When NASA scientists launched twin spacecraft to probe the Van Allen radiation belts last summer, they were expecting to study two rings of high-energy particles circling Earth. Instead they found three, overturning a 50-year-old model of the giant rings’ structure.
First discovered in 1958, the Van Allen belts have been thought to comprise two reservoirs of high-speed, electrically charged particles, corralled into separate doughnut-shaped rings by Earth’s magnetic field. The outer ring orbits at a distance of some 10,000–60,000 kilometers above Earth, and encircles an inner band of even more energetic particles, roughly 100–10,000 kilometers above Earth. That’s the configuration the belts were in when James Van Allen first spotted them using satellite data half a century ago, and that’s also the structure that NASA’s twin Van Allen Probes recorded when they began operation on 1 September 2012.
But just two days later, telescopes on the probes revealed the emergence of an additional, narrow belt of charged particles sandwiched between the inner ring and a now highly eroded outer ring. “It was so unexpected that we thought there was something wrong with the instrument,” says Daniel Baker, a space physicist at the University of Colorado in Boulder.