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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
http://www.oulu.fi/~spaceweb/textbook/radbelts.html (http://www.oulu.fi/%7Espaceweb/textbook/radbelts.html)).
Also, Dr. Steve Snowden has a brief description with a cool graphic at
http://heasarc.gsfc.nasa.gov/Images/rosat/misc_saad.html (http://heasarc.gsfc.nasa.gov/docs/rosat/gallery/misc_saad.html)
Hope this answers your question.
Regards,
Laura Whitlock
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