9 February 2006
If you have ever seen a mysterious ball of lightning chasing a cow or flying through your window during a thunderstorm, take comfort from the fact that you have witnessed a very rare phenomenon. Indeed, ball lightning -- a slow-moving ball of light that is occasionally seen at ground level during storms -- has puzzled scientists for centuries. Now, however, researchers in Israel have built a system that can create lightning balls in the lab. The work may not only help us to understand ball lightning but could even lead to practical applications that make use of these artificial balls (Phys. Rev. Lett. 96 045002).Ball lightning is thought to be a ball of plasma that is formed when a bolt of lightning hits the ground and creates a molten "hot spot". The ball can typically measure 30 centimetres across and can last for a few seconds. Although they are generally created during thunderstorms, Eli Jerby and Vladimir Dikhtyar from Tel Aviv University in Israel have now been able to make lightning balls in the lab using a "microwave drill". The device consists of the magnetron from a 600-watt domestic microwave oven and concentrates its power into a volume of just one cubic centimetre. The researchers inject the microwaves though a pointed rod into a solid substrate made from glass, silicon, germanium, alumina or other ceramics. The energy from the microwaves then produces a molten hot spot in the substrate. What the scientists then do is pull the microwave drill out of the solid, which drags the molten hot spot and creates a hot drop. The drop then becomes a floating fireball that measures about 3 centimetres across and lasts for some tens of milliseconds (see figure). "The fireball looks like a hot jellyfish, quivering and buoyant in the air," says Jerby. Although the composition of the laboratory fireballs still need to be verified, they seem to contain components of the substrate material in various phases, such as ions, neutral atoms and larger macroscopic particles. This is similar to natural lightning balls, which are thought to contain vaporized mineral grains from the soil that have been kicked into the atmosphere by a lightning strike. Moreover, the lab-produced fireballs appear to combine plasma and chemical oxidation and burning processes. Again, this is similar to naturally produced balls in which the vaporised sand grains are thought to react with oxygen in the air and burn to release light. "Our ability to generate such fireballs in a simple systematic manner may lead to techniques for synthesizing fireballs from solid materials," explains Jerby. He even hopes that the lab-generated fireballs could be used in practical applications such as coating, deposition, combustion and energy production.
About the author
Belle Dumé is science writer at PhysicsWeb