Editing South Atlantic Anomaly (section)

==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>