The Earth is surrounded by a pair of concentric donut-shaped clouds called the Van Allen radiation belts which, like magnetic bottle, store and trap charged particles from the solar wind. They are aligned with the magnetic axis of the Earth, which is tilted by 11 degrees from the rotation axis of the Earth, and are not symmetrically placed with respect to the Earth's surface. Although the inner surface is 1200 - 1300 kilometers from the Earth's surface on one side of the Earth, on the other they dip down to 200 - 800 kilometers. Above South America, about 200 - 300 kilometers off the coast of Brazil, and extending over much of South America, the nearby portion of the Van Allen Belt forms what is called the South Atlantic Anomaly. Satellites and other spacecraft passing through this region of space actually enter the Van Allen radiation belt and are bombarded by protons exceeding energies of 10 million electron volts at a rate of 3000 'hits' per square centimeter per second. This can produce 'glitches' in astronomical data, problems with the operation of on-board electronic systems, and premature aging of computer, detector and other spacecraft components.
The Hubble Space Telescope passes through the 'SAA' for 10 successive orbits each day, and spends nearly 15 percent of its time in this hostile region. Astronauts are also affected by this region which is said to be the cause of peculiar 'shooting stars' seen in the visual field of astronauts.
The illustration above is a picture of the relative location of the SAA from NASA. The data were collected by the South Atlantic Anomaly Detector (SAAD) aboard the ROSAT spacecraft. It consists of 10 cm² of Germanium and served as a particle background monitor.
Return toAsk the Astronomer