Scientists using NASA’s Fermi Gamma-ray Space Telescope have detected beams of antimatter produced above thunderstorms on Earth, a phenomenon never seen before.
Scientists think the antimatter particles were formed in a terrestrial gamma-ray flash (TGF), a brief burst produced inside thunderstorms and shown to be associated with lightning. It is estimated that about 500 TGFs occur daily worldwide, but most go undetected.
“These signals are the first direct evidence that thunderstorms make antimatter particle beams,” said Michael Briggs, a member of Fermi’s Gamma-ray Burst Monitor (GBM) team at the University of Alabama in Huntsville. He presented the findings Jan. 10 at the American Astronomical Society meeting in Seattle.
Fermi is designed to monitor gamma rays, the highest-energy form of light. When antimatter striking Fermi collides with a particle of normal matter, both particles immediately are annihilated and transformed into gamma rays. The GBM has detected gamma rays with energies of 511,000 electron volts, a signal indicating an electron has met its antimatter counterpart, a positron.
Although Fermi’s GBM telescope is designed to observe high-energy events in the universe, it is also providing valuable insights into this strange earthly phenomenon. The telescope constantly monitors the entire celestial sky above and the Earth below—and 130 TGFs have been identified since its launch in 2008.
The spacecraft was located immediately above a thunderstorm for most of the observed events, but in four cases, storms were far from Fermi. In addition, lightning-generated radio signals detected by a global monitoring network indicated that the only lightning at the time was hundreds or more miles away. During one TGF, which occurred on Dec. 14, 2009, Fermi was located over Egypt. But the active storm was in Zambia, some 2,800 miles to the south. The distant storm was below Fermi’s horizon, so any gamma rays it produced could not have been detected.
“Even though Fermi couldn’t see the storm, the spacecraft nevertheless was magnetically connected to it,” said Joseph Dwyer at the Florida Institute of Technology in Melbourne, Fla. “The TGF produced high-speed electrons and positrons, which then rode up Earth’s magnetic field to strike the spacecraft.”
The beam continued past Fermi, reached a location, known as a mirror point, where its motion was reversed, and then hit the spacecraft a second time just 23 milliseconds later. Each time, positrons in the beam collided with electrons in the spacecraft. The particles annihilated each other, emitting gamma rays detected by Fermi’s instruments.
Scientists long have suspected that TGFs arise from the strong electric fields near the tops of thunderstorms. Under the right conditions, they say, the field becomes strong enough that it drives an upward avalanche of electrons. Reaching speeds nearly as fast as light, the high-energy electrons give off gamma rays when they’re deflected by air molecules. Normally, these gamma rays are detected as a TGF.
NASA’s Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership. It is managed by NASA’s Goddard Space Flight Center in Greenbelt, Maryland. It was developed in collaboration with the U.S. Department of Energy, with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.
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