The South Atlantic Anomaly is a worrisome dip in the donut-shaped Van Allen Radiation Belts off the coast of South America.  It is caused by a more-or-less permanent mismatch between the spin axis of the earth and its magnetic dipole.  An 11-degree angle exists between the two axes and, in addition, there is a 300 […]
  1. The South Atlantic Anomaly is a worrisome dip in the donut-shaped Van Allen Radiation Belts off the coast of South America.  It is caused by a more-or-less permanent mismatch between the spin axis of the earth and its magnetic dipole.  An 11-degree angle exists between the two axes and, in addition, there is a 300 mile distance between the geometric center of the earth and the center of its magnetic dipole.
These relatively small mismatches result in a localized magnetic field strength that is only about half as strong as the worldwide average value of 0.5 Gauss.  The energetic protons in the lower Van Allen Radiation Belt are trapped in the earth’s magnetic field, and, as a result, they are pulled down toward the earth’s surface in the South Atlantic Anomaly.  When low-altitude satellites fly through the South Atlantic Anomaly, they experience dramatically increased levels of radiation. Astronauts, computer chips, and solid-state sensors must be protected from the extra radiation found in the South Atlantic Anomaly.  Mission planners avoid “space walks” over that part of the world, the Hubble Space Telescope is temporarily shut down, and laptop computers carried onboard the space shuttle and in the International Space Station sometimes malfunction.  In addition, some of the Globalstar mobile communication satellites are believed to have died earlier than predicted because of the extra radiation they experienced while gliding through the South Atlantic Anomaly. Strategies designed to cope with this region of enhanced radiation include extra hardening of the onboard electronics, the use of self-annealing gallium-arsenide circuit chips, careful mission planning to avoid the worst radiation-enhanced portions of space, and careful positioning of delicate components inside the spacecraft among other heavy and dense components. 2.  As you suspected, the South Atlantic Anomaly does move, expand, and change in intensity over relatively long periods of time.  Generally, it is expanding and moving predominantly in the southwestern direction.  Its field strength is also gradually weakening (which results in stronger doses of radiation for low-altitude satellites coasting though that region of space). The Van Allen Radiation Belts, which are composed primarily of highly energetic protons, electrons and positive ions, are trapped in the earth’s magnetic field.  They are shaped like gigantic donuts in space.  These enormous belts were first discovered in 1958 by the American physicist, Dr. James Van Allen, a professor at Iowa State University, shortly after the Explorer 1 spacecraft was lofted into space. The lower Van Allen Radiation Belt reaches its peak intensity at an altitude of about 1800 miles above the earth.  It consists primarily of protons and positive ions spiraling in long, lazy loops around the earth’s magnetic lines of flux.  A sphere two feet in diameter positioned at the center of the lower Van Allen Radiation Belt would be penetrated by 20 million of these energetic protons every second of its mission.  The upper Van Allen Radiation Belt reaches its peak intensity at about 10,000 miles above the earth.  It consists primarily of electrons at substantially lower energy levels. Most of the time when the various charged particles hurtling around the magnetic flux lines reach their northernmost or southernmost latitudes, an abrupt mirror-image reflection hurls them back into the opposite direction.  But near each pole a small number of them escape, plunge to lower altitudes, and impact the hydrogen, oxygen, and argon atoms in the Earth’s atmosphere.  These collisions create the Northern and Southern Lights.  Each type of atom is associated with its own characteristic colors. The GPS satellites are, unfortunately, positioned at an altitude of 12,500 miles near the peak intensity of the upper Van Allen Radiation Belt.  The intense radiation found there damages their silicon solar cells.  Consequently, over its 7.5-year design lifetime, a typical GPS satellite loses about 25 percent of its electrical generating capacity.  Near the peak of the sun’s 11-year sunspot cycle, the belts are pumped up by the larger number of charged particles in the solar wind.  Consequently, the GPs satellites’ power levels decline even more rapidly due to these enhanced levels of radiation. A solid metallic sphere 750 miles in radius lies at the center of the earth.  Surrounding it is a liquid metal shell that swirls around due to the Earth’s rotational motion.  This moving dynamo creates the earth’s rather complicated and variable magnetic field. About 90 percent of the earth’s magnetic field comes from its simple dipole that behaves like a bar magnet with a positive pole at one end and a negative pole at the other.  The remaining 10 percent comes from the earth’s various other non-dipole components (monopoles, quadrupoles, etc.). Over the past 150 years, (during which scientific measurements have been available), the dominating dipole component has decreased in magnetic field strength by about 6 percent.  This weaker dipole, in turn, causes a systematic increase in the size of the South Atlantic Anomaly as experienced by low-altitude satellites.  During each decade, on average, the center of the South Atlantic Anomaly moves predominantly in the southwestern direction at an average rate of 0.8 degrees of longitude.  This is equivalent to a movement of 48 statute miles per decade. Over much larger intervals, the earth’s magnetic field reverses polarity, rather abruptly, at unpredictable and irregular intervals:  the North Magnetic Pole becomes the South Pole and vice versa.  On the average, these reversals are 300,000 to 1,000,000 years apart. Unequivocal evidence highlighting these dipole reversals is provided by the large magnetized strips frozen into basalt layers lining the sea floor on opposite sides of the slowly spreading mid-Atlantic ridge. 3.  According to my sources, the best and most readily available information on the status, shape, and location of the South Atlantic Anomaly and related matters, comes from NASA’s Community Coordinated Modeling Center (CCMC).  The World Wide Web address of the center – which is maintained by NASA Goddard in Greenbelt, Maryland is: The letters “gsfc” in this address stand for the Goddard Space Flight Center.  I have taught short courses at the Goddard Space Flight Center for many years, but I do not know anyone who works at their modeling center.  Goddard’s headcount totals about 3000. If your organization plans to launch, operate or use satellites in space there are six different sources of radiation you may have to plan for and worry about: 1.      The various layers of the ionosphere 2.      The auroras (Northern and Southern Lights) 3.      The two major Van Allen Radiation Belts 4.      Periodic and intense solar proton events 5.      Cosmic rays originating in deep space 6.      Solar wind Fortunately, I often include material on these radiation sources in my “Orbital Mechanics” short courses, especially in the longer 5-day versions.  Discussions of the South Atlantic Anomaly are included in the portion that deals with the Van Allen Radiation Belts, including practical mitigation strategies that can help mission planner alleviate their worse detrimental effects. Tom Logsdon


  1. Nice post. A minor correction from an Iowa State University alum: James van Allen was a professor at the University of Iowa. He was a Hawkeye, not a Cyclone.

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