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By Deane Morrison
January 29, 2008
Fifty years ago this week, the United States entered the Space Age with the launch of its first satellite, Explorer I. The spacecraft made history by finding the first of two Earth-girdling radiation belts that threaten satellite electronics--and astronauts. To celebrate the anniversary, a University-led team used University-designed instruments to unlock one of the biggest mysteries of the Belts, which are named for their discoverer, James Van Allen. The researchers pinpointed the likely physical process that creates some of most destructive radiation in the Van Allen Belts, a necessary step toward NASA's goal of predicting and circumventing damage to spacecraft and space travelers. The culprit? The most powerful radio waves of their kind ever detected in the Belts. The researchers not only discovered the waves but showed that they are capable of accelerating electrons to near the speed of light--which gives the electrons enough energy to knock out computers, pierce spacesuits, and damage the tissues of astronauts--and that they can do it astonishingly fast. Their discovery of these "celestial tsunamis" appears in the journal Geophysical Research Letters. "No one has ever seen waves this big," says University physics professor Cynthia Cattell, who led the team. "They're more than 10 times bigger than what we knew about."
Shaped like two concentric pumpkin shells around the Earth, the Van Allen Belts are areas where electrons and other charged particles get trapped by Earth's magnetic field. The belts constantly shift and pulsate, but in general they are thickest above Earth's midsection. There, the center of the Inner Belt is about 6,000 miles up and the more active Outer Belt--where the high-powered radio waves were detected--is at about 16,000 miles.
NASA takes a keen interest in the physics of the Van Allen Belts because disturbances in them can knock out satellites for weather, GPS, communications, and spying. Astronauts en route to or from the moon must pass through the Belts, and the International Space Station (ISS) is currently of special concern because it spends up to 20 percent of any given day inside the Outer Belt where it curves down toward the Earth. The ISS is a laboratory for working out the problems that confront humans in space, and high-speed electrons are one of them. Astronauts in or visiting the ISS sometimes must perform spacewalks to repair the station, which leaves them extremely vulnerable.
The waves studied by Cattell and her colleagues are known as whistlers, a special type of radio-frequency wave that has been known since World War I. "When first discovered, they were being generated by lightning," says Cattell. "They could be heard through radio receivers as high pitches falling to lower." The newly found whistlers have a lot in common with the ocean waves off Waikiki Beach. Both pick up surfers--whether people or electrons--and transfer energy to them. Electrons that absorb enough energy from whistlers can hurtle along at up to 99 percent the speed of light, which translates to 184,000 miles per second. The most startling revelation was how fast it happens. It had been thought that multiple interactions between whistlers and electrons, taking place over a span of minutes or even tens of hours, were necessary. "But we saw that electrons can be energized in a tenth of a second," says Cattell.
The key to the discovery lay in a couple of identical instruments designed by University physicist Keith Goetz. They are aboard the twin spacecraft of NASA's STEREO mission, one orbiting ahead of Earth and the other orbiting behind. The idea is to use the widely separated spacecraft to study the sun in 3-D. STEREO was launched in October 2006.
"It's icing on the cake to get this discovery in the radiation belts when at the beginning, our prime mission was to study the sun." The focus of Goetz's instrument--called TDS, for time-domain sampler--is waves in the solar wind, a stream of charged particles flowing from the sun. The TDS's were intended to collect data after the two STEREO spacecraft had settled into their respective orbits. But that didn't stop Goetz from insisting that they be turned on early, when the two orbiters were still near Earth.
Cynthia Cattell is one of three University of Minnesota faculty recently elected fellows of the American Geophysical Union. The others are R. Lawrence Edwards, professor of geology and geophysics, and Renata Wentzcovitch, professor of chemical engineering and materials science.
AGU is a worldwide scientific community of 50,000 researchers, teachers, and students who advance the understanding of Earth and space for the benefit of humanity.
And so they were. And thus the antennas of the TDS were ready on December 12, 2006, when the big break came. On that day the two spacecraft sailed through the Outer Van Allen Belt in tandem, one about 84 minutes behind the other. During that short interval, the Outer Belt was hit by a "magnetospheric substorm," an explosive release of energy from the Earth's magnetic field. The substorm stirred up the massive whistlers, which were detected by the second STEREO spacecraft. The TDS was the first instrument ever to detect such large waves, and it was no accident. It was programmed to measure much more powerful radio waves over much shorter time intervals than instruments on previous missions and to regularly discard data on all but the biggest whistlers it detected. "It's a very smart instrument," Cattell observes. Using the University's Supercomputing Institute, Cattell and undergraduate Kris Kersten simulated interactions between whistlers and electrons and found that the whistlers they had detected in the Outer Belt were more than strong enough to accelerate electrons to near light-speed in a tiny fraction of a second. So those whistlers emerged as the prime suspect behind that kind of radiation. The finding was especially gratifying because it came as a bonus. "It's icing on the cake to get this discovery in the radiation belts when at the beginning, our prime mission was to study the sun," says Goetz.
The University will continue to play a major role in studying the radiation belts. Physics professor John Wygant will lead one of four teams designing the Van Allen Radiation Belt Storm Probes, a two-spacecraft NASA mission scheduled for launch in 2012. "The processes that form radiation belts are mysterious," he says. "We want to understand all the mechanisms that energize particles in them."