International Space Station experienced a transient problem as it passed through the anomaly.