South Atlantic Anomaly
The South Atlantic Anomaly (SAA) is an area where the Earth's inner Van Allen radiation belt comes closest to the Earth's surface. This leads to an increased flux of energetic particles in this region and exposes orbiting satellites to higher than usual levels of radiation. The effect is caused by the non-concentricity of the Earth and its magnetic dipole, and the SAA is the near-Earth region where the Earth's magnetic field is weakest.
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[edit] Position and shape
The Van Allen radiation belts are symmetric about the Earth's magnetic axis, which is tilted with respect to the Earth's rotational axis by an angle of ~11 degrees. The intersection between the magnetic and rotation axes of the Earth is located ~500 kilometres (300 mi) more to the North, above the centre of the Earth. Because of this tilt and translation, 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 shape of the SAA changes over time. Since its initial discovery in 1958,[3] 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 (300 mi), 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.
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]
[edit] Effects
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, caused by the trapped protons in the inner Van Allen belt. 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.[5] Astronauts are also affected by this region which is said to be the cause of peculiar 'shooting stars' (phosphenes) seen in the visual field of astronauts.[6] Passing through the South Atlantic Anomaly is thought[7] to be the reason for the early failures of the Globalstar network's satellites.
The PAMELA experiment, while passing through the SAA, detected antiproton levels that were orders of magnitude higher than those expected from normal particle decay. This suggests the Van Allen belt confines antiparticles produced by the interaction of the Earth's upper atmosphere with cosmic rays.[8]
NASA has reported that modern laptops have crashed when the space shuttle flights passed through the anomaly.[9]
[edit] See also
[edit] References
- ^ "ROSAT SAA". http://heasarc.gsfc.nasa.gov/docs/rosat/gallery/misc_saad.html. Retrieved 2007-10-16.
- ^ Stassinopoulos, E.G.; Staffer, C.A. (2007). Forty-Year Drift and Change of the SAA. NASA Goddard Spaceflight Center
- ^ Broad, William J. (June 5, 1990). "'Dip' on Earth Is Big Trouble In Space". New York Times. http://www.nytimes.com/1990/06/05/science/dip-on-earth-is-big-trouble-in-space.html?scp=1&sq=south%20atlantic%20anomaly&st=cse&pagewanted=1. Retrieved December 31, 2009.
- ^ "The South Atlantic Anomaly". Ask an Astrophysicist. http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/961004.html. Retrieved 2007-10-16.
- ^ "Hubble Achieves Milestone: 100,000th Exposure". STScI. 1996-07-18. http://hubblesite.org/newscenter/archive/releases/1996/25/text/. Retrieved 2009-01-25.
- ^ "What is the South Atlantic Anomaly?". Ask the Astronomer. http://www.astronomycafe.net/qadir/q525.html. Retrieved 2009-12-06.
- ^ "Space Intelligence News" (PDF). Ascend. March 2007. http://www.ascendworldwide.com/content/spacetrak/sinsample.pdf.
- ^ Adriani, O.; Barbarino, G. C.; Bazilevskaya, G. A.; Bellotti, R.; Boezio, M.; Bogomolov, E. A.; Bongi, M.; Bonvicini, V. et al (2011). "The Discovery of Geomagnetically Trapped Cosmic-Ray Antiprotons". The Astrophysical Journal Letters 737 (2): L29. arXiv:1107.4882v1. Bibcode 2011ApJ...737L..29A. doi:10.1088/2041-8205/737/2/L29.
- ^ Shuttle Computers Navigate Record of Reliability 2010-June-28. http://www.nasa.gov. Retrieved 2010-July-03
[edit] External links
- BBC News - Magnetic mysteries of Earth's Core (section: Magnetic flip) has a video showing the growth and movement of the South Atlantic Anomaly over the last 400 years.
