It turns out thunderstorms pack a much bigger punch than most people think. In the mid-1990s, we found that lightning and the associated electric field above a thunderstorm can be strong enough to produce a gamma ray blast detectable from space. These terrestrial gamma-ray flashes (TGFs) are believed to occur all over the world at a rate of about 500 per day. Researchers using NASA's Fermi Gamma-ray Space Telescope have discovered a similar, but previously undetected phenomena: the production of an antimatter beam from the top of these storms.
Fermi is built to observe and measure gamma rays from anywhere in the Universe, but it has also been used to look at terrestrial events as well. Over its first three years, Fermi has identified 130 TGFs. These TGFs have included gamma rays with an energy of 511 keV—the energy signature of an electron-positron annihilation event.
In all but four of these TGF events, the Fermi observatory was directly over a thunderstorm. Scientists hypothesize that the strong electrical field found at the top of thunderstorms can, under the right conditions, create "an upward avalanche of electrons." These electrons are accelerated to nearly the speed of light, and when they bounce off air molecules, they emit high-energy gamma rays—the TGFs.
So many high-energy gamma rays are created that some literally turn from energy to mass in the form of an electron-positron pair. These particles and antiparticles then reach orbit, triggering Fermi's sensors when the positron collides with its regular matter counterpart.
The times when an antimatter collision was detected and Fermi was not directly over a thunderstorm provide an interesting glimpse into where these particles go. On December 14, 2009, Fermi was located over Egypt and picked up the positron-electron annihilation's signature; however, the nearest electrical storm was occurring some 2,800 miles to the south in Zambia. This storm existed below Fermi's line of sight, below the horizon, so there was no way that the observatory could have picked up the TGF directly.
It seems that the particles emitted by the storm were being swept along in a helical motion, traveling in a south-to-north direction along the Earth's magnetic field lines. As they swept past the Fermi telescope, their signature was picked up. As they continued their path up the magnetic field line, they reached the ionosphere north of Fermi's location. Here, the particles bounced and began heading south again along the field line. A secondary signal was observed as the particles again washed over the Fermi a mere 23 milliseconds later.
This work has led the researchers to conclude that all TGFs are associated with electron/positron beams that spray high energy particles into space. Their work is set to be published in an upcoming edition of Geophysical Research Letters.
