Nanoparticles in ball lightning

For centuries, scientists have been perplexed by reported sightings of unusual and rarely observed luminous orbs that show up during electrical storms and can range up to a meter in diameter. Such “fireballs” shift and float and don’t stay still long enough to be properly imaged and probed. Scientists have been synthesizing fireballs in their laboratories for the past few decades in attempts to study their composition (see Physics Today, February 2007, page 22 ). The Abraham-son– Dinniss theory, which guides present-day experimentalists, suggests that ball lightning is simply a cloud of slowly burning silicon nanoparticles ejected from the soil after a lightning strike. Recently a French and Israeli research team gathered at the European Synchrotron Radiation Facility in France to make fireballs in a microwave cavity and probe them with 12.5-keV x rays at atmospheric pressure. The researchers created a hotspot by concentrating microwaves with a copper electrode, then touching that electrode to a borosilicate glass substrate. Upon retracting the electrode, a molten drop detached and vaporized into a buoyant fireball (see image). Small-angle x-ray scattering revealed that the particles contained in the fireball are approximately 50 nm in diameter. The researchers believe that those hot (on the order of 10³ K) nanoparticles emit electrons and form a dusty plasma. (J. B. A. Mitchell et al. , Phys. Rev. Lett. , in press.)