One end of the wire remains attached to the ground, while the coil in the moving rocket unwinds. This creates an electrically conductive link reaching part way to the cloud, Dwyer explained.
Usually the wire then explodes, and a positively charged lightning channel moves up toward the cloud, initiating a bolt along the wire's path.
Study leader Kasparian and his team aimed to replace copper wires with lasers—removing the need for rockets.
The laser beams free electrons from atoms in a long, thin channel of air—making it easy for an electrical charge to pass through the air between a cloud and the ground.
For controlling lightning, lasers would offer more flexibility than rockets, Kasparian said.
The easily redirected beams could send continuous pulses into clouds, diverting controlled lightning to safe targets.
The study was published Monday in the journal Optics Express.
The idea of laser-triggered lightning has been around since the 1970s. But earlier lasers could increase the conductivity of only a short channel of air, Kasparian said.
His team worked with engineers to develop a high-powered device that could boost the conductivity of a 110-yard (100-meter) channel of air and be used in the field—in this case, New Mexico's South Baldy Peak.
"It was the first laser of this class that was transportable for physics experiments," Kasparian said in a telephone interview from France.
During two thunderstorms in September 2004, the team sent ultrafast laser pulses into clouds and measured electrical activity using antennas around the experiment site.
Electrical activity in storm clouds is random, so the team analyzed statistical probabilities to make sure that the electrical events they measured were related to their laser pulses and not just coincidental.
Satisfied that they had caused electrical activity, the team now intends to build a transportable laser that is ten times stronger.
Kasparian said he hopes to use the new device to trigger a full-scale cloud-to-ground lightning bolt in the next few years.
In the event, mirrors or a strategically placed lightning rod would protect the laser apparatus from the triggered bolts, he said—an optimistic measure, because it assumes complete success.
A Step Toward What?
Florida Institute of Technology's Dwyer said research into cheaper and easier lightning-triggering methods is enormously important.
But he's not convinced this experiment has brought the scientific community closer to causing lightning with lasers.
"The results are certainly interesting. But I'm not sure how significant they are," Dwyer said.
But the team's approach was on the mark, he said.
"Rather than just talking about it and doing lab experiments, they're going out in the field and trying it.
"But it's not obvious what they measured," he added. "It's not obvious it has anything to do with initiating lightning."
Study leader Kasparian acknowledges the findings are preliminary and says future experiments will determine the real value of his team's work.
However, he said, the new findings will serve as the basis for future experiments and research.
"What is important is to have this small effect," he said. "Once you have seen a little effect, you can change parameters in the laser configuration" and try the experiment again.
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