Unusual third Van Allen belt explained

September 25, 2013 10:45 pm | Updated June 02, 2016 03:12 pm IST

The Van Allen radiation belts were discovered in 1958. They are gigantic donut-shaped belts of accelerated particles strung along the Earth’s magnetic field at 1,000-50,000 km above the surface. They consist of two distinct bands. The inner band consists of high-energy electrons and positive ions, and the outer band consists of high-energy electrons.

In February 2013, scientists reported that there was a third previously unobserved radiation belt, found lurking between the two original ones. Even more puzzling, this belt vanished after a month, leaving scientists wondering about its origins.


The answer may finally be here. Yuri Shprits, a researcher geophysicist at the University of California, Los Angeles, and his colleagues have performed calculations showing that the third belt was the result of an interaction between the outer belt and an injection of ions from a solar storm in September, 2012.

Shprits’s study, published in Nature Physics on September 22, also shows that the short-lived belt was composed entirely of ultra-relativistic electrons, i.e. travelling at close to the speed of light. The other belts have such electrons, too, but are not composed exclusively of them.

The team performed simulations with a model of the Earth’s radiation belts with respect to late August to early October, 2012.

They used information of space weather conditions as monitored and recorded by ground stations.

They showed that, during the solar storm, ions originating from a strong solar flare knocked out ultra-relativistic electrons from the outer belt almost to its inner edge. By the end, only a thin ring of ultra-relativistic electrons survived this.

“At ultra-relativistic energies, the physics to do with the wave-particle duality drives the radiation belts which allows for the formation of a vary narrow ring. This stayed unchanged for a whole month,” Shprits said in an email.

The simulations matched the observations from NASA’s Van Allen Probes mission, according some promise to this new theory of the mysterious ring.

Additionally, because so little is known about what accelerates particles in the radiation belts (a paper published in Science on August 30 suggests the cause is plasma waves originating from within the belts), Shprits’s work also sheds light on how those particles of different energies engage with conditions in space.

For instance, the electrons in the radiation belts have a wide range, from a “few hundred keV to multi-MeV,” Shprits wrote. 1 MeV, which stands for mega electron volt, is about the amount of energy corresponding to an electron accelerated to about 2,000 times its original mass.

“What we have discovered is that multi-MeV electrons form a new population that is driven by different physics and form very unusual structures in space,” he added.

He also cautioned that their ‘discovery’ was only the tip of the iceberg: “We still need to understand which physical processes are responsible for acceleration to ultra-relativistic energies.”

His and his team’s discovery could help engineers design better shielding for satellites that want to avoid radiation exposure to the Van Allen belts, as it could lead to anything from communication disruption to critical failure itself.

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