South Atlantic Anomaly

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The Van Allen radiation belts and the point of the South Atlantic Anomaly.

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. The result is that, for a given altitude, the radiation intensity is higher over this region than elsewhere. The SAA is produced by a "dip" in the Earth's magnetic field at that location, caused by the fact that the center of Earth's magnetic field is offset from its geographic center by 450 kilometers (280 miles). During Operation Argus, nuclear warheads were detonated at high altitudes in the SAA by the United States to research the possibility of destroying incoming ICBMs; the experiment proved that this technique had at least some possibility of damaging ICBM arming mechanisms.

The South Atlantic Anomaly is of great significance to satellites and other spacecraft that orbit at several hundred kilometers altitude and at orbital inclinations between 35° and 60°; 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.

An illustration of the SAA^[1] is shown in the image below. The figures show the maps of proton flux (number of protons registered by solid state detector with geometry factor 9.5×10^-3 cm² sr every second) at 850 km altitude. The maps are shown in geographic coordinates.

This image depicts:

the outer radiation belt - two red bands in top and bottom boundaries of images,
SAA - inner radiation belt - large red spot in the right part of images,
near-equatorial protons - band of proton flux enhancement near geomagnetic equator.

The SAA drifts to the west at a speed of about 0.3 degrees per year, and is noticeable in the figures 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.