Van Allen belts found to accelerate particles, not just trap them

Nature : The Van Allen radiation belts are a system of concentric rings of high-speed, high-energy particles encircling Earth. When the belts were discovered in 1958, it was thought that they captured and accelerated the particles themselves. That theory couldn’t explain why the energy and density of the belts appeared to vary relatively quickly, so then it was assumed that they trapped particles of already high energy emitted by the Sun. Now, the Van Allen Probes —a pair of identical satellites positioned to study the belts from different angles—have shown that the original theory is actually correct. One week after a major solar storm depleted the outer belt of most of its electrons, the satellites observed a 1000-fold increase in electron density in just 12 hours. That is significantly higher than possible if the high-energy electrons were only produced by the Sun and caught by the belts. The most likely explanation is that the belts include an electric field that is able to accelerate lower electrons when they are captured.