Gamma rays are the brightest, most powerful, explosions in the universe, often emitted from supernova or supermassive black holes. But these giant flashes can also have Earthly origins, stemming from intense storms — these are called terrestrial gamma-ray flashes. NASA's Fermi Gamma-ray Space Telescope (shown here in a May 2008 image), which launched in June 2008, can detect both types of gamma rays. The telescope can see the terrestrial type up to a distance of about 500 miles (800 kilometers).
On Dec. 14 2009 as Fermi passed over Egypt it spotted a TGF from a thunderstom in Zambia. The TGF flashed over spacecraft's horizon where Fermi couldn't actually "see it."
Terrestrial Gamma Rays
The TGFs are thought to begin in the intense electrical fields of thunderstorms. Within this field, the electrons get accelerated above the storm where the air is thin, ramping up to near light-speed. Here, an artist interpretation of electrons accelerating upward from a thunderhead.
Making Gamma Rays
When these electrons encounter an atom, they emit gamma rays, shown here in this artist impression of the powerful process.
Very rarely one of these gamma rays, traveling at near light-speed, will graze an atom, passing through its electron shell, and transform into a pair of particles — a normal matter electron and an antimatter electron, called a positron.
Along Magnetic Field Lines
So how did Fermi detect this gamma-ray flash above Zambia? Turns out, the gamma rays travel in straight lines, but the charged particles spiral along Earth's magnetic field lines, according to a NASA video.
As those high-energy particles spiraled upward, they hit the Fermi Gamma-Ray Space Telescope. The high-energy particles rode upward on magnetic field lines and then struck the spacecraft.
Fermi turns pink
The postirons annihilated when they struck electrons on the Fermi telescope, creating a flash of gamma rays, and, for an instant, the Fermi craft became a source of gamma rays, setting off its own detectors.
An illustration showing the launch of a gamma-ray flash from an intense storm on Earth, with (magenta) and high-energy electrons (yellow) shooting upward.
Here a terrestrial gamma-ray flash just 1.98 seconds old (right panel), revealing its electron/positron beam, moving from just 9.3 miles (15 kilometers) altitude to some 373 miles (600 km), where it may intercept such spacecraft as NASA's Fermi Gama-Ray Space Telescope. (Gamma rays are shown in magenta and high-energy electrons in yellow.)
Lots of Flashes
Researchers estimate there may be as many as 500 terrestrial gamma-ray flashes each day, according to a NASA video on the phenomenon.