The Earth temporarily gained a third radiation belt last year, NASA's Van Allen Probes mission has revealed.
Two other regions of trapped radiation have been known since 1958 to surround our planet, reacting to solar storms and space weather and sometimes endangering communications and GPS satellites, as well as humans in space. Now, it seems, there's sometimes a third.
It was discovered almost by accident. The Van Allen Probes consist of two identical spacecraft which are mapping out the region in detail. After they were launched on August 30 last year, the team decided to turn on the Relativistic Electron Proton Telescope (REPT) instrument a little earlier than planned, so that its observations would overlap with those of another mission called SAMPEX. It was a lucky decision.
"By the fifth day REPT was on, we could plot out our observations and watch the formation of a third radiation belt," says Shri Kanekal, deputy mission scientist for the Van Allen Probes.
"We started wondering if there was something wrong with our instruments. We checked everything, but there was nothing wrong with them. The third belt persisted beautifully, day after day, week after week, for four weeks."
At the end of this period a powerful interplanetary shock wave from the sun destroyed the third belt.
Spotting the third radiation belt has implications beyond the simple knowledge that a third belt is possible, says the team. It offers new clues as to what causes the changing shapes of the belts – which can sometimes swell dramatically in response to incoming energy from the sun, impacting satellites and spacecraft and potentially threatening manned space flight.
"Even 55 years after their discovery, the Earth's radiation belts still are capable of surprising us and still have mysteries to discover and explain," says Nicky Fox, Van Allen Probes deputy project scientist at Johns Hopkins University. "We thought we knew the radiation belts, but we don't."