Flashy Clouds

“There is way more going on in thunderstorms than we ever imagined.”
—Steve Cummer of Duke University, on recent research that revealed a new kind of gamma-ray emission in many thunderclouds that Cummer and colleagues call “flickering gamma-ray flashes,” or FGFs. The research team flew a retrofitted Cold War-era U2 spy plane in figure-eight patterns over tropical thunderclouds in the Caribbean and Central America. The extremely fast plane can fly more than twice as high as commercial aircraft, thereby providing unprecedented views of clouds and the source of the gamma rays. Prior research identified two types of radiation emitted from thunderclouds: moderate-intensity gamma-ray glows, which last from one to hundreds of seconds, and more intense terrestrial gamma-ray flashes (TGFs), which persist for tens to hundreds of microseconds. The new campaign not only revealed the FGFs, a previously undiscovered phenomenon resembling multipulse TGFs but with more pulses and longer durations—each FGF pulse lasts 20–250 milliseconds—but also found that they may be the missing link between gamma-ray glows and TGFs, which until now had been puzzling atmospheric electricity scientists with its absence. And unlike TGFs, FGFs are radio and optically silent, compounding their mystery. The study found that each FGF starts as an ordinary gamma ray glow, but then “suddenly increases exponentially in intensity and turns into an unstable, ‘flickering’ mode with a sequence of pulses.” The researchers also observed previously unseen short gamma radiation bursts—one kind that lasts for less than one-thousandth of a second, and another that appears in a sequence of about 10 bursts that repeat for about one-tenth of a second. “Those two new forms of gamma radiation are what I find most interesting,” Cummer says. “They don’t seem to be associated with developing lightning flashes. They emerge spontaneously somehow. There are hints in the data that they may actually be linked to the processes that initiate lightning flashes, which are still a mystery to scientists.” The new campaign observed some sort of gamma radiation in the clouds in nine out of ten flights, which was much more frequent than the researchers expected. “As it turns out, essentially all big thunderstorms generate gamma rays all day long in many different forms,” Cummer says. Additionally, the dynamics of the gamma-glowing clouds “starkly contradict the former quasistationary picture of glows, and rather resembles that of a huge gamma-glowing boiling pot both in pattern and behavior,” explains coauthor Martino Marisaldi of the University of Bergen. The research, which was published in two recent Nature articles, suggests that more than half of all thunderstorms in the tropics are radioactive, and that the low-level production of gamma radiation seems to help restrict the amount of energy that builds up inside a storm. [Source: Duke University]

CAPTION: The ER-2 research plane that flew the survey missions above the tropical lightning storms. [Image courtesy of NASA]