Editing South Atlantic Anomaly

{{Short description|Region where Earth's magnetic field is weakest relative to an idealised dipole}}
{{Coord|26.61|S|49.06|W|format=dms|type:landmark_scale:10000000|display=title}}

[[File:SAA 2020.png|thumb|upright=1.6|The strength of Earth's magnetic field, as of 2020 ({{val||e=-9|ul=T}})]]
The '''South Atlantic Anomaly''' ('''SAA''') is an area where [[Earth]]'s inner [[Van Allen radiation belt]] comes closest to [[Earth#Surface|Earth's surface]], dipping down to an altitude of {{convert|200|km|mi|-1}}. This leads to an increased [[flux]] of energetic particles in this region and exposes orbiting [[satellite]]s (including the [[International Space Station|ISS]]) to higher-than-usual levels of [[ionizing radiation]].

The effect is caused by the non-[[concentric objects|concentricity]] of Earth with its magnetic [[dipole]] and has been observed to be increasing in intensity recently.{{quantify|reason=Recent on what timescale – years, decades, centuries, millennia, millions of years? Article text does not discuss changes in intensity, only in size and location|date=November 2023}} The SAA is the near-Earth region where [[Earth's magnetic field]] 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]] at less than 32,000 [[nanotesla]] at sea level,<ref name="Pavon-Carrasco2016">{{cite journal |title=The South Atlantic Anomaly: The Key for a Possible Geomagnetic Reversal |journal=Frontiers in Earth Science |first1=F. Javier |last1=Pavón-Carrasco |first2=Angelo |last2=De Santis |volume=4 |at=40 |date=April 2016 |doi=10.3389/feart.2016.00040 |bibcode=2016FrEaS...4...40P|doi-access=free |hdl=10261/184585 |hdl-access=free }}</ref> which corresponds to the [[dipole|dipolar]] [[magnetic field]] at [[ionosphere|ionospheric]] altitudes.<ref name="Rao2010">{{cite book |url=https://books.google.com/books?id=lFVEvLGaV-MC&pg=PA125 |title=Global Navigation Satellite Systems: With Essentials of Satellite Communications |publisher=Tata McGraw-Hill |location=New Delhi |first=G. S. |last=Rao |page=125 |date=2010 |isbn=978-0-07-070029-1}}</ref> However, the field itself varies in intensity as a gradient.<ref name="Pavon-Carrasco2016" />{{rp|Figure 1}}

==Position and shape==
[[Image:South Atlantic Anomaly.svg|thumb|left|A cross-sectional view of the Van Allen radiation belts, noting the point where the South Atlantic Anomaly occurs]]
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 {{convert|450|to|500|km|mi|abbr=on}} 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 {{convert|200|km|mi|-1|abbr=on}} in altitude, and farthest from the Earth's surface over the north Pacific Ocean.<ref>{{Cite report |url=https://ntrs.nasa.gov/search.jsp?R=20160003393 |title=Forty-Year 'Drift' and Change of the SAA |publisher=NASA Goddard Spaceflight Center |first1=Epaminondas G. |last1=Stassinopoulos |first2=Michael A. |last2=Xapsos |first3=Craig A. |last3=Stauffer |date=December 2015 |id=NASA/TM-2015-217547, GSFC-E-DAA-TN28435}}</ref><ref name="Crotts2014">{{cite book |url=https://books.google.com/books?id=a0pCBAAAQBAJ&pg=PA168 |title=The New Moon: Water, Exploration, and Future Habitation |publisher=[[Cambridge University Press]] |first=Arlin |last=Crotts |page=168 |date=2014 |isbn=978-0-521-76224-3}}</ref>

{{multiple image
 | direction         = vertical
 | image1            = SAA field intensity.svg
 | caption1          = Intensity of the magnetic field in the center of the South Atlantic Anomaly, 1840 to 2020.
 | image2            = SAA area.svg
 | caption2          = Area of the South Atlantic Anomaly, 1840 to 2020.
}}
If Earth's magnetism is represented by a bar magnet of small size but strong intensity ("[[magnetic dipole]]"), 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]]. As a result, over northern South America and the south Atlantic, near Singapore's [[antipodal point]], the magnetic field is relatively weak, resulting in a lower [[magnetism|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.<ref>{{cite web |url=http://pwg.gsfc.nasa.gov/earthmag/magnQ&A1.htm#q15 |title=FAQ: "The Great Magnet, the Earth" |publisher=NASA |access-date=July 31, 2015}}</ref>

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.<ref name=NYT>{{cite news |url=https://www.nytimes.com/1990/06/05/science/dip-on-earth-is-big-trouble-in-space.html |title='Dip' on Earth is Big Trouble in Space |work=[[The New York Times]] |last=Broad |first=William J. |date=5 June 1990 |access-date=31 December 2009}}</ref> Additionally, the shape and particle density of the SAA varies on a [[Diurnal_cycle|diurnal]] basis, with greatest particle density corresponding roughly to local noon. At an altitude of approximately {{convert|500|km|mi|abbr=on}}, the SAA spans from {{nowrap|−50° to 0°}} geographic latitude and from {{nowrap|−90° to +40°}} longitude.<ref name="Ask">{{cite web |url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/961004.html |title=The South Atlantic Anomaly |series=Ask an Astrophysicist |publisher=NASA |date=4 October 1996 |access-date=16 October 2007 |archive-url=https://web.archive.org/web/20071105042835/http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/961004.html |archive-date=5 November 2007}}</ref> 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 [[Structure of the Earth#Core|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 radiation belt|Van Allen belt]] gets closer to the Earth, with a commensurate enlargement of the SAA at given altitudes.<ref>{{cite report|title=Swarm probes weakening of Earth's magnetic field|date=May 20, 2020 |url=https://www.esa.int/Applications/Observing_the_Earth/FutureEO/Swarm/Swarm_probes_weakening_of_Earth_s_magnetic_field
|publisher=ESA|access-date=February 5, 2023}}</ref> 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.<ref>{{Cite journal |last1=Jaqueto |first1=Plinio |last2=Trindade |first2=Ricardo I. F. |last3=Terra-Nova |first3=Filipe |last4=Feinberg |first4=Joshua M. |last5=Novello |first5=Valdir F. |last6=Stríkis |first6=Nicolás M. |last7=Schroedl |first7=Peter |last8=Azevedo |first8=Vitor |last9=Strauss |first9=Beck E. |last10=Cruz |first10=Francisco W. |last11=Cheng |first11=Hai |last12=Edwards |first12=R. Lawrence |date=15 March 2022 |title=Stalagmite paleomagnetic record of a quiet mid-to-late Holocene field activity in central South America |journal=[[Nature Communications]] |language=en |volume=13 |issue=1 |pages=1349 |doi=10.1038/s41467-022-28972-8 |issn=2041-1723 |doi-access=free |bibcode=2022NatCo..13.1349J |pmc=8924270 }}</ref>

The South Atlantic Anomaly seems to be caused by a huge reservoir of very dense rock inside the Earth called the [[Large low-shear-velocity provinces|African large low-shear velocity province]].<ref>{{Cite web |title=Scientists Are Getting Kinda Anxious About a Pothole in Space |url=https://www.popularmechanics.com/space/satellites/a43412493/nasa-tracking-dent-in-earths-magnetic-field/ |url-status=live |archive-url=https://web.archive.org/web/20230401193514/https://finance.yahoo.com/news/scientists-getting-kinda-anxious-pothole-153000350.html?guccounter=1&guce_referrer=aHR0cHM6Ly90LmNvLw&guce_referrer_sig=AQAAAHjVJtliTYTrQqo5NkVlxjaQBoELI9hOQwCp26cnl4X1Hxz0OdEKq9Ia5IINn0aaPV6JcBa7sFoDWQLG7d-Qp_e3UBE3lSqHaBl-Hhrhn32rBPQ6Dl_6Df4Q1omLwFZVKGGuPpYd_bDGRpkXlsa6d6LWy9nK7VRH6PaERm1nruTE |archive-date=2023-04-01 |access-date=2023-04-01 |website=Yahoo Finance |language=en-US}}</ref>

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 {{coord|26.61|S|49.06|W}} and drifting about 0.23°S 0.34°W per year.<ref>{{Cite journal |last1=Kovář |first1=Pavel |last2=Sommer |first2=Marek |date=January 2021 |title=CubeSat Observation of the Radiation Field of the South Atlantic Anomaly |journal=Remote Sensing |volume=13 |issue=7 |pages=1274 |doi=10.3390/rs13071274 |issn=2072-4292|doi-access=free |bibcode=2021RemS...13.1274K }}</ref>

==Intensity and effects==
[[File:Mission Control center of NASA.png|thumb|left|The location of the SAA is visible on the main screen at NASA's Mission Control Center in Houston.]]
The South Atlantic Anomaly is of great significance to astronomical [[satellite]]s 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 ionizing radiation, caused by the trapped protons in the inner Van Allen belt.

Measurements on [[Space Shuttle]] 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.<ref>{{cite journal |last1=Badhwar |first1=G. D |last2=Kushin |first2=V. V |last3=Akatov |first3=Yu A |last4=Myltseva |first4=V. A |date=1999-06-01 |title=Effects of trapped proton flux anisotropy on dose rates in low Earth orbit |journal=Radiation Measurements |language=en |volume=30 |issue=3 |pages=415–426 |pmid=11543145 |issn=1350-4487 |doi=10.1016/S1350-4487(99)00068-2 |bibcode=1999RadM...30..415B |url=https://www.sciencedirect.com/science/article/abs/pii/S1350448799000682}}</ref>

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 with its sensitive UV detectors while passing through the SAA.<ref>{{cite web |title=Hubble Achieves Milestone: 100,000th Exposure |publisher=[[Space Telescope Science Institute]] |date=July 18, 1996 |url=https://hubblesite.org/contents/news-releases/1996/news-1996-25.html |access-date=January 25, 2009}}</ref> Passing through the anomaly caused false alarms on [[Skylab]] [[Apollo Telescope Mount]]'s [[solar flare]] sensor.<ref name="elder">{{cite book |last=Elder |first=Donald C. |title=From Engineering Science to Big Science: The NACA and NASA Collier Trophy Research Project Winners |publisher=NASA |year=1998 |editor-last=Mack |editor-first=Pamela E. |series=The NASA History Series |chapter=The Human Touch: The History of the Skylab Program |id=SP-4219 |url=https://history.nasa.gov/SP-4219/Contents.html |chapter-url=https://history.nasa.gov/SP-4219/Chapter9.html}}</ref> Astronauts are also affected by this region, which is said to be the cause of peculiar "shooting stars" ([[phosphene]]s) seen in the visual field of astronauts, an effect termed [[cosmic ray visual phenomena]].<ref name="AskA">{{cite web |title=What is the South Atlantic Anomaly? |work=Ask the Astronomer |url=http://www.astronomycafe.net/qadir/q525.html |access-date=December 6, 2009}}</ref> Passing through the South Atlantic Anomaly is thought to be the reason for the [[Globalstar#First-generation satellite problems|failures of the Globalstar network's satellites]] in 2007.<ref>{{cite web |title=Space Intelligence News |publisher=Ascend |date=March 2007 |url=http://www.ascendworldwide.com/content/spacetrak/sinsample.pdf |url-status=dead |archive-url=https://web.archive.org/web/20070214142051/http://www.ascendworldwide.com/content/spacetrak/sinsample.pdf |archive-date=2007-02-14}}</ref>

The [[PAMELA detector|PAMELA]] experiment, while passing through the SAA, detected [[antiproton]] 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]]s.<ref>{{cite journal |title=The Discovery of Geomagnetically Trapped Cosmic-Ray Antiprotons |journal=The Astrophysical Journal Letters |last1=Adriani |first1=O. |last2=Barbarino |first2=G. C. |last3=Bazilevskaya |first3=G. A. |last4=Bellotti |first4=R. |last5=Boezio |first5=M. |last6=Bogomolov |first6=E. A. |last7=Bongi |first7=M. |last8=Bonvicini |first8=V. |last9=Borisov |first9=S. |last10=Bottai |first10=S. |last11=Bruno |first11=A. |last12=Cafagna |first12=F. |last13=Campana |first13=D. |last14=Carbone |first14=R. |last15=Carlson |first15=P. |last16=Casolino |first16=M. |last17=Castellini |first17=G. |last18=Consiglio |first18=L. |last19=De Pascale |first19=M. P. |last20=De Santis |first20=C. |last21=De Simone |first21=N. |last22=Di Felice |first22=V. |last23=Galper |first23=A. M. |last24=Gillard |first24=W. |last25=Grishantseva |first25=L. |last26=Jerse |first26=G. |last27=Karelin |first27=A. V. |last28=Kheymits |first28=M. D. |last29=Koldashov |first29=S. V. |last30=Krutkov |first30=S. Y. |display-authors=5 |volume=737 |issue=2 |at=L29 |date=August 2011 |doi=10.1088/2041-8205/737/2/L29 |bibcode=2011ApJ...737L..29A |arxiv=1107.4882}}</ref>

NASA has reported that modern laptop computers have crashed when [[Space Shuttle]] flights passed through the anomaly.<ref>{{cite web |title=Shuttle Computers Navigate Record of Reliability |publisher=NASA |last=Siceloff |first=Steven |date=June 28, 2010 |url=http://www.nasa.gov/mission_pages/shuttle/flyout/flyfeature_shuttlecomputers.html |access-date=July 3, 2010}}</ref>

In October 2012, the [[SpaceX CRS-1]] Dragon spacecraft attached to the International Space Station experienced a transient problem as it passed through the anomaly.<ref>{{cite news |title=Dragon enjoying ISS stay, despite minor issues |work=NASA Spaceflight |last=Bergin |first=Chris |date=October 19, 2012 |url=https://www.nasaspaceflight.com/2012/10/dragon-iss-stay-minor-issues-falcon-9-investigation/ |access-date=October 20, 2012}}</ref>

The SAA is believed to have started a series of events leading to the destruction of the [[Hitomi (satellite)|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.<ref>{{cite news |title=Japan's most powerful X-ray satellite is dead |work=[[Engadget]] |last=Moon |first=Mariella |date=April 29, 2016 |url=https://www.engadget.com/2016-04-29-rip-hitomi.html |access-date=April 29, 2016}}</ref>

==See also==
*[[Geomagnetic reversal]]
*[[Geomagnetic storm]]
*[[Large low-shear-velocity provinces]]
*[[Operation Argus]]
*[[Space weather]]

==References==
{{reflist}}

==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.

[[Category:Atlantic Ocean]]
[[Category:Magnetic anomalies]]
[[Category:Magnetic field of the Earth]]
[[Category:Space plasmas]]
[[Category:Geology of South America]]