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Team captures rare sightings of ‘jet’ lightning

This summer, an unusual phenomenon was recorded by Duke researchers not once, but twice: upside-down lightning bolts.

Along with a team of researchers, Steven Cummer, associate professor of computer science and assistant professor of electrical and computer engineering, captured rare bursts of “gigantic jets” on camera.

This uncommon meteorological event of upward lightning has sparked great interest among Cummer and his fellow Duke researchers and a member of FMA Research who also participated. FMA Research is composed of a group of scientists who monitor and analyze atmospheric patterns and phenomena. The National Science Foundation supported the lightning research.

“We were really surprised to see that we had captured a gigantic jet with our system,” Cummer said, adding that the group’s radio recordings are critical to fully understanding the mechanics of the phenomenon. Cummer noted that the captured images and radio should show that the “gigantic jets” are power-charged lightning that transfers electric charge from thunderclouds to the upper atmosphere—approximately 50 miles above ground.

Researchers were trying to understand the effects of “sprites” when they captured a one-second video recording of the gigantic jet along with its radio measurements. Sprites are a common type of electrical discharge in the upper atmosphere that is created as a response to a very strong lightning stroke between the clouds and the ground.

“I wouldn’t say that the discovery was captured accidentally since the camera was set up to routinely make observations, and a lot of experience is needed to direct the camera in the right direction,” said Gaopeng Lu, a postdoctoral associate in electrical engineering and a member of the research team.

Gigantic jets look much like regular lightning, but when they are 20 miles from the ground, they spread out in the shape of a fan. When the jets reach 50 miles above the ground, they are approximately 20 miles wide, Cummer said.

“Gigantic jets are a rare class of lightning phenomena that might share the same initiation process with regular lightning,” Lu said, adding that gigantic jets have a “marvelous appearance” because they extend to greater altitudes than regular lightning.

There are fewer than 20 documented reports of these uncommon gigantic jets in the last seven years. Because they most commonly occur during thunderstorms, it is difficult to know exactly how many have occurred. Most have been seen in tropical locations like the Caribbean or Taiwan, which was why it was surprising to document this phenomenon twice in Durham.

The term “gigantic jet” was coined in 2001 by researchers at National Cheng Kung University in Taiwan. Prior reports by pilots had been dismissed as imaginary and were later categorized as “sprites” or a smaller version of gigantic jets called “blue jets,” whose name was derived from their appearance in videos. The “gigantic jets” term was used to describe a much bigger version of blue jets that extended over a much higher range of altitude.

“This phenomenon is not completely understood because it is very rare,” said Jingbo Li, an electrical and computer engineering graduate student and a member of the research team. “Nobody has been able to analyze these images before because we were the first to capture radio measurements as opposed to just video or a picture.”

Cummer and his team are now using an intensified high-speed camera that can capture thousands of frames per second and a low-light color camera in addition to previously used technology. They hope that together, the equipment can show how the gigantic jets develop and what kind of light-producing processes happen inside it.

“We now know that powerful lightning can go up as well as down and we look forward to understanding what makes that happen,” Cummer said.

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