From Ben Franklin to Dr Frankenstein, lightning has long fascinated and enlightened scientists. Now NASA is studying another fascinating – and surprising – product of thunderstorms: gamma rays. NASA estimates that around a thousand gamma ray bursts are produced by storms each day and has been using the Fermi Gamma-ray Space Telescope to gain a better understanding of these high-energy events, which have tens of millions of times the energy of visible light.
Terrestrial Gamma-ray Flashes (TGF) were first noticed in 1992 by NASA's Compton Gamma-Ray Observatory. Lasting less than a millisecond, a TGF is invisible to the naked eye, yet as some of the highest-energy natural light on Earth, each has tens of millions times the energy of a lightning strike's visible flash.
TGFs are produced by storm clouds, including in thunderstorms, typhoons, hurricanes, and tropical storms. The violent winds that surge around a thunderhead cloud send snow and ice particles up and down until they act like a titanic electrostatic generator. When the charge is high enough, lightning arcs between the cloud and the earth.
So far, this is standard schoolbook physics, but if the electric field becomes strong enough, it produces a cascade effect that blasts electrons upward at near the speed of light. If some of these electrons get deflected by air molecules, they release a burst of gamma rays. Due to the peculiar conditions needed to create these bursts, meteorologists are interested in them as a way to study how hurricanes and other large storms work.
That's where NASA's Fermi Gamma-ray Space Telescope comes in. Launched in 2008, it has recorded over 4,000 TGFs at a range of up to 500 miles (800 km). In 2012, Fermi's Gamma-ray Burst Monitor was upgraded with new software that allowed it to directly download full-resolution gamma-ray data at a higher level of sensitivity, so many lower-energy TGFs were recorded. Now scientists are focusing on a few dozen that were produced by tropical storms, hurricanes and typhoons.
Working in collaboration with ground stations to gain precise location fixes, Fermi found that the TGF also produces a strong pulse of very low frequency radio waves, which were previously thought to be only associated with lightning, In addition, the scientists found that storm intensity wasn't the main factor in producing the gamma bursts. Some of the TGFs were generated in outer areas away from the most intense regions, and a weak system produced several bursts in one day.
In all, the NASA team looked at 37 TGFs, including from typhoons Nangka and Bolaven, Hurricane Paula, tropical storms Sonia and Emang and Hurricane Manuel, and the nascent Hurricane Julio in 2014. From these, they found that TGFs were most common in outer rainbands where the highest incidence of lightning takes place, and tended to occur as storms intensified.
"One result is a confirmation that storm intensity alone is not the key factor for producing TGFs," said Oliver Roberts, who led the study at the University College Dublin, Ireland, and is now at NASA's Marshall Space Flight Center in Huntsville, Alabama. "We found a few TGFs were made in the outer rain bands of major storms, hundreds of kilometers from the powerful eye walls at their centers, and one weak system that fired off several TGFs in a day."
The research was published in the Journal of Geophysical Research: Atmospheres.
The video below discusses how TGFs work.