Earth’s magnetic field acts like a protective shield around the planet, repelling and trapping charged particles from the Sun. However, there's an unusual weak spot in the field over South America and the southern Atlantic Ocean. This area, called the South Atlantic Anomaly (SAA), allows these particles to dip closer to the surface than normal.
What is the South Atlantic Anomaly?
The South Atlantic Anomaly (SAA) is an area where Earth's inner Van Allen radiation belt comes closest to Earth's surface, dipping down to an altitude of 200 kilometres (120 mi). 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°.
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. 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.
Changes Over Time
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 500 km (310 mi), the SAA spans from −50° to 0° geographic latitude and from −90° to +40° 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.
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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. 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.
Why The South Atlantic Anomaly Is There
Causes and Dynamics of the SAA
Earth is a bit like a bar magnet, with north and south poles that represent opposing magnetic polarities and invisible magnetic field lines encircling the planet between them. But unlike a bar magnet, the core magnetic field is not perfectly aligned through the globe, nor is it perfectly stable. That’s because the field originates from Earth’s outer core: molten, iron-rich and in vigorous motion 1800 miles below the surface.
As the core motion changes over time, due to complex geodynamic conditions within the core and at the boundary with the solid mantle up above, the magnetic field fluctuates in space and time too. These dynamical processes in the core ripple outward to the magnetic field surrounding the planet, generating the SAA and other features in the near-Earth environment - including the tilt and drift of the magnetic poles, which are moving over time.
“The magnetic field is actually a superposition of fields from many current sources,” said Terry Sabaka, a geophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Regions outside of the solid Earth also contribute to the observed magnetic field.
“The observed SAA can be also interpreted as a consequence of weakening dominance of the dipole field in the region,” said Weijia Kuang, a geophysicist and mathematician in Goddard’s Geodesy and Geophysics Laboratory.
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Impact on Satellites and Space Missions
Although the South Atlantic Anomaly arises from processes inside Earth, it has effects that reach far beyond Earth’s surface. The region can be hazardous for low-Earth orbit satellites that travel through it. If a satellite is hit by a high-energy proton, it can short-circuit and cause an event called single event upset or SEU.
This can cause the satellite’s function to glitch temporarily or can cause permanent damage if a key component is hit. In order to avoid losing instruments or an entire satellite, operators commonly shut down non-essential components as they pass through the SAA. The International Space Station, which is in low-Earth orbit, also passes through the SAA.
It is well protected, and astronauts are safe from harm while inside. However, the ISS has other passengers affected by the higher radiation levels: Instruments like the Global Ecosystem Dynamics Investigation mission, or GEDI, collect data from various positions on the outside of the ISS. “These events cause no harm to GEDI,” Blair said.
The International Space Station, orbiting with an inclination of 51.6°, requires extra shielding to deal with this problem. The PAMELA experiment, while passing through the SAA, detected antiproton levels that were orders of magnitude higher than expected.
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.
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Ongoing Research and Monitoring
In addition to measuring the SAA’s magnetic field strength, NASA scientists have also studied the particle radiation in the region with the Solar, Anomalous, and Magnetospheric Particle Explorer, or SAMPEX - the first of NASA’s Small Explorer missions, launched in 1992 and providing observations until 2012.
One study, led by NASA heliophysicist Ashley Greeley as part of her doctoral thesis, used two decades of data from SAMPEX to show that the SAA is slowly but steadily drifting in a northwesterly direction. “These particles are intimately associated with the magnetic field, which guides their motions,” said Shri Kanekal, a researcher in the Heliospheric Physics Laboratory at NASA Goddard.
The information Greeley and her collaborators garnered from SAMPEX’s in-situ measurements has also been useful for satellite design. The team assesses the current state of the magnetic field using data from the European Space Agency’s Swarm constellation, previous missions from agencies around the world, and ground measurements.
Sabaka’s team teases apart the observational data to separate out its source before passing it on to Kuang’s team. “This is similar to how weather forecasts are produced, but we are working with much longer time scales,” he said.
One such application that Sabaka and Kuang have contributed to is the International Geomagnetic Reference Field, or IGRF. “Even though the SAA is slow-moving, it is going through some change in morphology, so it’s also important that we keep observing it by having continued missions,” Sabaka said.
Recent Anomalies and Misinformation
A large wave anomaly was spotted between Antarctica and southern Africa for the second time in a month. It has sparked online speculation. A spokesperson for Ventusky told Newsweek that the video on Reddit appeared to be "manipulative" and that the issue was caused by a "model error."
"It seems to be addressing an error in the model that was previously refuted," the spokesperson said. In this case, the model is from the German Meteorological Service (DWD), with whom we are in contact and they have already resolved this error.
“Please note, that the model receives millions of data points from ships and buoys throughout the ocean. Problems can occur in such a large database. However, it may take time to determine the exact cause of this error."
The video on Reddit also referred to a number of other odd meteorological patterns near Bouvet Island, a volcanic island in the South Atlantic Ocean, commonly referred to as the most remote island on the planet.
These were not from the Ventusky app, but appear to have been captured by other meteorological data apps. However, Professor Roy Grainger, a reader in Atmospheric Physics at the University of Oxford, told Newsweek that these patterns appeared to be examples of vortex shedding.
Vortex shedding is caused when water or air flows around an object create a fluctuating, oscillating pattern of force behind it.
| Phenomenon | Description | Cause |
|---|---|---|
| South Atlantic Anomaly (SAA) | Weak spot in Earth's magnetic field | Processes within Earth's outer core |
| Wave Anomaly | Large wave patterns | Model errors in weather forecasting systems |
| Vortex Shedding | Oscillating pattern of force behind an object in fluid flow | Water or air flowing around an object |
The changing SAA provides researchers new opportunities to understand Earth’s core, and how its dynamics influence other aspects of the Earth system, said Kuang.
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