Most tropical lightning storms are radioactive

While it didn't take long for researchers to figure out that these radioactive supercharged particles were coming from thunderstorms, how commonly the phenomenon happened remained a mystery. Satellites weren't built to find gamma radiation coming from Earth, and they had to be in just the right place at just the right time to do so.

After years of making do with platforms not ideal for the task, a group of scientists secured an opportunity to fly a retrofitted U2 spy plane owned by NASA over storms to take a proper look. In two new papers published October 3 in Nature, the team discovered that gamma radiation produced in thunderstorms is far more common than anyone thought and that the dynamics creating the radiation hold a treasure trove of mysteries yet to be solved.

"There is way more going on in thunderstorms than we ever imagined," said Steve Cummer, the William H. Younger Distinguished Professor of Engineering at Duke University, who was a coauthor on both papers. "As it turns out, essentially all big thunderstorms generate gamma rays all day long in many different forms."

The general physics behind how thunderstorms create high-energy flashes of gamma radiation is not a mystery. As thunderstorms develop, swirling drafts drive water droplets, hail and ice into a mixture that creates an electric charge much like rubbing a balloon on your shirt. Positively charged particles end up at the top of the storm while negatively charged particles drop to the bottom, creating an enormous electric field that can be as strong as 100 million AA batteries stacked end-to-end.

When other charged particles -- such as electrons -- find themselves in such a strong field, they accelerate. If they accelerate to high enough speeds and happen to strike an air molecule, they knock off more high-energy electrons. The process cascades until the collisions have enough energy to create nuclear reactions, producing extremely strong and extremely fast flashes of gamma rays, antimatter and other forms of radiation.

But that's not the end of the thunderstorm gamma radiation story. Aircraft flying close to thunderstorms have also seen a faint glow of gamma radiation coming from clouds. These storms seem to have enough energy to produce a low-level simmering of gamma radiation, but something prevents it from creating an explosive burst like a popping corn kernel.

"A few aircraft campaigns tried to figure out if these phenomena were common or not, but there were mixed results, and several campaigns over the United States didn't find any gamma radiation at all," Cummer said. "This project was designed to address these questions once and for all."

The research group secured the use of a NASA ER-2 High-Altitude Airborne Science Aircraft. A retrofitted U2 spy plane left over from the Cold War, it flies over twice as high as commercial aircraft and about three miles above most thunderstorms. It's also extremely fast, giving the team the opportunity to pick the exact thunderstorms they thought were most likely to produce results.

"The ER-2 aircraft would be the ultimate observing platform for gamma-rays from thunderclouds," said Nikolai Østgaard, professor of space physics at the University of Bergen in Norway and lead investigator of the project. "Flying at 20 km [12.4 miles], we can fly directly over the cloud top, as close as possible to the gamma-ray source."

Because the ER-2 was the perfect solution and the team was going to fly over the right storms, the researchers figured that if these phenomena were rare, then they'd barely see any at all. But if they were common, then they'd see a lot.

And they saw a lot.

Over the course of a month, the ER-2 flew 10 flights over large storms in the tropics south of Florida, and 9 of them yielded observations of this simmer of gamma radiation, which was also more dynamic than expected.

"The dynamics of gamma-glowing thunderclouds starkly contradicts the former quasi-stationary picture of glows, and rather resembles that of a huge gamma-glowing boiling pot both in pattern and behavior," said Martino Marisaldi, professor of physics and technology at the University of Bergen.

Given the size of a typical thunderstorm in the tropics, which get much larger than storms at other latitudes, this suggests that more than half of all thunderstorms in the tropics are radioactive. The researchers postulate that this low-level production of gamma radiation acts like steam boiling off a pot of water and limits how much energy can be built up inside.

The researchers were equally excited to see numerous examples of short duration and intense gamma radiation bursts coming from the same thunderstorms. Some of these were precisely like those that were originally detected by the NASA satellites. These almost always occurred in conjunction with an active lightning discharge. This suggests that the large electric field created by lightning is likely supercharging the already high-energy electrons, enabling them to create high-energy nuclear reactions.

But there were also at least two other types of short gamma radiation bursts that had never been seen before. One type is incredibly short, less than a thousandth of a second, while the other is a sequence of about 10 individual bursts that repeat over the course of about a tenth of a second.

"Those two new forms of gamma radiation are what I find most interesting," Cummer said. "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."

If there is anybody out there worried about getting turned into the Hulk by all of this gamma radiation, Cummer added, they shouldn't be. The amount of radiation being produced would only be dangerous if a person or object were quite close to the origination source.

"The radiation would be the least of your problems if you found yourself there. Airplanes avoid flying in active thunderstorm cores due to the extreme turbulence and winds," Cummer said. "Even knowing what we now know, I don't worry about flying any more than I used to."

This research was supported by the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. 320839 and the Research Council of Norway under contracts 223252/F50 (CoE) and contract 325582.