Earth's Van Allen belts still offering up surprises

Posted by Eric Betz
on Monday, January 5, 2015

NASA's twin Van Allen probes launched in 2012 and almost immediately discovered a new radiation belt surrounding Earth. // JHUAPL/NASA
Our planet's magnetic field holds in a toroidal band of radiation in a belt around Earth, which protects us from solar eruptions, growing and shrinking as it's excited. That find is known as the first great discovery of the Space Age. And despite Americans getting to space second, the discovery was ours.

The Soviet Union had shocked the world with Sputnik. When they followed their feat with a second craft, the data it captured contained evidence of these radiation belts, but the data was received by the Australians, who asked the Russians to share the secret to its encryption. The Russians refused and demanded the data, which the Australians did not turn over. So when the United States launched its Explorer 1 in 1958, scientists were surprised by the finding.

The main belts start at an altitude around 600 miles (1,000 kilometers) above Earth and stretch to some 37,000 miles (60,000km). Their particles originate in the solar wind and from cosmic rays.

NASA launched the two identical Van Allen Probe spacecraft to study the little explored region and confirm what scientists thought they already knew. But even after 60 years, the belts were holding back new surprises.

"Something happened that puzzled me and alarmed me and had me wondering if the instrument was working properly," says Daniel Baker of the University of Colorado, Boulder, who did his doctorate with James Van Allen and gave the first science talk at the 225th American Astronomical Society meeting Monday in Seattle.

But Baker and others soon learned the instruments were not malfunctioning. Almost immediately, NASA's probes had turned up a new radiation belt. "The textbooks always said two belts, and here we had three belts," he says.

The finds aren't done yet, either. Astronomers also discovered the belts have what appears to be an impenetrable barrier, preventing the most energetic particles from reaching Earth. The find was first announced in the journal Nature in November. Baker says it was made possible because measurements of the most energetic electrons had not previously been available.

The probes showed a sharp edge on the inner side of the outermost belt, which stops even the fastest, highest-energy electrons. Baker says that scientists looked at earthly radio transmissions to see if they could be causing the effect, but soon realized only nature would be capable of such high energy. They now suspect Earth's plasmasphere.

"When you look at really energetic electrons, they can only come to within a certain distance from Earth," Shri Kanekal, deputy mission scientist for the probes, said in a release initially announcing the discovery. "This is completely new. We certainly didn't expect that."

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