Context

• NASA is actively monitoring a strange anomaly in Earth's magnetic field: a giant region of lower magnetic intensity in the skies above the planet, stretching out between South America and southwest Africa.
• This vast, developing phenomenon, called the South Atlantic Anomaly, has intrigued and concerned scientists for years.

 SAA

• The South Atlantic Anomalyis a weak spot in Earth's magnetic field, which protects the planet from high doses of solar wind and cosmic radiation.
• This anomaly exists because the Earth's inner Van Allen radiation beltcomes closest to the planet’s surface, causing an increased flux of energetic particles.
• In turn, this anomaly also causes technical disturbances in satellites and spacecraft orbiting Earth. 
• The area of the SAA is confined by the intensity of Earth's magnetic field at less than 32,000 nanotesla at sea level, which corresponds to the dipolar magnetic field at ionospheric altitudes. However, the field itself varies in intensity as a gradient.

Van Allen radiation belt 

• A Van Allen radiation beltis 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. 
• Earthhas two such belts, and sometimes others may be temporarily created. The belts are named after James Van Allen, who is credited with their discovery.
• Earth's two main belts extend from an altitudeof about 640 to 58,000 km above the surface, in which region radiation levels vary.
• Most of the particles that form the belts are thought to come from solar wind and other particles by cosmic rays.
• By trapping the solar wind, the magnetic field deflects those energetic particles and protects the atmospherefrom destruction.
• The belts are in the inner region of Earth's magnetic field. The belts trap energetic electronsand protons.
• Other nuclei, such as alpha particles, are less prevalent.
• The belts endanger satellites, which must have their sensitive components protected with adequate shielding if they spend significant time near that zone.
• In 2013, the Van Allen Probesdetected a transient, third radiation belt, which persisted for four weeks.
• Apollo Astronauts going through the Van Allen Belts received a very low and non-harmful dose of radiation.

Closest/Farthest from Earth’s surface

• 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 450 to 500 km (280 to 310 mi) 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 200 km (120 mi) in altitude, and farthest from the Earth's surface over the north Pacific Ocean.  

https://www.sciencealert.com/nasa-is-tracking-a-huge-growing-anomaly-in-earths-magnetic-field

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