Mysterious anomaly weakening Earth's magnetic field is older than we thought

New analysis shows Earth's magnetic field has a long history of acting up.

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Jackson Ryan was CNET's science editor, and a multiple award-winning one at that. Earlier, he'd been a scientist, but he realized he wasn't very happy sitting at a lab bench all day. Science writing, he realized, was the best job in the world -- it let him tell stories about space, the planet, climate change and the people working at the frontiers of human knowledge. He also owns a lot of ugly Christmas sweaters.
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The blue region represents the South Atlantic Anomaly.  

Division of Geomagnetism, DTU Space/ESA

Earth's magnetic field has been acting a little wonky. One particular region, stretching from Africa to South America over the South Atlantic ocean, has experienced an intense weakening since the 1950s. This has caused some concern, because the magnetic field acts like a protective blanket for the planet, shielding us from the charged particles zipping around in space. 

Over the last few years the region, known as the South Atlantic Anomaly (SAA), has experienced an increased weakening and seems to be splitting in two, which some scientists have suggested could portend a reversal of our magnetic poles, playing havoc with electricity and navigation systems.

But a new study, published in the Proceedings of the National Academy of Sciences on Monday, suggests the strange changes in the region may not be as unusual as they seem -- in fact, the SAA may behave this way quite regularly, if you're on the universe's clock. 

Researchers from the University of Liverpool analyzed rock from volcanic eruptions that took place between eight and 11 million years ago. Previous research has shown the anomaly has existed for a thousand years or more, but the Liverpool team were able to look even further back in time. 

The team collected 225 samples from 46 different locations across Saint Helena, an island in the South Atlantic that lies within the SAA and is home to two volcanoes that formed eight and 11 million years ago. 

Volcanic rock provides a window through time, allowing researchers to understand what the magnetic field looked like eons ago. When lava flows cool down and harden, the minerals "trapped" inside record the intensity and the direction of the magnetic field at the location at that time.  

"Our study provides the first long term analysis of the magnetic field in this region dating back millions of years," said Yael Engbers, a PhD student at the University of Liverpool. "It suggests that the South Atlantic Anomaly is a recurring feature and probably not a sign of an impending reversal."

The researchers hypothesize their finding also support the idea interactions in Earth's innermost layers are likely responsible for the weakening of the field. Scientists have discovered a huge region of rock in the Earth's mantle underneath the anomaly, dubbed the African Large Low Shear Velocity Province (ALLSVP). Changes in the geochemical makeup of this ALLSVP may be causing the changes to the SAA.

"This brings us closer to linking behaviour of the geomagnetic field directly to features of the Earth's interior," said Engbers.

Understanding the anomaly will be important for space-based research and could affect the safety of satellites in the future. A Japanese space agency telescope known as Hitomi is believed to have been affected by the anomaly. In 2016, as Hitomi passed over the SAA, it suffered a communication problem. After the communication problem, Hitomi lost altitude and broke apart. The magnetic field is weaker over the SAA because the Earth's magnetic axis is slightly off-center. Because of this, the inner radiation belt gets closer to the Earth, so spacecraft in low-Earth orbit which traverse the region are more exposed to the perils of space radiation trapped there. 

Updated July 21: Clarified SAA as a region in opening paragraphs. An earlier version of this article incorrectly stated the anomaly caused spacecraft to be exposed to more radiation. This has been amended.