The South Atlantic Anomaly (SAA) refers to the area where the Earth's inner Van Allen radiation belt The Van Allen radiation belt is a torus of energetic charged particles around Earth, which is held in place by Earth's magnetic field. This field is not uniformly distributed around the Earth. On the sunward side, it is compressed because of the solar wind, while on the other side it is elongated to around three earth radii. This creates a cavity comes closest to the Earth Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets. It is sometimes referred to as the World, the Blue Planet,[note 6] or by its Latin name, Terra.[note 7]'s surface. This leads to an increased flux One could argue, based on the work of James Clerk Maxwell, that the transport definition precedes the more recent way the term is used in electromagnetism. The specific quote from Maxwell is "In the case of fluxes, we have to take the integral, over a surface, of the flux through every element of the surface. The result of this operation is 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 Concentric objects share the same center, axis or origin with one inside the other. Circles, tubes, cylindrical shafts, disks, and spheres may be concentric to one another. Concentric objects generally have different radii, as concentric objects with the same radius are equal of the Earth and its magnetic dipole Dipoles can be characterized by their dipole moment, a vector quantity. For the simple electric dipole given above, the electric dipole moment points from the negative charge towards the positive charge, and has a magnitude equal to the strength of each charge times the separation between the charges. For the current loop, the magnetic dipole, and the SAA is the near-Earth region where the Earth's magnetic field Earth's magnetic field is approximately a magnetic dipole, with the magnetic field S pole near the Earth's geographic north pole (see Magnetic North Pole) and the other magnetic field N pole near the Earth's geographic south pole (see Magnetic South Pole). The cause of the field can be explained by dynamo theory. Magnetic fields extend infinitely, is weakest.

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. The intersection between the magnetic and rotation axis of the Earth is located ~500 kilometres The kilometre , symbol km is a unit of length in the metric system, equal to one thousand metres and is therefore exactly equal to the distance travelled by light in free space in 1⁄ 299,792.458 of a second. It is the conventionally used measurement unit for expressing distances between geographical places in most of the world; notable (300 mi A mile is a unit of length in a number of different systems. In contemporary English, a mile most commonly refers to the statute mile of 5,280 feet , the survey mile of 5,280 survey feet (1,609.3472 metres (5,280.01 ft)) or the nautical mile of 1,852 metres (6,076.12 ft). It is about a third of the old measurement, the league) 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 South Atlantic Anomaly is of great significance to astronomical satellites In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavor. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon and other spacecraft A spacecraft is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and space tourism. Spacecraft and space travel are common themes in works of science fiction that orbit In physics, an orbit is the gravitationally curved path of one object around a point or another body, for example the gravitational orbit of a planet around a star 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, each time. The International Space Station The International Space Station is an internationally developed research facility that is being assembled in low Earth orbit. On-orbit construction of the station began in 1998 and is scheduled for completion by late 2011. The station is expected to remain in operation until at least 2015, and likely 2020. With a greater mass than that of any, orbiting with an inclination Inclination in general is the angle between a reference plane and another plane or axis of direction of 51.6°, requires extra shielding to deal with this problem. The Hubble Space Telescope The Hubble Space Telescope is a space telescope that was carried into orbit by a space shuttle in April 1990. It is named after the American astronomer Edwin Hubble. Although not the first space telescope, Hubble is one of the largest and most versatile, and is well-known as both a vital research tool and a public relations boon for astronomy. The does not take observations while passing through the SAA.^[3] 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.^[4]

The shape of the SAA changes over time. Since its initial discovery in 1958,^[5] 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 A day is a unit of time equivalent to approximately 24 hours. It is not an SI unit but it is accepted for use with SI. The SI unit of time is the second 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.^[6] 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 The interior structure of the Earth, similar to the outer, is layered. These layers can be defined by either their chemical or their rheological properties. The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. Scientific understanding of Earth's 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]} 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 A geomagnetic reversal is a change in the orientation of Earth's magnetic field such that the positions of magnetic north and magnetic south become interchanged. These events often involve an extended decline in field strength followed by a rapid recovery after the new orientation has been established. These events occur on a scale of tens of.^{[citation needed]}

See also

• Geomagnetic reversal A geomagnetic reversal is a change in the orientation of Earth's magnetic field such that the positions of magnetic north and magnetic south become interchanged. These events often involve an extended decline in field strength followed by a rapid recovery after the new orientation has been established. These events occur on a scale of tens of
• Geomagnetic storm A geomagnetic storm is a temporary disturbance of the Earth's magnetosphere caused by a disturbance in space weather. Associated with solar coronal mass ejections , coronal holes, or solar flares, a geomagnetic storm is caused by a solar wind shock wave which typically strikes the Earth's magnetic field 24 to 36 hours after the event. This only
• Space weather Space weather is the concept of changing environmental conditions in near-Earth space. It is distinct from the concept of weather within a planetary atmosphere, and deals with phenomena involving ambient plasma, magnetic fields, radiation and other matter in space. "Space weather" often implicitly means the conditions in near-Earth space
• Van Allen Belts The Van Allen radiation belt is a torus of energetic charged particles around Earth, which is held in place by Earth's magnetic field. This field is not uniformly distributed around the Earth. On the sunward side, it is compressed because of the solar wind, while on the other side it is elongated to around three earth radii. This creates a cavity
• Operation Argus Operation Argus was a series of nuclear weapons tests and missile tests secretly conducted during August and September of 1958 over the South Atlantic Ocean by the United States's Defense Nuclear Agency, in conjunction with the Explorer 4 space mission. Operation Argus was conducted between the nuclear test series Operation Hardtack I and

References

1. ^ "ROSAT SAA" (HTML). http://heasarc.gsfc.nasa.gov/docs/rosat/gallery/misc_saad.html. Retrieved 2007-10-16.
2. ^ Stassinopoulos, E.G.; Staffer, C.A. (2007), Forty-Year Drift and Change of the SAA, NASA Goddard Spaceflight Center
3. ^ "Hubble Achieves Milestone: 100,000th Exposure". STScI. 1996-07-18. http://hubblesite.org/newscenter/archive/releases/1996/25/text/. Retrieved 2009-01-25.
4. ^ "What is the South Atlantic Anomaly?" (HTML). Ask the Astronomer. http://www.astronomycafe.net/qadir/q525.html. Retrieved 2009-12-06.
5. ^ 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.
6. ^ "The South Atlantic Anomaly" (HTML). Ask an Astrophysicist. http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/961004.html. Retrieved 2007-10-16.

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