Saturday, March 1, 2008

Anatomy of a Lightning Ball

Not many people get to see ball lightning, but those who do never forget it. Imagine a glowing orb suddenly materializing in front of you, possibly sizzling or exuding a bluish mist and an acrid smell. The globe may be larger than a beach ball and dart through the air, perhaps hovering occasionally as if considering its next move. The ball may also roll or bounce along the ground, climb utility poles, and skitter along power lines. As it travels, the fiery sphere may destroy electrical equipment, ignite fires, and even singe animals or people.

After only 10 seconds or so, the apparition typically vanishes abruptly. Some balls flick out in silence, like a lamp turning off. Others burst with sharp bangs and fiery streamers.
Despite half-a-millennium’s worth of anecdotal reports and two centuries of scientific investigations, no one yet can say for sure just what ball lightning is. Lately, however, a small group of researchers has developed theories and reported experimental results that appear to explain some features of ball lightning that previous models couldn’t account for.
Most eyewitness reports point to ordinary lightning as the trigger, but other electric discharges have also been implicated. What happens next depends on the theorist.
These researchers agree that an aerosol, a suspension of fine particles in air, is present in the balls. The particles react chemically and interact electromagnetically. Some of the theorists, however, picture a radiant network of filaments—a “fluff ball of fire,” as one scientist described it. Another contends that the aerosol is an acid mist and that it encloses a gaseous, hot core of reactive chemicals. In all the models, the aerosol’s action is critical in explaining the litany of often astonishing eyewitness accounts.
Alternative to plasmas
Documented sightings of ball lightning date back to the Middle Ages. A Russian databank includes about 10,000 reports from the past several decades. In recent years, as science failed to decipher the phenomenon, pseudoscientific explanations have abounded. Those include matter-antimatter annihilations, clumps of the exotic dark matter of the universe, and spontaneous bursts of nuclear fusion.
Ball lightning has “a big kook following and a scientific following because it’s one of the great unexplained mysteries,” says Martin A. Uman of the University of Florida in Gainesville, who has studied lightning for some 30 years.
Of the many scientific theories of ball lightning, most depict the phenomenon as some kind of plasma, or hot gas of electrons and positively charged atomic or molecular ions. That’s a reasonable expectation since ball lightning generally has been reported to occur along with thunderstorms whose ordinary lightning bolts ionize the air, creating columns of plasma along their paths.
Nonetheless, pure-plasma models for ball lightning are plagued by difficulties. “None of them works,” scowls Graham K. Hubler, a physicist and materials scientist at the Naval Research Laboratory (NRL) in Washington, D.C. He saw ball lightning 42 years ago as a 16-year-old, and he has never forgotten the experience. “You’re just so startled you can’t move,” he recalls of the moment a whitish-yellow ball about the size of a tennis ball suddenly appeared in front of him one night in a park in upstate New York.
One major challenge to the plasma explanation for ball lightning is that plasma always expands unless great pains are taken to confine it. Fusion researchers “build enormous [reactors called] tokamaks to do that sort of thing—to contain a plasma for a second” within a magnetic field for nuclear fusion experiments, Hubler notes (SN: 3/18/00, p. 191).
“Hot plasma in air has two tendencies—to disappear and to go up,” says physical chemist David J. Turner of Condensation Physics in Huntingtown, Md., a retired electric utility researcher who became interested in ball lightning while studying the behavior of ions in steam. The oppositely charged particles that make up the plasma tend to rapidly recombine, quickly annihilating it. Moreover, the buoyancy of hot plasma in air, which would make a ball rise, doesn’t jibe with ball lightning’s hovering, rolling, and flying horizontally, Turner adds.
He suggests that one way out of the conundrum is to add features of an aerosol to a plasma theory of ball lightning. An aerosol’s additional material can form a structure, host long-lasting chemical reactions, store electric charges, and otherwise account for observed ball-lightning properties, Turner and others argue.
Says Turner, “I don’t think you can explain all the properties [of ball lightning] without accepting that it’s an aerosol-related phenomenon.”
Dirty secret
The notion that aerosols may be a part of ball lightning goes back to at least the 1970s, but it’s currently winning unprecedented attention.

Some of the theories don’t include a plasma after the original lightning strike. Two years ago, chemical engineers proposed a specific and plausible mechanism by which a lightning strike on soil could produce an aerosol type of ball lightning. John Abrahamson of the University of Canterbury in Christchurch, New Zealand, and James Dinniss, who’s now at the household chemicals firm Lever Rexona in Petone, New Zealand, described their hypothesis in Nature and reported on experiments that seemed to support it.
Hubler said in a commentary accompanying the report, that the model is the first that “can explain most aspects of ball lightning.”
Besides giving a boost to aerosol interpretations of ball lightning, the findings prompted people around the world—many of them scientists or engineers—to contact Abrahamson with previously undocumented eyewitness accounts of the phenomenon. The findings also sparked further research, as well as new collaborations among the handful of aerosol ball–lightning investigators.
A variety of articles on ball lightning, including descriptions of the current aerosol theories and a collection of the new eyewitness reports, appears in the January Philosophical Transactions of the Royal Society London A. Abrahamson, who was guest editor of the one-theme issue, says it “probably doubles the number of [published] observations . . . by scientifically trained people.”
“You could say that Abrahamson and [his] theory have revived interest in ball lightning in general,” says Uman. The model invented by Abrahamson, a specialist in reducing the dust content of factory air, and Dinniss, his former student, has been dubbed by Hubler as the fluff-ball model. They revisited an old hypothesis that ball lightning might contain a fine network of metal particles. They built upon that idea by formulating a specific sequence of chemical reactions that could be triggered by a lightning strike on soil and generate such networks.
Specifically, they proposed that when a lightning bolt vaporizes silicon dioxide—a common mineral in soil—reactions with carbon compounds transforms it into nanometer-scale pure-silicon droplets. Such reactions also underlie the smelting of many rocky ores into metal.
Once formed, the silicon particles would react with oxygen and become coated with an insulating skin. In the highly charged atmosphere of an electrical storm, the oxide-coated nanoparticles would then pick up polarized electrical charges and form loose-knit networks of filaments—the “fluff balls” of Hubler’s commentary.
By calculating the heat and light that those wispy, charge-laden balls would generate, the team determined that a plausible ball roughly the size of a basketball would last 3 to 30 seconds and glow like a 100-watt bulb—conditions often reported by those who have observed ball lightning. Without a protective layer, oxidation of bare metal nanoparticles would be expected to proceed more quickly and at higher temperatures than observations of typical ball lightning have described. That oxide layer ought to keep fresh oxygen from diffusing too quickly to the underlying silicon. That slows the reactions and reduces the energy output.
Uman says the theory is promising, but he suspects that it has gaps. He and fellow lightning researchers have been within 100 meters of ground strikes hundreds of times, he notes. They should have seen ball lightning at least a few times if the New Zealanders’ theory is correct, Uman argues. What’s more, both he and Hubler say, the New Zealand team’s model for how the charged filaments assemble into ball-shaped structures is less than compelling.
Abrahamson counters that additional support for the model has recently come from other fields, particularly from microgravity experiments on granular materials. In fact, in the May Journal of Electrostatics, he and geologist John Marshall of the SETI Institute in Mountain View, Calif., will present a novel explanation—with supporting experimental evidence—of how electrical forces can build filament networks.
Different strokes
Six years before Abrahamson and Dinniss published their model, Vladimir L. Bychkov of the Russian Academy of Sciences’ Institute for High Temperatures in Moscow proposed that ball lightning consists of a loose, porous aggregate of particles. In his theory, the heat and light come mainly from electric effects, not oxidation.
Bychkov presents the latest version of his theory alongside an updated version of the New Zealand model in the Transactions issue on ball lightning. In Bychkov’s theory, lightning can transform many organic materials in the environment—not just metal residues from the soil—into airborne polymer threads. Once that happens, he surmises, the threads could tangle up into a spongy ball. As long as the materials in the tangles are electrical insulators, or dielectrics, such a ball can hold electric charges in place and permit huge buildups of energy on the ball’s surface, Bychkov argues. The energy is stored in well-separated patches—a mosaic—of positive charges and negative charges, he says.
Once a ball forms, it could yield heat and light when high voltages begin breaking down gases near the surface. That process could create the orange or blue coronas that some observers have witnessed. The enormous charge buildup also could intermittently force electric currents through some of the threads in the ball itself, making them glow like light-bulb filaments.
Turner’s theory, also updated in Transactions, holds that ball lightning contains a hot plasma as its main energy source and that the sphere maintains its shape without any network of interconnected filaments. Instead, electrically charged ions from the plasma drift outward and cool, collecting water molecules along the way. This hydration of the ions transforms them into acidic moisture droplets—aerosol particles. Ultimately an electrically charged shell of those droplets encloses the plasma, all the while absorbing ions from it and causing the internal pressure of the plasma within the shell to fall. The resulting inward pressure from the air maintains the ball’s shape.
Both Bychkov and Turner claim that their theories can account for the small but significant number of reports of so-called high-energy ball lightning. In those reports, the fiery orbs land in liquids and boil them away or sear through glass, metal, trees, or even people.
As first proposed 2 years ago, the New Zealand team’s model could not accommodate the higher-energy balls. But now, Abrahamson argues that his model can explain high-energy balls under certain unusual conditions, such as when there’s a lightning strike or a powerful electric discharge on a fuse box or other closely confined metal object. The resultant balls would be richer in metal fuel than those produced by a strike on soil and therefore would burn hotter.
Abrahamson notes a precedent for this. U.S. military scientists have devised as potential missile decoys for warplanes balls of aerogel (SN: 12/14/96, p. 383) —an extremely porous and lightweight substance—whose surfaces inside and out are coated with a thin film of iron. Normally packed in inert gas, the balls spontaneously oxidize when they hit the air, emitting missile-fooling infrared radiation. NRL’s Celia I. Merzbacher, one of the inventors of the balls, describes them in the Transactions issue.
Seeing is believing
“If we want to understand ball lightning, we need to be able to make and control it in the laboratory,” says Turner in his Transactions report.
“This is the acid test of any theory,” Abrahamson agrees.
As a starting point for tests of aerosol theories, scientists note that lightning strikes have long been known to create glassy walled, hollow tubes just under the ground’s surface. The tubes, known as fulgurites (SN: 3/20/93, p. 184), form where lightning discharges melt and vaporize soil along their paths. For at least 30 years, researchers have suspected that materials from such cavities might play a role in ball lightning.

In Soviet experiments reported in 1977, laboratory researchers used up to 12,000 volts to vaporize the inner walls of tubes of ice or plastic that served as models of fulgurites. Once enough pressure built up in the tube to rupture a thin plastic diaphragm, brilliant balls up to 400 mm in diameter came flying out. Although they were in the size range of natural ball lightning, the specimens were much too bright and lasted only a few milliseconds, the scientists reported.
Subsequent experiments by Bychkov and his colleagues also have produced fiery balls. For example, electric discharges vaporized material from the walls of wax or plastic tubes. When that plasma hit a metal, tiny balls glowing yellow or yellowish-red appeared, but none of them lasted longer than a fraction of a second.
Instead of plastic and other surrogates for soil, Abrahamson and Dinniss tested their hypothesis with actual dirt. They packed it onto shallow, electrically conductive platters and zapped it with up to 20,000 volts. As they had predicted, chains of nanometer-scale particles formed. To find them, the scientists pumped the air above the soil beds through filters just as the electrical discharge took place. Using an electron microscope, the researchers detected thread-like chains caught in the filters.
In more-recent experiments described in the Transactions issue, Abrahamson and his colleagues carried out discharges on deeper, narrower soil beds. The beds were insulated on top so that a fulgurite-like cavity could form when the soil vaporized. Although none of 24 tests produced luminous balls, two of the discharges generated short-lived, donut-shape puffs of material, like glowing smoke rings. Those shining loops may be precursors to ball lightning formation, Abrahamson conjectures.
Neither he nor Turner has had the privilege of seeing ball lightning. “I do badly want to see it,” Turner confesses.
Adds Abrahamson, “I won’t be satisfied until we’ve got all the conditions right to achieve a [soccer-ball]-sized ball lightning in the lab.” With luck, they or some other fortunate scientist may soon transform ball lightning from a rare apparition into an everyday acquaintance.

1 comment:

Jiri said...

Ball lightning had hit the path behind our summer house. Everything around - trees, branches, green grasss - caught fire for a second or two. All the el.wires in the house were burnt. There was a coal-like stone left on the path - I´ve got it -if somebody is interested in having it in collection?