Ball Lightning

Introduction to Ball Lightning

Ball lightning is a phenomenon that has been reported for many hundreds of years all over the world, yet even to this day there is no definite scientific explanation for it. The failure to even prove its existence means it is more at home in books such as 'Mysteries of the Unexplained' and 'UFOs, Crop Circles and other Strange Phenomena' rather than a school textbook. This is not to say that there has been no serious scientific discussion; indeed, there are many theories and hypotheses put forward to explain ball lightning, yet none are considered to be conclusive and there is no generally accepted physical theory.

Ball lightning is most often observed during or around the time of thunderstorms, though this is not always the case. It manifests as a small fiery ball, typically of a size between a tennis ball and a football. Many eyewitnesses report the ball moving slowly around in various manners; some travel along the ground at a steady walking pace, up walls and poles, some hover above the ground, and many have been described as entering a building through windows. Sometimes the glass is undamaged; in other cases the glass becomes melted or perforated. After a short time the ball dissipates, either just fading out like a switched off street lamp, or releasing its energy in an explosion.

However, these properties vary widely in many accounts. The lifetime can range from just a few seconds to several minutes. Some are reported to exist when there is no thunderstorm present. The size also varies from that of a golf ball to 100 metres in diameter.[REF]

Properties of Ball Lightning

In the past few decades, around 10,000 reported sighting of ball lightning have been collated in a Russian data bank, detailing accounts of eye witnesses mostly from Russia and Asia.[REF] Taking this number and the fact that it does not contain accounts from places such as USA and elsewhere, it would seem that the phenomenon is actually fairly frequent. Certainly some of the accounts will be falsified or mistaken, but the sheer number of sightings lends credence to the reality of the phenomenon. Further to this, when reporting an observation of ball lightning, a non-scientist may well be able to describe what was seen visually but leave out important details that might be required for a satisfactory model. For these reasons, many of the theories proposed are based on a selected number of reports that reinforce their ideas. Conflicting reports that indicate a property not predicted by a particular model are often simply ignored. It is also possible that a particular property that does not fit a specific model has been mistakenly attributed to ball lightning when in fact it is an entirely unrelated phenomenon.

Taking these reports into consideration, there are 3 major properties of ball lightning that must be incorporated into any adequate theory:

• The lifetime of the ball, i.e. an explanation as to how the ball exists for a relatively large, finite amount of time, as opposed to a single 'flash' of lightning.
• An explanation for the apparent motion of the ball, as to why it travels at a slow speed but in varied directions, and at varying heights.
• Related to both the above, the energy contained in the ball to provide the motion and lifetime, and an explanation for the reported explosions at the end of the ball's life. Theories can be placed into one of two categories based on their energy considerations: those that suggest the ball forms with a finite amount of energy, and when this is used up the ball disappears, and those that suggest that the ball's energy is supplied externally.

Theories

An early theory popular with those sceptical of ball lightning was the belief that it was simply an optical after-effect of a normal lightning strike. Similar to when a camera flashes, an image is left on the eye in the form of a 'glowing' ball. This could certainly be used to explain some of the properties reported, such as the motion and colour of the ball in many, but not all, cases. However, a great deal of observations are made by a group of people who report the same path taken, and a lifetime greater than one would associate with such an optical illusion. It is unlikely that this is a true explanation for the phenomena but it is certainly possible that some people can mistake it for ball lightning.

For those that believe that ball lightning is more than just an optical illusion, the most readily acceptable explanation is that it is some sort of plasma, a state similar to a gas but with ionised electrons and positive atomic or molecular ions. Ordinary lightning is known to ionise the air, creating columns of plasma, so this is not an unreasonable stretch for ball lightning. The plasmoids created by a single lightning stroke impacting on a metallic surface do not last long enough as the ions quickly lose energy and recombine. A major problem with pure-plasma based theories is that plasma is very hard to contain; like a gas, its tendency is to expand. Attempts to create a confined plasma state artificially only succeed for a very small lifetime, and the infrastructure and technology required to do so are very large. A further property of plasma is its buoyancy which would cause it to rise in air - this could explain why ball lightning has been observed to rise but does not account for its motion in the majority of cases, i.e. in a horizontal manner along, or near to, the ground.

Despite these problems, plasma based theories do begin to describe some of the observed properties of ball lightning. One proposed refinement of this class of theory is to add features of an aerosol to it. An aerosol is a suspension of fine particles in air that can interact electromagnetically and also react chemically. The additional material in an aerosol provides a more stable structure for the ball, allowing it to sustain long term chemical reactions and store charges.

Nanoparticle Networks

In 2000, J. Abrahamson [REF] published a paper detailing his theory of ball lightning production. When a lightning strikes soil, it causes the silicon dioxide component of the soil to be reduced (the oxygen has been removed from the compound) by the carbon component of the soil (forming carbon dioxide). This leaves a silicon vapour. When the vapour is cooled, in the aftermath of the lightning strike, it condenses, forming a structure of reduced nanoparticles. These nanoparticles can then oxidise, forming an insulating skin, and it is this chemical process that provides the energy source for the ball lightning, and explains its luminescence. The oxide coated particles pick up charged particles present in the atmosphere of an electrical storm and form a network of filaments.

Using this model of an oxidising filament network, Abrahamson calculated that for a ball of diameter 300mm, roughly the size of a basketball, the observed lifetime should be in the range of 3 to 30s, for a formation temperature of 1100-1300K. This fits with many reported sightings and seems reasonable. The model also predicts a rise in temperature at the end of the ball's lifetime, which causes the network to melt and so the ball will disintegrate - as is observed. The colour radiated at these temperatures is consistent with observations. If the ball forms at a temperature above 1100K it should be visible immediately, and if it forms below this temperature it will have a longer lifetime and be visible later on as its temperature increases. This also accounts for sightings of ball lightning away from actually lightning strokes. The model also accounts for higher energy ball lightning networks which can arise from a different carbon source, such as a rubber seal or a plastic window. The metal vapour is released with a higher concentration of metal than air.

However, this model is not without faults. M. A. Uman of the University of Florida [REF], who has been studying lightning effects for more than thirty years, noted that he and his fellow researchers have been close to lightning strokes many hundreds of times. If Abrahamson's theory is correct, then it would be expected that they would have seen ball lightning on more than a few occasions.

Polymer Composites

Figure 1 - Polymer composite theory of ball lightning consisting of separated charged regions.

A similar theory was proposed by V. L. Bychkov [REF] six years earlier than Abrahamson's, where ball lightning is composed of polymer-composite material. Bychkov noted that ball lightning is most often observed during the summer when thunderstorms are more frequent. This coincides with an increase in green vegetation. When lightning strikes the ground, it creates polymer threads in the air from organic materials in the soil such as lignin and cellulose, not just metallic residues as in Abrahamson's model. The threads become entangled and form a spherical structure. If the threads are insulating material, they can hold electric charges in place which causes a build up of energy on the surface of the ball. The charge is held in well separated regions in a mosaic like pattern. The distance between each region is much greater than the diameter of each region. This framework is charged by ions that are produced by lightning discharges in air.

The mass of the polymer composite is given by: [REF]

where m₀ and r₀ are the mass and size of the monomer respectively, RSP is the radius of the structure, and D is the fractal dimension, typically 2.5 ± 0.3 for polymers. This predicts a mass of about 0.15g for a 100mm sphere of cellulose, and around 3.2g for a silicon dioxide framework. Due to the small mass involved, the ball will levitate due to the Coulomb forces of the Earth's electric field. The net charge, q, of the surface of the structure is given by:

where M_- and M₊ are the reduced masses of negative and positive air ions, e is the electronic charge, T is the gas temperature and N1 is the number of charged links in the polymer structure. According to this equation, ball lightning has a net negative charge at usual atmospheric conditions. Choosing typically observed values of 100mm diameter balls levitating at about 0.4m above the ground, q is calculated to be ~ - 8 x 10 ^-7 C and the electric field E ~ 2.1 kV m^-1. Such conditions are met during a thunderstorm.

Bychkov explains that with this model, ball lightning can be formed in or around electronic devices such a telephone sockets and television sets as they contain polymer and ceramic-dielectric insulating components. Ball lightning has indeed been observed in some cases to appear from inside a telephone socket. Localised high electric fields destroy the polymeric material which then evaporates, forming polymer threads. These are then charged by a locally generated plasma, and more and more material is aggregated into the ball lightning. Discharges appear on the surface of the ball causing it glow, at which point it becomes visible. The ball will then exit the object through a hole.

Bychkov compared the lifetimes predicted by his model with those observed for the most common size, under 800mm diameter, and found a reasonable agreement, and also that the lifetime depended on the diameter, as was also observed. For small diameters, there was a discrepancy between the theoretical lifetime and the statistical lifetime which was put down to the formation of less powerful discharges over the surface than was considered in the theory.

The polymer-composite theory succeeds in explaining many of the properties exhibited by ball lightning, including its motion, energy and lifetime. Specifically, Bychkov set out to explain the high energy density of some observed ball lightning and the dependence of lifetime on diameter and as one might expect his theory fits in with statistical data on these points. This theory also explains the origin of ball lightning away from a particular lightning stroke, as it can form inside electronic devices if the right conditions are met.

Abrahamson's original theory could not account for high energy ball lightning but his revised theory allows for the possibility. In certain extreme conditions, such as when lightning strikes or a large electric discharge takes place on a confined metal surface, there will be a higher concentration of metal fuel than there would be for an ordinary strike on soil, and so the resultant ball lightning would have a higher temperature and higher energy density.

The Electrochemical Theory

A very different theory to the above two was proposed by D. J. Turner in which the main energy source in ball lightning is a very hot plasma, and no network of filaments is required in order to maintain shape. The charged ions in the plasma drift outwards and cool - as one would expect - but along the way they pick up water molecules from the air. The hydrated ions are acidic and become a fine suspension of moisture droplets, i.e. an aerosol. The plasma becomes surrounded by a shell of these charged droplets, causing the internal pressure to fall by absorbing ions from inside. The resultant pressure from the surrounding air maintains the shape of the ball lightning.

As Turner himself notes in his comments, [REF] this model assumes that there is a steady inflow of the surrounding air in order to stabilise the water droplets due to drag forces. There is no clear indication of how such an inflow could occur, perhaps due to nitric acid production. Alternatively, there could be another factor governing to stability of the shells, such as an electrostatic mechanism. Until this is fully developed, the electrochemical theory cannot be considered to fully explain ball lightning.

One area in which all the presented theories fail in is in lab reproduction of ball lightning. In order for a theory to truly be described as a success, it must be tested in a laboratory to create a stable ball lightning. Various attempts have been made to do this, but so far the only successes have been in creating fiery balls that last for no more than a few seconds; certainly not the longer lifetimes predicted by any of the models, and many are much smaller than typically observed ball lightning. However, in a Soviet experiment reported in 1977, 12,000 Volts were used to vaporise the inner walls of plastic tubes. These tubes were used to simulate fulgurites, hollow tubes formed under the surface when lightning strikes and melts the soil along their path. The pressure built up inside the tube until it was great enough to break through a plastic barrier, and glowing balls of diameter 400mm came flying out. Unlike observed ball lighting, they had an extremely high luminescence and therefore an extremely short lifetime of just a few milliseconds. Abrahamson tested his theory on actual soil, and noted that polymer chains did indeed form as his model predicted. Unfortunately the chains never formed in such a way as to be classified as ball lightning.

Bringing it all together

Currently there is no decisive theory of exactly what ball lightning is. Many of the models presented go some way to explaining observed behaviour, but none can explain every reported property. It has been said [REF] that a problem with many of the scientific theories is that each one combines at best a few different fields of science. A plasma physicist is more likely to prefer a plasma-based theory of ball lightning whereas a physical chemist may prefer the electrochemical theory or a polymer based theory. Instead, some scientists have noted that as many as 10 different fields may be required to fully explain ball lightning. One thing is for certain, given the major differences between the theories presented here, not all can be the true explanation for ball lightning. It would seem currently that the polymer composite theory is the best developed, accounting for the majority of observed properties. However, its failure to reproduce the phenomenon in the lab indicates it still has some way to go. Perhaps some of the ideas of the electrochemical model can be incorporated, along with other theories, and we can truly get an understanding of this interesting phenomenon.

Hopefully some day soon, ball lightning will no longer be a 'Mystery of the Unexplained'.

References

1. See picture and caption, Anatomy of a Lightning Ball, http://www.sciencenews.org/20020209/bob8.asp

2. Abrahamson, J., A.V. Bychkov, and V.K. Bychkov. 2002. Recently reported sightings of ball lightning: Observations collected by correspondence and Russian and Ukrainian sightings. Philosophical Transactions of the Royal Society A 360, pp 11-35.

3. Abrahamson, J., and J. Dinniss. 2000. Ball lightning caused by oxidation of nanoparticle networks from normal lightning strikes on soil. Nature 403, p 519.

4. See Anatomy of a Lightning Ball, http://www.sciencenews.org/20020209/bob8.asp

5. Bychkov, V.L. 2002. Polymer-composite ball lightning. Philosophical Transactions of the Royal Society A 360 p 37.

6. Chabra, A., Hermann, H. J. and Landau, D. P., 1986. Fractal dimensions of frameworks and clusters in kinetic model of gel forming. Fractals in Physics (Proc. 6th Trieste Symp. on Fractals in Physics) Elsevier, pp 179-183.

7. Turner, D.J. 2002. The fragmented science of ball lightning (with comment). Philosophical Transactions of the Royal Society A 360, pp 107-152.

8. See Ball Lightning Scientists remain in the dark, http://www.newscientist.com/news/print.jsp?id=ns99991720

Bibliography

Ball Lightning Theme Issue
Philosophical Transactions of the Royal Society A 360

Anatomy of a Lightning Ball: Science News Online, Feb 9, 2002
http://www.sciencenews.org/20020209/bob8.asp

Scott's Ball Lightning Hypothesis
http://www.ball-lightning.info

Scientific American: Ask the Experts
http://www.sciam.com/askexpert_question.cfm?articleID=000CC3F9-66E4-1C71-9EB7809EC588F2D7&catID=3