Fermi Spots Antimatter in Thunderstorms

Analysis by Jennifer Ouellette
Thu Jan 13, 2011 01:07 AM ET



Mother Nature seems to be giving expensive man-made particle accelerators like the Large Hadron Collider a run for their money these days.

Scientists at the American Astronomical Society (AAS) meeting in Seattle announced this week that NASA's Fermi Gamma-Ray Space Telescope has spotted particles of antimatter being flung into space during thunderstorms in the Earth's atmosphere. It's the first direct evidence of a phenomenon that scientists have suspected since 2009.

Antimatter is the same as regular matter except that each particle has an opposite charge. So while an electron has a negative charge, its antimatter counterpart, a positron, has a positive charge and they annihilate each other when they get too close.

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However, it's rare to find antimatter in our universe, since it annihilates almost instantly when it comes into contact with matter. Scientists can only make tiny amounts, even in the LHC -- much to the disappointment of some Star Trek fans, who still dream of antimatter-powered spaceships like the Enterprise.

Now it's looking as if antimatter might not be quite so exotic as we thought; thunderstorms might routinely produce antimatter beams via a byproduct of lightning known as a terrestrial gamma ray flash (TGF).

Until quite recently, scientists believed that most gamma-rays are produced by cataclysmic events in distant galaxies, such as the death of supermassive stars. But thanks to the advent of gamma-ray telescopes, like Fermi, we've known about terrestrial gamma radiation since 1994 --"probably the last place we ever expected to see [them]," Gerald Fishman, an astrophysicist at NASA's Marshall Space Flight Center, told Discovery News last year.

One way to create such short bursts of gamma rays is through the collision -- and subsequent annihilation, as the mass of those particles converts into energy -- of an electron and a positron. The Fermi Space Telescope found hints in 2009 of the telltale pattern associated with positrons in terrestrial gamma rays stemming from lightning storms.
500x_thunderstorms-make-antimatter

Under the right conditions, strong electric fields -- the kind typically found near the tops of thunderstorms -- can trigger an upward avalanche of high-energy electronics that in turn interact with the atmosphere to create a natural "particle accelerator," particularly when combined with the cosmic rays that routinely bombard Earth's atmosphere. (Such avalanches may also be what triggers lightning.) All those high-energy electrons emit gamma rays whenever they're deflected by air molecules in the atmosphere, and these are usually detected as a TGF.

But how can terrestrial gamma rays reach Fermi's orbit in space? It turns out that this avalanche of electrons can produce so many gamma rays that some transform into electrons and positrons, and these in turn are hurled out of the atmosphere completely. Those antimatter beams are then picked up by Fermi's instruments.

In this case, the telltale signal is gamma-rays at energies of 511,000 electron volts -- evidence that an electron/positron pair has annihilated.

The Fermi Space Telescope is so sensitive to these gamma ray bursts that it doesn't even need to be positioned right over a thunderstorm to detect the TGFs. In one case, the telescope was orbiting over Egypt, but the TGFs it detected came from an active storm in Zambia, a good 1,800 miles south. "Even though Fermi couldn't see the storm, the spacecraft nevertheless was magnetically connected to it," Florida Institute of Technology's Joseph Dwyer said in NASA's press release.

"The TGF produced high-speed electrons and positrons, which then rode up Earth's magnetic field to strike the spacecraft."

This doesn't mean we now know everything about producing large quantities of antimatter (sorry, hardcore Star Trek fans -- yes, it would be awesome), or even about TGFs and lightning's unique role in this process. But it's still a pretty nifty result, and I, for one, remain in awe of the extraordinary power of Mother Nature.