April 24 (UPI) -- For the first time, researchers have analyzed terrestrial gamma-ray flashes produced by tropical storms. Terrestrial gamma-ray flashes, or TGFs, are one of most intense forms of light naturally produced on Earth.
Since 2008, NASA's Fermi Gamma-ray Space Telescope has recorded more than 4,000 TGFs, each of which last less than a millisecond and produce several million times the energy of natural light. Until now, scientists hadn't studied flashes produced by tropical storms, hurricanes or typhoons.
"One result is a confirmation that storm intensity alone is not the key factor for producing TGFs," NASA researcher Oliver Roberts said in a news release. "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."
Scientists hypothesize that TGFs are produced by a strong electric forces in upper regions of thunderstorm clouds. The forces trigger an upward cascade of electrons traveling at nearly the speed of light. As the rush of electrons bump into air molecules, they emit gamma rays.
The latest research -- detailed earlier this year in the Journal of Geophysical Research: Atmospheres -- showed tropical systems don't produce TGFs markedly different from those generated by smaller storms. In fact, smaller storms can often generated many more flashes than larger systems.
Scientists found TGFs in tropical systems were mostly generated within the storm's outer rain bands. TGFs in smaller storms can originate from any part of the storm.
Researchers also found rates of flashes increased as larger storm systems intensified. As tropical systems grow in size and strength, their clouds are pushed higher into the atmosphere where intense electrical fields can develop.
Among the surveyed storms, 2014's Hurricane Julio was the most prolific TGF-producer.
"In our study, Julio holds the record for TGFs, firing off four within 100 minutes on Aug. 3, 2014, another the day after, and then no more for the life of the storm," Roberts said. "Most of this activity occurred as Julio underwent rapid intensification into a tropical depression, but long before it had even become a named storm."