Flash! Lightning emits X-rays

Anyone who has heard a radio crackle during a storm knows lightning emits radio signals. But in a series of unique experiments that involved firing wire-trailing rockets into storm clouds, a team of Florida researchers has found that “triggered” lightning also emits waves of energy much higher up the frequency scale – X-rays, or possibly gamma rays or relativistic electrons.From the University of Florida:FLORIDA RESEARCHERS: LIGHTNING EMITS X-RAYS
Modern-day Ben Franklins use rockets to settle 80-year-old debate

Jan. 30, 2003
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GAINESVILLE, Fla. — Anyone who has heard a radio crackle during a storm knows lightning emits radio signals.

But in a series of unique experiments that involved firing wire-trailing rockets into storm clouds, a team of Florida researchers has found that “triggered” lightning also emits waves of energy much higher up the frequency scale – X-rays, or possibly gamma rays or relativistic electrons.

Set to be reported Friday in the journal Science, the finding comes on the heels of a similar discovery for natural lightning reported last year, suggesting that all lightning emits such so-called energetic radiation. Not only might the discovery finally settle a question that has been debated for 80 years, it also is among the rare instances where such high-energy, high-frequency radiation has been reported in atmospheric conditions. Vacuum tubes in doctor’s office machines can produce X-rays on Earth, but otherwise X-rays and gamma rays generally occur only in outer space, where they are propagated by such extraordinary events as supernova explosions.

“I think it’s really exciting,” said Martin Uman, a lightning expert and director of the University of Florida’s International Center for Lightning Research and Testing, where the experiments were done. “We didn’t expect to see anything at all, and then, all of a sudden, with almost every lightning stroke, we had X-rays.”

Debate over whether lightning emits energetic radiation dates back to the 1920s, when Nobel Prize physicist Charles Thomson Rees Wilson first predicted the phenomenon. Numerous researchers have attempted to confirm or refute his prediction, but results have proved inconclusive. That’s because natural lightning is devilishly hard to study, said Joe Dwyer, the lead investigator on the project and an assistant professor of physics and space science at the Florida Institute of Technology.

While there is no practical application for the discovery, it enhances the basic understanding of lightning, which aids in development of lightning protection systems, he said. X-rays, gamma rays and relativistic electrons travel only a few hundred yards at most through the air at sea level, Dwyer said.

Because no one knows where lightning will strike, obtaining accurate measurements can be a matter of extraordinary luck rather than repeatable experiment. Researchers also have been hamstrung by the difficulty of distinguishing interference from true measurements. As a result, Dwyer said, “there have been a whole long series of results, with roughly half positive and half negative.”

Dywer set out to obtain a more conclusive result. Rather than chase natural lightning, he turned to the UF engineering college’s lightning research and testing center in rural North Florida. Researchers at the facility, located at Camp Blanding near Starke, spark lightning by launching slender rockets from batteries of steel tubs 2,400 feet toward passing storm clouds. Each of the rockets trails a thin, Kevlar-coated wire designed to conduct the lightning back to the targeted strike point on the ground.

Ground zero for the triggered lightning is the rocket launch tower. Dwyer installed a carefully constructed detection system about 75 feet from this tower. The system, contained in a heavy aluminum box with thick sides that blocked out all signals except energetic radiation, consisted in part of two photo multiplier tube detectors, standard equipment for measuring the radiation. Two detectors were used so that one could act as a control.

Besides triggering the lightning, the UF group provided critical measurements of its current and field strength. Manning the system from July through September, the researchers triggered multiple lightning flashes. Each flash typically contained several return strokes, or individual lightning events that occur too quickly for the human eye to distinguish. The detector recorded energetic radiation in 87 percent of 37 such strokes, showing it occurred at the beginning or just before each stroke – the moment when the charge moved down from the cloud and contacted the ground just before the stroke.

“It’s right before the visible stroke occurs – that appears to be when the energetic radiation is being produced,” Dwyer said. “Nobody really understands completely how this is happening.”

This phase of the lightning process is known as the “dart leader” and also is present in natural lightning extending from clouds to the ground, which suggests the findings likely apply to all cloud-to-ground lightning, Uman said. It’s also important the observations occurred near sea level, because the lower the altitude, the harder it is for energetic radiation to generate and propagate, he said.

Earth-orbiting satellites have recorded energetic radiation apparently associated with thunderstorms. But few expected to see it produced at near ground level, Dwyer said. “People didn’t think the electric fields were strong enough or that the length scales were long enough,” he said.

Dwyer’s project is part of a five-year, $410,000 Young Investigator Award from the National Science Foundation, research performed in connection with related NSF-sponsored research at UF. He plans to return to the lightning research and testing center next summer to continue the investigation. At the top of his priority list: narrowing down whether the energetic radiation produced by lightning consists specifically of X-rays, gamma rays, energetic electrons or some combination of the three. UF researchers are preparing an improved set of supporting instruments for the experiment.

“We can go out there every summer,” Dwyer said. “So it’s finally become an experimental science where we can do experiments and test theories, and it never really was that before.”

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