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| This long-exposure photograph shows a triggered lightning event. The initial copper wire burn glows green, while nine subsequent return strokes are more purplish. Image Courtesy of University of Florida, Florida Institute of Technology, and Southwest Research Institute |
GAINESVILLE, Florida -- For the first time, scientists have successfully imaged thunder, visually
capturing the sound waves created by artificially triggered lightning at the International Center for Lightning Research and
Testing at the University of Florida in Gainesville, Florida, taking advantage of
the Sunshine State’s claim to the most lightning strikes per year in the U.S.
Although people see it as a flashing bolt, lightning begins as a
complex process of electrostatic charges churning around in storm
clouds. These charges initiate step leaders, branching veins of
electricity propagating down, which subsequently lead to a main
discharge channel. That channel opens a path to nearly instantaneous
return strokes, which form the lightning flash as we see it. By studying
the acoustic power radiated from different portions of the lightning
channel, researchers can learn more about the origins of thunder as well
as the energetic processes associated with lightning.
“Lightning strikes the Earth more than 4 million times a day, yet the
physics behind this violent process remain poorly understood,” said Dr.
Maher A. Dayeh, a research scientist in the Southwest Research Institute (SwRI) Space Science and
Engineering Division who collaborated with groups from the University of New Hampshire, Florida
Institute of Technology, and the University of Florida for this research. “While we understand the general mechanics of
thunder generation, it’s not particularly clear which physical processes
of the lightning discharge contribute to the thunder we hear. A
listener perceives thunder largely based upon the distance from
lightning. From nearby, thunder has a sharp, cracking sound. From
farther away, it has a longer-lasting, rumbling nature.”
The technique involves launching a
small rocket trailing a grounded copper wire into thunderclouds. The
copper wire provides a conductive channel and creates a predictable path
for lightning, allowing scientists to precisely focus their instruments
and perform repeatable experiments close to the discharge channel.
Using SwRI internal research funding, Dayeh led a proof-of-concept
experiment to image the acoustic signature of thunder.
Dayeh designed a large, sophisticated array of microphones to study
the acoustic signature of thunder. Fifteen microphones, spaced one meter
apart, were lined up 95 meters away from the rocket launch pad where
the triggered lightning would strike. To image the vertical profile of
the bolt, he used post-signal processing techniques and directional
amplification of the data signals captured by the microphone array.
“At first I thought the experiment didn’t work,” said Dayeh. “The
initial constructed images looked like a colorful piece of modern art
that you could hang over your fireplace. But you couldn’t see the
detailed sound signature of lightning in the acoustic data.”
However, when Dayeh looked at the different sound frequency bands, he
saw that the images cleared up at higher frequencies. The technique
revealed a distinct signature of thunder generated by the lightning
strike. Future experiments could allow scientists to study the probable
acoustic signatures of current pulses, step leader branches, and
discharge channel zigzags independently.