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.”
