Scientists have for the first time discovered that antimatter is regularly being produced on Earth - by thunderstorms.
NASA's Fermi Gamma-ray Space Telescope has detected beams of antimatter formed in a terrestrial gamma-ray flash (TGF), a brief burst produced inside thunderstorms and known to be associated with lightning. It's estimated that as many as 500 TGFs occur daily worldwide.
"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.
When antimatter striking Fermi collides with a particle of normal matter, both particles are immediately annihilated and transformed into gamma rays.
And 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.
"In orbit for less than three years, the Fermi mission has proven to be an amazing tool to probe the universe. Now we learn that it can discover mysteries much, much closer to home," said Ilana Harrus, Fermi program scientist.
The spacecraft was located immediately above a thunderstorm for most of the 130 TGFs it's identified so far, but in four cases, storms were some way off. Lightning-generated radio signals indicated that the only lightning at the time was hundreds or more miles away.
During one TGF, which occurred on December 14, 2009, Fermi was located over Egypt. However, the active storm was in Zambia, some 2,800 miles to the south. It 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. "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.
Under the right conditions, it seems, the electric fields near the tops of thunderstorms become strong enough to drive 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.
But it now seems that the cascading electrons can produce so many gamma rays that they blast electrons and positrons right out of the atmosphere.
"The Fermi results put us a step closer to understanding how TGFs work," said Steven Cummer at Duke University. "We still have to figure out what is special about these storms and the precise role lightning plays in the process."