Dusty fireball: can lab-made blob explain ball lightning?

Science News, Jan 19, 2008 by Davide Castelvecchi

By trapping and X-raying a mysterious kind of artificial fireball, researchers have demonstrated a technique that may help answer whether chemical reactions power the ball lightning occasionally seen in nature.

The fireballs first showed up in the late 1990s in the lab of Vladimir Dikhtyar and Eli Jerby, engineers at Tel Aviv University in Israel. The researchers were testing their newly invented type of drill. Made in part out of pieces from conventional microwave ovens, the drill has a tip that concentrates microwave radiation into a 2-millimeter-wide spot that can pierce many materials and liquefy its way through.

One day, as the researchers extracted the drill tip from a sample, a glowing blob unexpectedly blew out of the molten material. The blob made its way back inside the drill and into the microwave generator. "It caused a lot of damage," Jerby says.

With some tinkering, the researchers learned how to reproduce the phenomenon with consistency by drilling into glass. They also found a way to cage a fireball and sustain it for up to several minutes by zapping it with additional microwaves inside a glass-walled "oven" the size of a tissue box.

Like other, similar phenomena that scientists have learned to create in the lab, the fireballs only partly resemble natural ball lightning. Ball lightning appears after lightning strikes soil, not after microwaves strike glass. It's often the size of a basketball or larger, tends to float in midair or bounce on the floor, and can last several seconds or even minutes. By contrast, Dikhtyar and Jerby's fireballs were only centimeters wide, tended to travel upward, and, if left alone, vanished within 30 milliseconds.

Still, the researchers were interested in testing one of the more plausible among the many mechanisms scientists have proposed to explain ball lightning (SN: 02/09/02, p. 87). In 2000, chemical engineers John Abrahamson and James Dinniss of the University of Canterbury in Christchurch, New Zealand, suggested that when lightning strikes soil and creates a plasma--a glowing gas of ions and electrons--a dust of microscopic particles can also form. In the dust particles, carbon reacts with silicon dioxide, releasing silicon that then recombines with oxygen, which emits the energy that keeps the plasma glowing.

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Dikhtyar and Jerby teamed up with scientists in France to measure how their plasma scattered an intense beam of X rays. As reported in an upcoming Physical Review Letters, the researchers found particles around 50 nanometers wide. That "supports the Abrahamson and Dinniss model," Abrahamson says, and the particle size "lies in the range that we observed after simulated lightning strikes on soils."

Lightning expert Martin Uman of the University of Florida in Gainesville says the result is interesting, "but it's problematic whether it has anything to do with ball lightning."