What were you doing last September? The charged particles that dance around Earth were busy. Unbeknown to most earthlings, a previously unseen ring of radiation encircled our planet for nearly the whole month – before being destroyed by a powerful interplanetary shock wave.
We already knew that two, persistent belts of charged particles, called the Van Allen radiation belts, encircle Earth. The discovery of a third, middle ring by NASA's twin Van Allen probes, launched in August 2012, suggests that these belts, which have puzzled scientists for over 50 years, are even stranger than we thought. Working out what caused the third ring to develop could help protect spacecraft from damaging doses of radiation.
Charged particles get trapped by Earth's magnetic field into two distinct regions, forming the belts. The inner belt, which extends from an altitude of 1600 to 12,900 kilometres, is fairly stable. But the outer belt, spanning altitudes ranging from 19,000 to 40,000 kilometres, can vary wildly. Over the course of minutes or hours, its electrons can be accelerated to close to the speed of light, and it can grow to 100 times its usual size.
Mystery acceleration
No one is sure what causes these "acceleration events", although it seems to have something to do with solar activity interacting with the Earths' magnetic field.
"That's one of the key things the probes are in place to understand," says Dan Baker of the University of Colorado, Boulder. "How does this cosmic accelerator, operating just a few thousand miles above our head, accelerate electrons to such extraordinarily high energies?"
When the Van Allen probes started taking data on 1 September 2012, one of these mysterious events was already under way. "We came in the middle of the movie there," Baker says. But otherwise, he says, "What we expected was what we saw when we first turned on: two distinct belts, separated."
That changed a day later when, to the team's surprise, an extra ring developed between the inner and outer ones. "We watched it develop right before our eyes," Baker says. The new, middle ring was relatively narrow, and its electrons had energies between 4 and 7.5 megaelectronvolts - about the same as in the outer Van Allen belt during an acceleration event.
Although the outer ring displayed its characteristic inconstancy, the new middle ring barely budged for nearly four weeks. Then a shock wave, probably linked to a burst of solar activity, wiped it out in less than an hour on 1 October.
Spacecraft malfunctions
It's not clear where the middle ring came from, Baker says, although it was probably related to the acceleration event. The electrons could have been stripped from the outer Van Allen belt, funnelled back towards the Earth and got trapped in the middle on the way, or they could have been energised from closer to Earth and shot up to higher altitudes.
Figuring out what happened could be important to protecting spacecraft from radiation damage, says Yuri Shprits of the University of California in Los Angeles, who was not involved in the observations but is crafting a theoretical explanation that he hopes to publish soon. "It truly presents us with a very important question, and very important puzzles," he says.
There were no specific spacecraft malfunctions during September that can be directly linked to the new belt, says Shprits. However satellite operators will want to know if such belts are common and if they pose more of a risk.
With no other examples of a transient belt caught so far, it's too soon to answer all those questions, Baker says. "We only have one in captivity," he says. "We're still trying to figure out exactly how it works."
Journal reference: Science, DOI: 10.1126/science.1233518
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