South Atlantic Anomaly
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The South Atlantic Anomaly (SAA) is an area where
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
's inner
Van Allen radiation belt The Van Allen radiation belt is a zone of energetic charged particles, most of which originate from the solar wind, that are captured by and held around a planet by that planet's magnetosphere. Earth has two such belts, and sometimes others ma ...
comes closest to
Earth's surface Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
, dipping down to an altitude of . This leads to an increased
flux Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications in physics. For transport phe ...
of energetic particles in this region and exposes orbiting
satellite A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...
s (including the ISS) to higher-than-usual levels of
ionizing radiation Ionizing (ionising) radiation, including Radioactive decay, nuclear radiation, consists of subatomic particles or electromagnetic waves that have enough energy per individual photon or particle to ionization, ionize atoms or molecules by detaching ...
. The effect is caused by the non- concentricity of Earth with its magnetic
dipole In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways: * An electric dipole moment, electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple ...
and has been observed to be increasing in intensity recently. The SAA is the near-Earth region where
Earth's magnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from structure of Earth, Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from ...
is weakest relative to an idealized Earth-centered dipole field.


Definition

The area of the SAA is confined by the intensity of
Earth's magnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from structure of Earth, Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from ...
at less than 32,000 nanotesla at sea level, which corresponds to the dipolar
magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
at
ionospheric The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...
altitudes. However, the field itself varies in intensity as a gradient.


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 approximately 11°. The intersection between the magnetic and rotation axes of the Earth is located not at the Earth's center, but some away. Because of this asymmetry, the inner Van Allen belt is closest to the Earth's surface over the south Atlantic Ocean where it dips down to in altitude, and farthest from the Earth's surface over the north Pacific Ocean. If Earth's magnetism is represented by a bar magnet of small size but strong intensity ("
magnetic dipole In electromagnetism, a magnetic dipole is the limit of either a closed loop of electric current or a pair of poles as the size of the source is reduced to zero while keeping the magnetic moment constant. It is a magnetic analogue of the Electri ...
"), the SAA variation can be illustrated by placing the magnet not in the plane of the Equator, but some small distance North, shifted more or less in the direction of
Singapore Singapore, officially the Republic of Singapore, is an island country and city-state in Southeast Asia. The country's territory comprises one main island, 63 satellite islands and islets, and one outlying islet. It is about one degree ...
. As a result, over northern South America and the south Atlantic, near Singapore's
antipodal point In mathematics, two points of a sphere (or n-sphere, including a circle) are called antipodal or diametrically opposite if they are the endpoints of a diameter, a straight line segment between two points on a sphere and passing through its cen ...
, the magnetic field is relatively weak, resulting in a lower repulsion to trapped particles of the radiation belts there, and as a result these particles reach deeper into the upper atmosphere than they otherwise would. The shape of the SAA changes over time. Since its initial discovery in 1958, 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 , the SAA spans from geographic latitude and from longitude. The highest intensity portion of the SAA drifts to the west at a speed of about 0.3° 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° 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 The Van Allen radiation belt is a zone of energetic charged particles, most of which originate from the solar wind, that are captured by and held around a planet by that planet's magnetosphere. Earth has two such belts, and sometimes others ma ...
gets closer to the Earth, with a commensurate enlargement of the SAA at given altitudes. During the Middle Holocene, the Earth's magnetic field in the region occupied by the SAA was relatively calm and quiescent, contrasting with its present day activity. The South Atlantic Anomaly seems to be caused by a huge reservoir of very dense rock inside the Earth called the African large low-shear velocity province. The position of the anomaly can be that of the maximum magnetic flux or that of the centroid of the flux, which is less sensitive to sampling noise and more representative of the feature as a whole. In January 2021, the centroid was located near and drifting about 0.23°S 0.34°W per year.


Intensity and effects

The South Atlantic Anomaly is of great significance to astronomical
satellite A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...
s and other
spacecraft A spacecraft is a vehicle that is designed spaceflight, to fly and operate in outer space. Spacecraft are used for a variety of purposes, including Telecommunications, communications, Earth observation satellite, Earth observation, Weather s ...
that
orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...
the Earth at several hundred kilometers altitude; these orbits take satellites through the anomaly periodically, exposing them to several minutes of strong ionizing radiation, caused by the trapped protons in the inner Van Allen belt. Measurements on
Space Shuttle The Space Shuttle is a retired, partially reusable launch system, reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. ...
flight STS-94 have ascertained that absorbed dose rates from charged particles have extended from 112 to 175 μGy/day, with dose equivalent rates ranging from 264.3 to 413 μSv/day. The
International Space Station The International Space Station (ISS) is a large space station that was Assembly of the International Space Station, assembled and is maintained in low Earth orbit by a collaboration of five space agencies and their contractors: NASA (United ...
, orbiting with an
inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Eart ...
of 51.6°, requires extra shielding to deal with this problem. The
Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...
does not take observations with its sensitive UV detectors while passing through the SAA. Passing through the anomaly caused false alarms on
Skylab Skylab was the United States' first space station, launched by NASA, occupied for about 24 weeks between May 1973 and February 1974. It was operated by three trios of astronaut crews: Skylab 2, Skylab 3, and Skylab 4. Skylab was constructe ...
Apollo Telescope Mount The Apollo Telescope Mount, or ATM, was a crewed Sun, solar observatory that was a part of Skylab, the first American space station. It could observe the Sun in wavelengths ranging from soft X-rays, ultraviolet, and visible light. The ATM was ...
's
solar flare A solar flare is a relatively intense, localized emission of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and ot ...
sensor. 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, an effect termed cosmic ray visual phenomena. Passing through the South Atlantic Anomaly is thought to be the reason for the failures of the Globalstar network's satellites in 2007. The PAMELA experiment, while passing through the SAA, detected
antiproton The antiproton, , (pronounced ''p-bar'') is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived, since any collision with a proton will cause both particles to be annihilated in a burst of energy. The exis ...
levels that were orders of magnitude higher than expected. This suggests the Van Allen belt confines antiparticles produced by the interaction of the Earth's upper atmosphere with
cosmic ray Cosmic rays or astroparticles are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the ...
s. NASA has reported that modern laptop computers have crashed when
Space Shuttle The Space Shuttle is a retired, partially reusable launch system, reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. ...
flights passed through the anomaly. In October 2012, the
SpaceX CRS-1 SpaceX CRS-1, also known as SpX-1, was SpaceX, SpaceX's first operational cargo mission to the International Space Station, under their Commercial Resupply Services (CRS-1) contract with NASA. It was the third flight for the uncrewed SpaceX Dra ...
Dragon spacecraft attached to the International Space Station experienced a transient problem as it passed through the anomaly. The SAA is believed to have started a series of events leading to the destruction of the Hitomi, Japan's most powerful X-ray observatory. The anomaly transiently disabled a direction-finding mechanism, causing the satellite to rely solely on gyroscopes that were not working properly, after which it spun out of control, losing its solar panels in the process.


See also

*
Geomagnetic reversal A geomagnetic reversal is a change in the Earth's Dipole magnet, dipole magnetic field such that the positions of magnetic north and magnetic south are interchanged (not to be confused with North Pole, geographic north and South Pole, geograp ...
*
Geomagnetic storm A geomagnetic storm, also known as a magnetic storm, is a temporary disturbance of the Earth's magnetosphere that is driven by interactions between the magnetosphere and large-scale transient Plasma (physics), plasma and magnetic field structur ...
*
Large low-shear-velocity provinces Large low-shear-velocity provinces (LLSVPs), also called large low-velocity provinces (LLVPs) or superplumes, are characteristic structures of parts of the lowermost mantle, the region surrounding the outer core deep inside the Earth. These provi ...
* Operation Argus *
Space weather Space weather is a branch of space physics and aeronomy, or heliophysics, concerned with the varying conditions within the Solar System and its heliosphere. This includes the effects of the solar wind, especially on the Earth's magnetosphere, ion ...


References


External links

* {{cite news , url=https://www.bbc.com/news/uk-14678002 , title=Magnetic mysteries of Earth's Core , work=BBC News Section "Magnetic flip" contains a video showing the growth and movement of the South Atlantic Anomaly over the last 400 years. Atlantic Ocean Magnetic anomalies Magnetic field of the Earth Space plasmas Geology of South America