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Space boffins having a poke around in the Van Allen Radiation Belts - two concentric rings encircling the Earth in which large numbers of high-energy charged particles are dashing around - have confirmed a theory about how the particles are accelerated to ultra-high energies.

Since the Van Allen belts were first discovered, astronomers have been trying to trace the source of the super-acceleration.

Now NASA's twin space probes have beamed back research showing the particles may be supercharged by local acceleration (speeding up local particles rather than those sourced from outside the radiation belt) in much the same way as hurricanes are powered by the evaporation of warm ocean water on Earth.

Part of the Van Allen probes' mission was to answer this very question in order to help boffins to improve their space weather predictions.

"Until the 1990s, we thought the Van Allen belts were pretty well-behaved and changed slowly," Geoff Reeves, radiation belt scientist at Los Alamos National Laboratory, said in a statement.

"With more and more measurements, however, we realised how quickly and unpredictably the radiation belts change. They are basically never in equilibrium, but in a constant state of change."

Reeves and a team had two theories about how the particles are sped up: local or radial acceleration. Radial acceleration (directed toward the centre of the "belt circles") would mean the electrons were transported and accelerated from "a source population located outside the radiation belts". Local acceleration would mean that lower-energy electrons sourced from inside the belts themselves had been accelerated deep in the heart of the circles.

Researchers found in new results from the probes that the second theory - local acceleration - was more likely. The team saw a rapid increase in energy in October last year, but particles weren't starting at high altitude and falling towards Earth, as would be expected in radial acceleration. A radial model would mean the particles would move perpendicular to the magnetic fields around Earth from low magnetic strength into the areas of higher strength closer to the planet - speeding up as the field strength increases.

Instead, the tempo was stepped up for particles in the middle of the belts and gradually spread both inward and outward, showing that local energy comes from electromagnetic waves carrying energy from other particles in the same region of space.

"These new results go a long way toward answering the questions of where and how particles are accelerated to high energy," said Mona Kessel, Van Allen Probes program scientist.

Now boffins have to try to determine which of the electromagnetic waves are affecting the particles. The probes, which are able to distinguish between different types of waves, are currently in the process of gathering that data. ®

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