Thunderstorm Gamma Ray Generators

High-energy events in outer space and thermonuclear reactions within the sun are considered the main source of gamma radiation on Earth, but engineers at Duke University's Pratt School of Engineering have found gamma rays coming from thunderclouds in the Earth's atmosphere. Steve Cummer, an assistant professor of electrical and computer engineering at the Pratt School of Engineering, explained, "All of this comes as a huge surprise. These are higher energy gamma rays than come from the sun. And yet here they are coming from the kind of terrestrial thunderstorm that we see here all the time."

In 1994 scientists using the Compton Gamma Ray Observatory satellite detected gamma rays originating near the Earth's surface that were connected to lightning. These were referred to a terrestrial gamma ray flashes--TGFs. Cummer and his team identified lightning episodes they could link in time and place to TGFs. As part of their study of thunderstorms, they installed sensitive receivers in DukeForest, an outdoor research reserve near the university's campuses. One receiver detects very low frequency and extremely low frequency RF signals in the 50-30,000 hertz and 3-3,000 hertz bands. Another instrument senses even lower frequencies--less than 0.1 to 400 hertz.

Researchers were surprised to find TGFs being generated by much smaller thunderstorms than would be required if runaway breakdown was creating a high-altitude high-energy electron beam that generated TGFs when it interacted with the atmosphere. The Duke engineering news release Gamma Rays From Thunderstorms? said a report on the TGFs published April 30 in the Journal Geophysical Research Letters suggested, "runaway breakdown at a much lower altitude, created within 'strong fields in or just above the thundercloud,' could have triggered the TGFs instead." Steve Cummer said, "It still almost certainly has to be runaway breakdown that's creating these. The only real possibility is that it's much closer to the cloud top, and linked to something else happening inside the cloud."

The research also found that the TGFs occurred 1.24 milliseconds before their associated lightning strikes. Cummer commented, "That was something we absolutely were not expecting. But the coincidence between the lightning and the TGFs we found is too good to be random. So, even if the TGFs precede the lightning, they are in some way connected."

For more information on this interesting topic, see Gamma Rays From Thunderstorms?. Duke University also has a video describing the research.