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Thunderstorms that shoot antimatter.

NASA's Fermi Gamma-ray Space Telescope is watching some of the most extreme phenomena in the distant universe, but little did the Fermi team expect it to be hit from below. Nevertheless, in what lightning researcher Steven Cummer (Duke University) calls "One of most exciting discoveries in geosciences in quite a long time," Fermi has discovered that thunderstorms on Earth sometimes shoot beams of antimatter up into space.

Antimatter particles match familiar particles such as protons and electrons but have the opposite electric charge. When particles of matter and antimatter meet, they annihilate each other to produce gamma rays. The process can also work in reverse. When a high-energy gamma-ray hits or skims by something, it can decay into a particle-antiparticle pair. But as Cummer told an American Astronomical Society meeting in January, "The idea that any planet can produce antimatter and beam it into space in narrow beams sounds like science fiction."

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Positrons (antielectrons) were sensed by Fermi's Gamma-ray Burst Monitor (GBM) coming from "terrestrial gamma-ray flashes" that occur above thunderstorms. TGFs, discovered in 1994, are brief, upward sprays of gamma rays that typically last a thousandth of a second. An estimated 500 occur around the world each day, mostly unnoticed.

Fermi has detected 130 TGFs. In four of them, positrons also struck the satellite. Apparently, some of the upward gamma rays are sufficiently energetic to convert into electron-positron pairs, and when this happens near the border of space, the positrons flying onward become trapped along Earth's magnetic field lines to travel in a narrow, curved stream. Occasionally a stream intersects Fermi.

The gamma rays themselves presumably originate when electrons driven by lightning are somehow accelerated to nearly the speed of light before they can strike atoms of air. Cummer pointed out that despite decades of research, scientists still don't know exactly how lightning is produced or propagates in clouds. "This discovery has very important implications for understanding lightning itself," he said in making the announcement. "A new result like this gives us important clues about what's happening."

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Title Annotation:News Notes
Publication:Sky & Telescope
Geographic Code:1USA
Date:Apr 1, 2011
Words:340
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