Scientists fired rockets into raging Florida thunderstorms and learned that lightning produces not just bright flashes and a lot of noise, but also intense bursts of radiation believed to be X-rays.

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Scientists fired rockets into raging Florida thunderstorms and learned that lightning produces not just bright flashes and a lot of noise, but also intense bursts of radiation believed to be X-rays.

Many theorists had long thought such emissions from lightning impossible.

The discovery illustrates another of Nature`s many ways of generating high-energy particles, of which X-rays are one type. Black holes, fast-spinning stars and even entire galaxies are prolific producers of similar energetic radiation.

It also shows how little scientists know about lightning.

"Here`s this really spectacular fireworks display that Nature provides," said study leader Joseph Dwyer of the Florida Institute of Technology, "and we really don`t understand it."

High-tech kite-on-a-string

No scientist doubts the power of lightning (nor do those who`ve been struck and lived to tell about it).

Also impressive is lightning`s range of emissions. The visible light is obvious, of course. Yet an unseen bolt many miles away can induce instant and irritating crackles in your car that make even Bob Dylan sound good, an example of radio noise emitted by the flash. Radio and light energy are both relatively weak forms of electromagnetic radiation. X-rays and gamma rays are on the high end of the same spectrum.

To induce lightning, researchers have created a high-tech version of Ben Franklin`s kite experiment. They attach a spool of copper-Kevlar wire to a chest-high rocket and affix the other end to a specially designed launch pad.

Then they wait for one of Florida`s many summer thunderstorms to pass overhead and intensify.

When things get really gloomy, and lightning is on the verge of busting loose, the scientists shoot a rocket up about 2,000 feet (610 meters), Dwyer explained in a telephone interview. As expected, the storm takes advantage of the direct connection to the ground.

Nature takes over

A process that was already imminent now proceeds naturally along a path crafted by the scientists. A leader of charged particles reaches from the rocket to the clouds. Then a leader heading the other direction slithers down the path of least resistance from cloud to ground, passing through the rocket.

"This makes an electrical connection from the cloud down through the wire," Dwyer explained in a telephone interview. "A big current then flows through the wire and completely vaporizes the wire."

The rocket generally survives this torture and parachutes back to the ground, but Dwyer points out that the fate his team was tempting involves a risk NASA would never take, at least not after Apollo 12 was struck by lightning in 1969.

"You`re giving lightning a place to strike when you launch a rocket," Dwyer said.

Back to the experiment: Once a path is established, a powerful return stroke -- the main lightning bolt -- shoots back into the sky, Dwyer said. (Most lighting does not travel down to Earth, but instead involves the transfer of charged particles from one cloud to another or from the ground to the clouds. Learn more.)

Just prior to the main flash, a device on the surface, sitting roughly 80 feet (25 meters) from the launch pad, measured a sharply increased flow of X-rays, gamma-rays or electrons moving at nearly the speed of light, something scientists call relativistic. The results were similar in 31 of 37 separate lightning flickers.

Just like in space

There is much to learn following the unique experiment. Dwyer and his colleagues can`t say for sure how the presumed X-rays are produced.

Here`s what they think is going on: Electrons ambling freely through the air cannot move very fast, because they keep bumping into air molecules. Yet these free electrons, as they are called, behave counterintuitively. Speed them up, and they are impeded less by neighboring molecules. Once some momentum is in place, the electrical field of a lightning storm is then free to accelerate the electrons to nearly the speed of light.

"We`re assuming that`s what is happening, because nobody has a better idea," Dwyer said.

It is also not clear exactly which form of particles the energetic radiation represents. The instrument measures total energy received in a microsecond of time and cannot distinguish between 100 X-rays and a single, higher-energy gamma ray. Either way, the output can be likened to a small-scale version of processes that create X-rays and gamma rays across the universe.

"This is a fundamentally new mechanism for accelerating particles," Dwyer said. "You can think of it as one more thing Nature has in its tool box for accelerating particles, just like in space."

The results will be detailed in the Jan. 31 issue of the journal Science.

Airline safety and more

The experiment was conducted last summer at the University of Florida`s Lightning Research Laboratory, which also investigates how lightning can cripple power distribution systems. NASA also contributes to the work in the interest of learning more about where and how lightning strikes and how it could affect safety of shuttles and other rockets.

Researchers at the lab are also creating safety guidelines for airplane construction that considers what happens when an airliner is struck by lightning "and the magnetic field is wandering around the plane," said Martin Uman, a University of Florida researcher who leads the lab. The report will go to the FAA, which will determine whether to suggest or require compliance among manufacturers.

Uman is also a co-author of the new report. He said the lab launches about 70 rockets in a typical summer, triggering a lightning strike just less than half the time. Each strike, he explained, actually generates about 10 to 12 individual flickers that can be studied separately.

More to come

The search for X-rays generated by lightning dates back to the 1920s.

But lightning strikes in unpredictably specific locations. X-rays and gamma rays are absorbed by the atmosphere and don`t travel far (which is why telescopes studying these emissions must be located in space). So the search had come up mostly empty until now.

Philip Krider, an atmospheric physicist at the University of Arizona, said the work by Dwyer and Uman is the first rocket experiment to show relativistic electrons are indeed produced by lightning. A similar finding previously suggested the possibility after observations of three natural lightning strikes in New Mexico.

Krider, who was not involved in the work, said this experiment and more like it will improve understanding of lightning and the various forms of energy produced in a strike. It could also help researchers better understand mysterious "sprites," which shoot up into space above thunderstorms.

Zentek International