Scientists have been baffled for centuries about the strange drifting balls of light that appear occasionally during thunderstorms. Theories put forward so far suggest that this “ball lightning” is either a moving electrical discharge or that it is some kind of self-contained object. Now, research from an Israeli group is making the latter seem more likely. The scientists have created artificial fireballs and then used the European Synchrotron Radiation Facility (ESRF) in Grenoble to analyse their composition.
Although a rare phenomenon, ball lightning has been glimpsed by several thousand people around the world, each of whom seems to recount a different set of properties. Diameters range from a centimetre to over a metre, colours are anything from red to white to blue, while motion encompasses both vertical descent and horizontal meandering. The fireballs have even been seen entering buildings via chimneys and windows. However, such sightings are always unexpected, so there is seldom an opportunity to observe ball lightning systematically.
A number of research groups are therefore trying to recreate ball lightning in the lab. Two years ago, electrical engineers Eli Jerby and Vladimir Dikhtyar of Tel Aviv University in Israel were able to make artificial fireballs by focusing microwaves onto substrates made from silicon and other solids placed inside a shoebox-sized cavity. They melted part of the silicon by delivering microwaves through a metal tip that, when pulled away, could drag some vaporized silicon with it. This created a column of fire that eventually detached to form a buoyant, quivering fireball coloured orange, red and yellow.
Now, Jerby and Brian Mitchell from the University of Rennes 1 in France — together with other researchers from the ESRF, Tel Aviv and Rennes — have tested the composition and properties of their fireballs by installing the cavity, which contains a substrate of borosilicate glass, in a beamline at the ESRF. After passing X-rays through the cavity and generating diffraction patterns, they discovered that the fireballs contain about 109 particles per cm3, each of which has an average diameter of about 50 nm (Phys. Rev. Lett. 100 065001).
They believe that this observation supports a theory put forward by John Abrahamson, a chemical engineer at the University of Canterbury in New Zealand, proposing that ball lightening occurs when ordinary lightning vapourizes carbon and silicon oxides within soil, allowing the carbon to chemically reduce the silicon into its elemental form. The silicon atoms then cool, condense and group together into nanoparticles, which oxidise in the surrounding air and give off thermal radiation.
This latest research does not solve the mystery of ball lightning, however. Whereas many witnesses have reported glowing orbs that persist for several seconds — sightings that back up Abrahamson’s theory — Jerby and Dikhtyar’s fireballs glow for just 30–40 ms once the microwave source is turned off.
The Israeli group speculate that the mechanism responsible for the longevity of ball lightning in Abrahamson’s theory is masked in their experiments. Abrahamson points out that as the silicon nanoparticles oxidize, the rate at which further oxygen molecules can reach the silicon diminishes, thereby slowing the dissipation of the nanoparticles’ chemical energy. Jerby says that this oxidation process may occur while the silicon is being illuminated with microwaves and that their chemical energy is therefore almost spent by the time the microwave source is removed.
In addition to this problem of fireball lifetime, Abrahamson’s theory has still to explain exactly how ball lightning can pass through windows, walls and other objects. Jerby, however, is optimistic that these problems can be overcome and that eventually real ball lightning will be recreated in the laboratory. He also believes that his group’s work could have important practical applications, such as producing nanoparticles directly from solid materials.
