The South Atlantic Anomaly
(SAA) refers to a zone above South America
and the south part of the Atlantic ocean
where the Earth
's magnetic field
is perturbed. This perturbation
is the result of the fact that the magnetosphere
is decentered with respect to the Earth's surface, and is furthermore inclined at 11 degrees with respect to the Earth's axis of rotation. The Van Allen radiation belts
are thus not centered around the Earth, and as a result, they are found at an altitude of 1100 to 1300 km on one side of the Earth, and of 200 to 800 km on the other.
Since the Van Allen belts trap and store charged particles from the solar wind, any spacecraft passing through the SAA is subject to be bombarded by protons with energies in excess of 10 GeV, at a rate of 3000 hits/cm2s. This of course can cause serious dammage and glitches to electronic components and human DNA, unless measures are taken to protect equipment and astronauts.
Spacecrafts in low Earth orbit with orbital inclinations of 35 to 60 degrees periodically go through the SAA for non-negligible periods of time. For instance, the Hubble Space Telescope spends 15% of its time in the SAA, and the ISS will fly right through it quite often too.