South Atlantic Anomaly

Van Allen radiation belt

The South Atlantic Anomaly (SAA) is an area where the Earth’s inner Van Allen radiation belt (a protective zone of energetic charged particles originating from the solar wind that are captured by Earth’s magnetic field) comes closest to the Earth’s surface, dipping down to an altitude of 120 miles. 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. 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 symmetrical about the Earth’s magnetic axis, which is tilted with respect to the Earth’s rotational axis by an angle of approximately 11°. The intersection between the magnetic and rotation axes of the Earth is located not at the Earth’s center, but some 280 to 310 miles away. 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 120 miles in altitude, and farthest from the Earth’s surface over the north Pacific Ocean.

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 daily basis, with greatest particle density corresponding roughly to local noon. 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 kilometres 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, an effect termed the ‘cosmic ray visual phenomena.’ NASA has reported that modern laptops have crashed when Space Shuttle flights passed through the anomaly.

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