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University of Florida

Contact Information:

Writer:
Aaron Hoover, (352) 392-0186, ahoover{at}ufl.edu

Sources:
Martin Uman, (352) 392-0913, uman{at}ece.ufl.edu
Joe Dwyer, (321) 674-7208, dwyer{at}pss.fit.edu

Jan. 30, 2003

FLORIDA RESEARCHERS: LIGHTNING EMITS X-RAYS

Modern-day Ben Franklins use rockets to settle 80-year-old debate

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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