SEATTLE, WASHINGTON—Thunderstorms produce beams of antimatter. That's the surprising finding reported here yesterday at the 217th meeting of the American Astronomical Society. Scientists already knew about flashes of high-energy gamma-rays from Earth, which are associated with large thunderstorms. Every day, about 500 of these terrestrial gamma-ray flashes (TGFs) are produced worldwide by accelerated electrons interacting with air molecules. Now, astrophysicists working with NASA's Fermi Gamma-ray Space Telescope have found that some of the high-energy gamma-ray photons from TGFs are converted into pairs of electrons and positrons, the positively charged antiparticles of electrons. Every now and then, the orbiting space telescope is hit by some of these antimatter particles, which rush through Earth's magnetic field. When the positrons collide with electrons in the atoms that make up the spacecraft, they annihilate each other, producing gamma-ray photons with a telltale energy in the process. The role of lightning in the production of gamma rays and antimatter is still unclear, but the new discovery might help physicists better understand the mysterious TGF's.
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