Space telescope catches antimatter from terrestrial thunderstorms

When an antimatter particle strikes Fermi, it will collide with a particle of normal matter. Both particles are immediately annihilated and transformed into gamma rays of characteristic energy. The GBM detected gamma rays with energies of 511,000 electron volts, a signal indicating that an electron has met its antimatter counterpart -- a positron. For comparison, visible light has energies between 2 and 3 electron volts.

The scientists think that the antimatter particles were formed in a terrestrial gamma-ray flash (TGF), a brief burst of gamma rays produced inside thunderstorms that is somehow associated with lightning. "These gamma photons have typical energies of 20-40 million electron volts, and these are usually detected as TGF," says Andreas v. Kienlin, scientist at the MPE who led the development of the Fermi GBM.

Theorists estimate that about 500 TGFs occur worldwide each day, but most go undetected. Even though Fermi's GBM is designed to observe high-energy events in the distant universe, the GBM team has identified 130 TGFs since Fermi's launch. 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 (4,500 km) to the south. Because the distant storm was below Fermi's horizon, any gamma that rays it produced could not have been detected by the GBM. High-speed electrons and positrons produced in the TGF, however, could travel along the Earth's magnetic field to strike the spacecraft and produce a pulse of gamma rays that was picked up by Fermi's GBM. "This signal is the first direct evidence that thunderstorms make antimatter particle beams," said Michael Briggs, a member of the GBM team at the University of Alabama.

The detection of positrons thus proves that copious high-energy particles are indeed being ejected from the atmosphere. In fact, scientists now think that all TGFs emit electron/positron beams. "However, we still don't know how these TGFs are produced, and we also do not quite understand how the classical lightning forms," says Jochen Greiner from the MPE, the German principal investigator of the GBM. Even though turbulence in thunderclouds can produce large voltages, they are not strong enough to ionize the air and to lead to sparks. TGFs could provide the trigger.

A paper on the findings will appear in a forthcoming issue of Geophysical Research Letters.