Besides lightning, it looks as though thunderstorms produce antimatter beams as well.

Scientists using NASA's Fermi Gamma-ray Space Telescope have detected beams of antimatter that produced above thunderstorms on Earth, something never seen before.

The researchers think the antimatter particles were formed in a terrestrial gamma-ray flash, a brief burst produced inside thunderstorms associated with lightning. It is estimated that about 500 such flashes occur every day around the world. 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 team at the University of Alabama in Huntsville, in a statement.

When antimatter collides with a particle of normal matter, both particles immediately are annihilated and transformed into energy, forming gamma rays -exactly what Fermi is designed to detect. The telescope has detected gamma rays with energies of 511,000 electron volts, which indicates an electron has met its antimatter counterpart, a positron.

Fermi is designed to observe high-energy events in the universe, but it is also providing valuable insights into this earthly phenomenon. The satellite monitors both the sky and Earth, and has identified 130 terrestrial gamma ray flashes since its launch in 2008.

The spacecraft was located immediately above a thunderstorm for most of the observed flashes, but in four cases, storms were far from Fermi. In addition, lightning-generated radio signals detected by a global monitoring network indicated the only lightning at the time was hundreds of miles away or more.

During one flash, 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 directly.

But the gamma flash produced a beam of high-speed electrons and positrons, which then rode up Earth's magnetic field, allowing them to strike the spacecraft even from over the horizon. 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 23 milliseconds later. Each time, positrons in the beam collided with electrons in the spacecraft. The particles annihilated each other, emitting the gamma rays detected by Fermi.

Scientists long have suspected gamma flashes arise from the strong electric fields near the tops of thunderstorms. Under the right conditions, 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 terrestrial gamma flash.

But the cascading electrons can produce so many gamma rays that they blast electrons and positrons clear out of the atmosphere. This happens when the gamma-ray energy transforms into a pair of particles: an electron and a positron. It's these particles that reach Fermi's orbit.

The detection of positrons shows many high-energy particles are being ejected from the atmosphere. In fact, scientists now think that all terrestrial gamma flashes emit beams of electrons and positrons. A paper on the findings has been accepted for publication in Geophysical Research Letters.