Washington, July 16 : Engineering researchers at University of Florida and Florida Institute of Technology have now come closer to discovering the source of X-rays emitted by lightning, in what may one day help predict where lightning will strike.
It has long been believed that thunderstorms and lightning make X-rays, but the evidence for the same had not been achieved despite decades of research. In 2001 and 2002, researchers at New Mexico Institute of Mining and Technology, the University of Florida and Florida Tech reported solid confirmation that lightning does indeed produce large quantities of X-rays.
Martin Uman, a professor of electrical and computer engineering, said that scientists worldwide have been seeking to understand and explain the phenomenon since then.
"From a practical point of view, if we are going to ever be able to predict when and where lightning will strike, we need to first understand how lightning moves from one place to the other. At present, we do not have a good handle on this. X-rays are giving us a close-up view of what is happening inside the lightning as it moves," said Joseph Dwyer, a professor in the department of physics and space sciences at FIT.
For their study, the researchers used an array of electric field and X-ray detectors at a UF/FIT-operated lightning research facility in North Florida to hunt the source of X-rays emitted by lightning strokes and concluded: As the lightning comes down from the cloud toward the ground in 30- to 160-foot stages known as "steps" in a "step leader" process, the X-rays shoot out just below each step, mere millionths of a second after the step completes.
"Nobody understands how lightning makes X-rays. Despite reaching temperatures five times hotter than the surface of the sun, the temperature of lightning is still thousands of times too cold to account for the X-rays observed. It's obviously happening. And we have put limits on how it's happening and where it's happening," said Uman.
The New Mexico Tech researchers detected high-energy radiation from natural lightning. The UF/FIT's International Center for Lightning Research Laboratory, located on a military base in Clay County, triggers lightning using wire-trailing rockets fired into passing storm clouds. In 2002, Uman's team showed "triggered lightning" produces X-rays.
In this research, electrical engineering doctoral student Joey Howard, the paper's lead author, and other UF/FIT researchers used a series of electric field detectors and sodium iodide X-ray detectors to try to probe X-rays more closely.
In the 2002 paper, the UF/FIT researchers confirmed that X-rays are produced by the stepped leader in natural lightning. However, in the latest paper, Uman said that they narrowed the production of X-rays to the beginning of each step of the step leader, based on data gathered from one natural lightning strike and one triggered strike,.
"We could see when the electric field arrived at the sequence of stations, and it was the same with the X-rays. We then went back and calculated what the source location was for the field and the X-ray," said Uman.
According to Dwyer, this research is one more step toward using X-rays to understand how lightning travels.
"A spark that begins inside a thunderstorm somehow manages to travel many miles to the ground, where it can hurt people and damage property. Now, for the first time, we can actually detect lightning moving toward the ground using X-rays. So just as medical X-rays provide doctors with a clearer view inside patients, X-rays allow us to probe parts of the lightning that are otherwise very difficult to measure," he said.
While Uman said that the research will continue with more expensive, faster and more sensitive X-ray detectors, but there is one area of future interest, i.e. whether lightning strikes to airplanes could produce X-rays harmful to passengers.
An article detailing the UF and FIT team's findings appears in the online edition of Geophysical Research Letters, published by the American Geophysical Union.