Scientists have for the first time imaged thunder, visually capturing the sound waves created by artificially triggered lightning, according to a new study.
Researchers from Southwest Research Institute (SwRI) are presenting the images at a joint meeting of American and Canadian geophysical societies in Montreal, Canada, May 3-7.
Lightning strikes the Earth more than 4 million times a day, yet the physics behind this violent process remain poorly understood," Dr. Maher A. Dayeh, a research scientist in the SwRI Space Science and Engineering Division, said in a statement. "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."
When we picture lightning, usually a flashing bolt comes to mind. And while that is often the case, it really starts out 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.
Although lightning is always accompanied by the sound of thunder, distant lightning may be seen but be too far away for the thunder to be heard.
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.
"Thunder and lightning are fascinating, wild, and unpredictable," said Dayeh. "Because of their erratic nature, the phenomena are best studied using triggered events." (Scroll to read on...)
So Dayeh and his colleagues used a technique in which they launched a small rocket trailing a grounded copper wire into thunderclouds. The copper wire provides a conductive channel and creates a predictable path for lightning. This allowed scientists to precisely focus their instruments and perform repeatable experiments close to the discharge channel.
The team conducted their experiments in Florida, supposedly the state that sees the most lightning strikes per year in the United States.
To study the acoustic signature of thunder, Dayeh designed a large, sophisticated array of microphones. Fifteen microphones in all, spaced one meter apart, were lined up 95 meters (312 feet) away from the rocket launch pad where the triggered lightning would strike. To image the vertical profile of the bolt, his team 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," Dayeh explained. "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, despite the jumbled mess it looked like at first, it turns out the images cleared up at higher frequencies. The technique revealed a distinct signature of thunder generated by the lightning strike.
In the future, scientists hope to conduct more experiments that could shed light on the probable acoustic signatures of current pulses, step leader branches, and discharge channel zigzags independently.
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