The Van Allen probes were launched with an array of different sensors to study the radiation belts, which can interfere with satellite operations. NASA says the discovery was a lucky one. Planetary scientist Daniel Baker made a last-minute request that his team's Relativistic Electron Proton Telescope (REPT) be turned on earlier than scheduled, hoping to overlap observations with another mission called SAMPEX.
Almost immediately, the REPT instrument caught sight of a number of additional high energy particles trapped in the two known Allen Belts, and over the next several days, those particles aligned themselves into a third, high-energy band embedded in the outer Van Allen belt, about 11,900 to 13,900 miles above Earth's surface. The stable ring of radiation formed on Sept. 2 and lasted more than a month.
"We started wondering if there was something wrong with our instruments," said Shri Kanekal, deputy mission scientist for the Van Allen Probes and coauthor on the paper describing the results, published in this week's issue of Science. "We checked everything, but there was nothing wrong with them. The third belt persisted beautifully, day after day, week after week, for four weeks."
Then the newly discovered third belt abruptly vanished. A spike in solar wind caused an interplanetary shockwave that appears to have dispersed the super-high-energy electrons comprising the temporary ring.
It's still unclear how the radiation belt appeared. Van Allen mission scientists suspect it was created by solar wind tearing particles away from the outer Van Allen belt. "It looks like its existence may have been bookended by solar disturbances," Baker said.