South Atlantic Anomaly
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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. For a given altitude, the radiation intensity is greater within this region than elsewhere. The Van Allen radiation belts are symmetric with the Earth's magnetic axis, which is tilted with respect to the Earth's rotational axis by an angle of ~11 degrees. Because of this tilt, the inner Van Allen belt is closest to the Earth's surface over the south Atlantic ocean, and farthest from the Earth's surface over the north Pacific ocean.[2]
The South Atlantic Anomaly is of great significance to astronomical satellites and other spacecraft that orbit the Earth at several hundred kilometers altitude; 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°, requires extra shielding to deal with this problem. The Hubble Space Telescope does not take observations while passing through the SAA.[3]
The shape of the SAA changes over time. Since its initial discovery in the late 1950s, the southern limits of the SAA have remained roughly constant while a long-term expansion has been measured to the northwest, the north, the northeast, and the east. Additionally, the shape and particle density of the SAA varies on a diurnal basis, with greatest particle density corresponding roughly to local noon. At an altitude of approximately 500 km, the SAA spans from -50° to 0° geographic latitude and from -90° to +40° longitude.[4] The highest intensity portion of the SAA drifts to the west at a speed of about 0.3 degrees per year, and is noticeable in the references listed 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.[citation needed]
Current literature suggests that a slow weakening of the geomagnetic field is one of several causes for the changes in the borders of the SAA since its discovery. As the geomagnetic field continues to weaken, the inner Van Allen belt gets closer to the Earth, with a commensurate enlargement of the SAA at given altitudes.[citation needed] Some scientists, including Dr. Pieter Kotze, head of the geomagnetism group at the Hermanus Magnetic Observatory in the southern Cape, believe that the anomaly is a side effect of geomagnetic reversal.[citation needed]
[edit] See also
[edit] References
- ^ "ROSAT SAA" (HTML). http://heasarc.gsfc.nasa.gov/docs/rosat/gallery/misc_saad.html. Retrieved on 2007-10-16.
- ^ Stassinopoulos, E.G.; Staffer, C.A. (2007), Forty-Year Drift and Change of the SAA, NASA Goddard Spaceflight Center
- ^ "Hubble Achieves Milestone: 100,000th Exposure". STScI. 1996-07-18. http://hubblesite.org/newscenter/archive/releases/1996/25/text/. Retrieved on 2009-01-25.
- ^ "The South Atlantic Anomaly" (HTML). Ask an Astrophysicist. http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/961004.html. Retrieved on 2007-10-16.
