As the Sun's churning surface lets loose a belch of white-hot flame, it sends out a storm of radiation that washes over the solar system. Luckily for us, Earth's magnetic field shields us from most of these deadly rays. But overhead, something strange and lethal is happening when the solar wind bombards the Earth. A band of radioactive particles circling the planet, called the outer Van Allen belt, starts to charge up like a rail gun. It whips electrons along on its circular racetrack at a breakneck pace—near light speed. The powerful band ebbs and flows with solar radiation, but until today, nobody could be sure how it was creating such swift and energetic particles.

"This is like watching a natural particle accelerator in space," says Geoffrey Reeves, a magnetic field researcher at Los Alamos National Laboratory.

Reeves and a team of scientists published research today in the journal Science describing the bizarre way the outer Van Allen belt—which orbits around the Earth like a giant doughnut—accelerates electrons to more than 99.9 percent of the speed of light. Discounting light itself, Reeves says, "these electrons are the fastest things the Earth creates naturally." And they aren't simply a high-velocity curiosity: They pose a threat to the International Space Station and to commercial satellites. The particles can burst through the protective shielding—causing temporary computer failures—and cause degradation to vital onboard equipment such as solar panels.

Surfing Radio Waves

Reeves and his team, with the help of a pair of newly launched NASA satellites called the Van Allen Probes, showed that the electrons in the belt gain their breathtaking velocity by hitching rides on radio waves. Daniel Baker, an astrophysicist with the study from the University of Colorado at Boulder, says that as solar radiation crushes against the Earth's magnetic field, a surge of radio and other waves begin to pump through the Van Allen belts. By chance, some of the radio waves are the exact frequency as electrons already twirling through the belt at slow speeds. "So the electrons hitch a ride—sort of surfing on these waves—and can be carried to these high speeds," Baker says. "It's a subtle but very powerful interaction."

Geoff Reeves, Los Alamos National Laboratory

Reeves says the new NASA satellites, launched last August, are the main reason researchers ID'd this radio wave surfing. "We had the right instruments, the satellites were in the right orbit, and we had two of them," he says. Previous satellites used to measure the Van Allen belts lacked the tools or orbit to take successive measurements deep within the hazardous core of the belts. They could get a snapshot, but not a moving picture, of how the Van Allen belts evolve and respond to solar wind. The heavily shielded Van Allen probes repeatedly darted through the heart of the Van Allen belt, quickly measuring the entire range of slow to speedy electrons.

Knowing how Earth's natural electron accelerator works is important for more reasons than just satisfying scientific curiosity. Because of the Van Allen belts' doughnut-like shape and constant expansion and contraction with solar wind, their high-energy electrons can impact almost every satellite in orbit. "It's a rare satellite that isn't affected by this," Baker says, "and we have numerous examples through the last couple of decades where we've seen absolutely confirmed evidence that this is a serious operational problem."

Still, Jean-Luc Froeliger, the vice president of satellite engineering at Intelsat, which operates the world's largest commercial satellite fleet, cautions that while the Van Allen belts' high energy particles can do long-term damage to satellites, all modern satellites are built to withstand this particle barrage with extra shielding covering vital components, and major damage is rare. "I don't know any commercial satellite that has been totally destroyed by high energy elections," Froeliger says, "but there is certainly an impact, in that the solar arrays degrade the more a satellite's bombarded by particles."

Forecasting Space Storms

The researchers hope that by understanding the Van Allen belt, they can predict when it will be at its worst, and develop methods to keep our satellites better protected. Figuring out how the electrons accelerate is the first step. "If we want to be able to predict when the radiation belts are going to get very intense, we have to know what's the source of the energy," Reeves says.

The Van Allen belts are still by no means fully understood. Vassilis Angelopoulos, a magnetic field expert at the University of California, Los Angeles, who was not involved in the study, points out that while we've discovered how the electrons are accelerated, we still don't know how solar radiation creates these surfable radio waves. "What generates the waves to begin with still remains to be solved," Angelopoulos says. "While we're putting to rest one of the important questions about the Earth's [radiation belts], there is still a lot of work to be done to reach a predictive capability within our models."

Reeves argues that in a world increasingly reliant on all the technologies satellites foster—from GPS to telecommunications—working to protect satellites is an essential goal. "Even when you swipe your credit card at the gas station," he says, "that transaction will go through a satellite before it goes to your bank."

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