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The Question

(Submitted March 07, 1997)

In a previous question you have told us what the boundaries of the South Atlantic Anomaly are. Can you please tell me what causes the SAA?

The Answer

The South Atlantic Anomaly is of particular interest to us in high-energy astrophysics. This is a region of very dense radiation ("high particle flux") above the Atlantic Ocean off the coast of Brazil. The particle flux is so high in this region that often the detectors on our satellites must be shut off (or at least placed in a "safe" mode) to protect them from the radiation.

The South Atlantic Anomaly comes about because the Earth's field is not completely symmetric. If we were to represent it by a compact magnet (which reproduces the main effect, not the local wiggles), that magnet would not be at the center of the Earth but a few hundred miles away, in the direction away from the "Anomaly." Thus the anomaly is the region most distant from the "source magnet" and its magnetic field (at any given height) is thus relatively weak. The reason trapped particles don't reach the atmosphere is that they are repelled (sort of) by strong magnetic fields, and the weak field in the anomaly allows them to reach further down than elsewhere (see also https://wiki.oulu.fi/display/SpaceWiki/Radiation+belts).

Also, Dr. Steve Snowden has a brief description with a cool graphic at http://heasarc.gsfc.nasa.gov/Images/rosat/misc_saad.html

Hope this answers your question.

Regards,
Laura Whitlock
for Ask an Astrophysicist

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