Texas State Researcher Sheds Light on Ball Lightning

San Marcos, Texas, Nov. 20—It glows bright blue-white or orange. It seems to float in the air. And it may help to solve one of nature's most spectacular mysteries: the phenomenon of ball lightning.
When Texas State engineering professor Karl Stephan read that some researchers at Israel's Tel Aviv University had produced objects that acted in many ways like ball lightning, he decided to try the experiment himself. "I've never witnessed ball lightning in person, but the subject has fascinated me for years," Stephan says. "So when we figured out that reproducing their experiment was possible in our lab, we decided to do it." Stephan's collaborator, Prof. John A. Pearce of U. T. Austin's Department of Electrical and Computer Engineering, directs the Process Energetics Laboratory at UT's Pickle Research Campus, which has the specialized high-power microwave equipment Prof. Stephan required.

"Real ball lightning probably has nothing to do with microwaves," Stephan explains. "But microwaves are a convenient way to concentrate a lot of energy into a small volume, which is also apparently what happens with ball lightning." Unlike ordinary lightning, which is just a giant electric spark lasting a few thousandths of a second, ball lightning takes the form of a glowing ball that ranges in size from a softball to a beach ball, and lasts several seconds or more. Often sighted in connection with thunderstorms, ball lightning can come down chimneys, pass through closed windows, and disappear silently or explode suddenly. Although many researchers over the last few decades have been able to produce laboratory effects that resemble ball lightning, none of the laboratory imitations show most of the characteristics of the real thing.
The Tel Aviv University researchers Vladimir Dikhtyar and Eli Jerby were not originally trying to make ball lightning. They were investigating the use of high-power microwaves in drilling hard materials such as ceramics. When they noted that occasionally a glowing orange ball would rise from the drill and float to the ceiling of their drilling chamber, they designed a special setup just to produce the fireballs. Publicity about the paper they published in Physical Review Letters in February of 2006 attracted Prof. Stephan's attention.
"At first, I tried to duplicate their experiment exactly, down to the drill and everything," Stephan recalls. "But when I couldn't get the drill to work, I substituted some aluminum foil for the ceramic material, and then things started to happen." Eventually, Stephan found that two tungsten welding rods were enough to start a fireball inside his setup. He can produce fireballs at will by touching the rods together and drawing them apart with microwave power applied to the system. As the rods move apart, a small blue glow between them gradually enlarges to a blue-white arc, and then detaches from the rods and hovers on the aluminum chamber ceiling. The fireball, a cone-shaped object an inch or two tall, persists as long as microwave power is applied, but disappears as soon as the energy source is shut off.

Stephan's fireball is actually a type of matter known as a plasma, similar to the plasmas in neon signs and fluorescent tubes. "While plasmas are pretty well understood in some ways," Stephan says, "their behavior in open air and how energy moves around in them is still hard to predict." Experiments like Stephan's can explore the conditions needed for a ball-lightning-like object to exist in air. The fireball produced in Stephan's experiment resembles a candle flame, in that air appears to move into it from the bottom and exits near the top. It can be blown out like a candle flame as well.

The paper describing Prof. Stephan's research was published in the November 2006 issue of Physical Review E and is posted on the American Physical Society's website at . Two movies showing the fireballs are available for download by clicking on the note in the paper's Reference 11 and following the instructions.