South Atlantic Anomaly
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The South Atlantic Anomaly (or SAA) is the region where Earth's inner van Allen radiation belt makes its closest approach to the planet's surface. Thus, for a given altitude, the radiation intensity is greater within this region than elsewhere. The van Allen radiation belts are symmetric with the Earth's magnetic axis, which is tilted with respect to the Earth's rotational axis by an angle of ~11 degrees. Additionally, the magnetic axis is offset from the rotational axis by ~450 kilometers (280 miles). Because of the tilt and offset, the inner van Allen belt is closest to the Earth's surface over the south Atlantic ocean, and furthest from the Earth's surface over the north Pacific ocean. [1]
Some believe that the anomaly is a side effect of geomagnetic reversal.[who?] This may result from a misunderstanding of the extant literature, which mentions a slow weakening of the geomagnetic field as one of several causes for the changes in the borders of the SAA since its discovery. What is true is that as the geomagnetic field continues to weaken, the inner van Allen belt will get 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 each time. The International Space Station, orbiting with an inclination of 51.6°, required extra shielding to deal with this problem. The Hubble Space Telescope does not take observations while passing through the SAA.
The size of the SAA increases with increasing altitude. The shape of the SAA changes over time. Since its initial discovery in the late 1950s, 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, the SAA spans from -50° to 0° geographic latitude and from -90° to +40° longitude. [2]
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.
