Great balls of fire




Дата канвертавання26.04.2016
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“Great balls of fire”- the story (so far) of ‘Ball Lightning’ by David Johnson
Many years ago, before the spread of Christianity, many people throughout northern Europe believed in the stories of Norse mythology to explain the events occurring around them. One of the main Gods of these ancient tales was Thor, the God of fertility, who was said to fly through the sky in a chariot above the clouds. Thor was worshipped as the protector of all that was good, and was said to be in a constant battle with Thrym, the king of the evil giants. Thor’s main weapon against the forces of evil was his almighty hammer, the famous ‘hammer of the gods’, which he used to slay his enemies. It was believed that when Thor was in combat he swung his hammer with such force that it made lightning and thunder, and thus was a sign of Thor protecting the people of ancient times.
Through the advance of science, we now know that thunder and lightning are created by electromagnetic activity in our atmosphere, rather than the clashes of gods at war.
When moist air near the ground is heated, its density decreases and it rises by convection. As these ‘air-parcels’ rise, they expand and cool, causing moisture to condense into water droplets. If the process happens many times, cumulus clouds are produced, and these can combine to form cumulonimbus- storm clouds. Inside these clouds convection effects are particularly pronounced, and these effects can cause a separation of charge, probably due to collisions. Heavier particles become negatively charged, whilst lighter particles become positively charged. As the heavier particles fall to the bottom of the cloud, and the lighter particles rise to the top, a separation of charge occurs, and an electric field is established between both the top and bottom of the cloud, and the bottom of the cloud with the ground (due to the attraction of positive ions in the Earth to the negative charges in the bottom of the cloud). Ionisation of the air between these regions may occur, creating a path for an electrical discharge- in other words, a huge spark we know as lightning.
History is littered with similar examples of mystical or paranormal events, only to be explained later by rigorous scientific enquiry. Indeed, today such occurrences are common as ever, with hundreds of reported sightings of UFOs or ghosts later debunked as meteorites, weather balloons, or local atmospheric events such as will-‘o-the-wisp.
However one phenomenon which has so far evaded our complete understanding is ball lightning.
Ball lightning tends to occur (though not always) around the same time as an electrical storm. It is usually described as a luminous orb of diameter between 20 and 50cm[1], and unlike lightning strikes which last for a fraction of a second, can vary in duration from a few seconds up to a number of minutes in some reports. It has been observed ‘hovering’, remaining in a static position for the duration of its lifetime, but more frequently appears to float along the ground, often bouncing off objects. Some reports describe ball lightning burning holes through windows and wreaking damage to property, whilst others describe it seemingly ‘passing through’ solid materials. The phenomena is often associated around the moment a lightning strike hits the ground, although in some cases it has been reported to form in enclosed spaces
There are numerous historical references and reports of ball lightning dating back over hundreds of years, however it was only during the scientific era when this strange event was given a closer and more serious scrutiny.
One of the earliest and most famous scientific recordings of ball lightning was made by the colleagues of Professor Georg Richman a German scientist living in Saint Petersburg, Russia. The year was 1753, and like many other scientists at the time, Richman was fascinated by the experiments of Benjamin Franklin, only undertaken a year earlier. Franklin had proven that storm clouds were electrified by flying a kite with a metal key into a storm. The key and kite line provided a conducting path between the cloud and earth, and sparks were observed to travel down the line. Prof. Richman was attempting to recreate this experiment using his own lightning rod. However, as a storm neared and Richman approached the


FIG 1: impression of the electrocution

of Proffessor Richman by ‘ball lightning’[i 1].

conducting rod a pale blue ‘ball of fire’ was formed on the conducting rod. This ball was described to be ‘as big as a fist’, and it collided with Richman in the forehead, killing him silently and instantly (FIG 1). Later, when Richman’s body was examined, a small protrusion was seen on the forehead, and his left shoe had been burst open, indicating the points of entry and exit of the ball lightning[2].


The first scientific survey of ball lightning was conducted by Dominique Francois Jean Arago, the French physicist and astronomer, in 1838. Arago compared twenty reports of the phenomena, and remarked the improbability of ball lightning being mistaken for linear lightning. He justified this by claiming that it was highly unlikely for an observer to witness linear lightning directed straight at them, and thus appearing cylindrical, as this would surely result in the observer being struck[3].

Another account which is scientifically noteworthy was reported by Michael Fitzgerald, an Irish land surveyor to the Royal Society. The event occurred in a peat bog in Donegal, Ireland, in 1868. Fitzgerald stated that he had witnessed a bright red glowing orb in daylight, of 50cm diameter, bobbing silently over the valley, and disappearing by a stream twenty minutes later. When Fitzgerald examined the path left by the object, he found a square hole 6 metres wide where the object had first touched the ground, followed by a trench of upturned peat 100 metres long. Also, where the object had disappeared, 25 metres of the bank of the stream had been torn away. Although this claim sounds outlandish, scientists have revisited the site, and found the square hole and trench created by the object, exactly as Fitzgerald had described. Carbon dating techniques were used, and it was shown that the peat had been disturbed to a consistent manner of that described, verifying the account[4].


A more recent, and terrifying account was given by Tom Dewis, a flight engineer on a VC-10 aircraft in 1982. The flight was en route to Nairobi from Rome, and after travelling through some cumulonimbus clouds, a dim yellow light appeared in front of the aircraft’s nose. This light expanded in circumference until it filled the whole windscreen. A bright flash was suddenly experienced, followed by the smell of ozone. Two attendants in the galley witnessed a ball of fire, the size of a football, leaving the flight deck and rolling down the aisle to the rear of the aircraft when it disappeared. Upon later inspection, there was no damage to the windscreen, and no sign of the object at the rear of the craft[5].

Such descriptions are analogous to the growing body of reported ball lightning events.



Unfortunately due to the unpredictability of these events, ball lightning has rarely been caught on camera or videotape, and scientists have had limited luck reproducing ball lightning in the lab. Therefore, the phenomena has many critics who do not believe in its existence, rather believing that the object witnessed can be explained by other, more understood atmospheric events. Indeed many reported sightings of ball lightning do carry the hallmarks of known phenomena.
The event which appears to be mistaken for ball lightning more than any other is ‘St Elmo’s fire’. This occurs on the ends of objects such as ship masts and antenna in an atmospheric electric field (such as that set up by a thunderstorm). This phenomena occurs when the electric field around the object in question causes the ionisation of air molecules, leading to a discharge of light[7]. The light produced is bluish white due to the oxygen and nitrogen in earth’s atmosphere, and appears roughly spherical with a diameter of approximately 10cm. Although the phenomena can last several minutes, it is distinct from ball lightning in the sense that it can not move independently; the flame always remains in contact with the conducting object.
Ignis fatus, or ‘will-o’-the-wisp’ is also often mistaken for ball lightning. This phenomenon occurs over bogs and swamps, or warm moist areas where soil has been recently upturned. A bluish flame of ghostly appearance, typically 10cm high and 5cm wide is seen to bob and swing during twilight and night time hours. The object is thought to occur by the chemical reactions of gases such as methane and phosphine, released by decaying matter. However will-o’-the wisp can not account for ball lightning appearing away from swamps, such as in enclosed spaces.
Extraterrestrial objects can often cause visual effects which could be mistaken for ball lightning. The majority of these involve objects entering and interacting with our atmosphere. Meteoroids are small dust particles or debris, which may be caught by earth’s gravitational field. When this debris enters the atmosphere, it may experience heating due to friction with gas particles, which produces visible light. Meteors are larger objects, which appear as ‘shooting-stars’ in the atmosphere. If the meteor is large enough, it can form a Bolide- a fireball flying through the sky which can have the apparent diameter of the sun or the moon to an observer. However the trajectory of such objects is almost always straight, which does not account for the non-linear nature of the movement of ball lightning.
Celestial bodies such as the sun or the moon could also be misidentified as ball lightning, particularly when viewed through broken cloud. Other bodies which are visible to the naked eye include Mercury, Mars, Saturn, and Venus. Of these, Venus appears the brightest, with a maximum magnitude of -4.4, which can often be seen during daylight. Venus is best observed in the southern hemisphere, and the image can be larger and particularly pronounced due to atmospheric distortion, created by the variable refractive index of our atmosphere.
However all of the above mentioned celestial bodies would appear static to the eye, which again differs from many reports of ball lightning having an irregular motion.

To explain this irregular motion of ball lightning, some critics use the example of nocturnal insect swarms moving through an intense electric field (again, such as those created in thunderstorms). The insects shells’ can act as a dielectric conductor and electrical discharges can occur at the sharp ends of the shells[8].


Weather balloons have also been mistaken for ball lightning, particularly when sun is below the observer’s horizon, lighting the balloon from below.
Although both of the above would explain the movement of ball lightning, they are not consistent with reports of ball lightning occurring in confined spaces, and cannot explain the observed ‘penetration’ of solid objects by this phenomenon.
So although all of the above events can often explain some reported sightings of ball lightning, they do not explain the formation of spherical, luminous, and persistent glowing orbs, of a mobile nature which rarely exhibit upward movement. Numerous theories have been hypothesised on the nature of ball lightning since the subject came under scientific scrutiny one hundred and fifty years ago by Arago. However, progress has been painfully slow, and there is still no general consensus on the physical mechanism responsible for ball lightning amongst scientists. With this in mind, the first International Symposium of Ball Lightning (ISBL) was held in 1988 and in subsequent years, in order to bring the global scientific community closer together on this subject.
The goal of these symposia is to establish a ball lightning theory which describes the many general characteristics of ball lightning observations. Such a theory must establish the relationship between ball lightning and electrical storms. This is because many reports describe the formation of ball lightning to coincide with a nearby cloud to ground lightning flash. The theory must also describe the constant luminosity that ball lightning emits. This could be of incandescent form, where the emission is caused by raising the temperature of the object, or luminescence, where the emission occurs for other reasons, such as the electronic excitation of a gas. Another general characteristic of reports is the constancy of size of the sphere. This is generally between 20-50cm, and such a theory would have to balance the outward pressure force of the sphere, with any radial attractive forces. One of the most difficult characteristics for scientists to explain is the apparent independent motion of ball lightning. Any successful theory must be able to account for this, as well as the apparent absence of upward movements caused by convection. The last aim of a ball lightning theory would be to explain the decay of such an event. Reports describe both violent explosions, and sudden silent decays, so both would need to be accounted for.
The majority of proposed ball lightning theories assume the object to be created when a lightning strike hits the ground. Of these, Plasma models have been a popular source of discussion amongst theorists. Plasma is a hot, highly ionised gas, often referred to as the fourth state of matter. The reason for this models’ popularity is that when a lightning strike occurs, it ionises surrounding air, creating pockets of plasma[9] which could radiate luminescent light. However, plasma models have been plagued with difficulties, mainly due to the duration of their existence. Plasma tends to expand and recombine to form neutral atoms and molecules unless there is a mechanism to combat the process. The difficulty in containing plasma can be seen in mankind’s attempts to harness the power of nuclear fusion; huge reactors are constructed in order to confine stable plasma for a matter of seconds. Theorists have attempted to overcome this problem by suggesting that because the plasma would be in an electric field created by the thunderstorm, the moving charges in the plasma could create a self contained magnetic field, opposing the diffusion of the object. However, this model has proven extremely difficult to recreate in the lab, and does not explain why hot plasma would not rise due to convection. Other suggestions to the plasma model include ball lightning being a combination of impure plasma mixed with a fine aerosol of particles, and the sphere having multiple sheets of opposite charge, separated by insulating layers. Again, the complexity of these theories has left them beyond the reach of the laboratory scientist.


A recent theory which has produced more tangible results was published in 2000. The theory proposed that when lightning strikes the earth, chemical energy is stored in minerals found in the soil such as silicon, and these are ejected into the atmosphere, creating a filamentary network[10]. This is because these chains of nano-scale particles react with the air, and form an oxide coating. As there is a strong electric field produced by the storm, these coated particles will attract opposite electrical charges, and a network of filaments would be set up, converting the chemical energy of the minerals into heat and light. The coating of the minerals reduces further oxygen from oxidising the silicon[11], giving a constant energy output, much like those described by witnesses of ball lightning.
The team who proposed the theory showed experimentally that when soil and soil/wood combinations were exposed to a ‘lightning-like discharge’, that the ejected silicon particles did develop an oxide coating, but no ball lightning was observed[12]. However since the publication of the theory, two other teams have produced striking results to back up this model of ball lightning.
The first was a team in Israel who were experimenting with a microwave drill, which created hotspots on materials by focusing energy through a drill-bit[13]. It was found that when the drill bit was removed, globs of molten material were ejected which burst into flames. When the experiment was conducted on plates of silicate in a microwave chamber, these flames formed floating spheres a few centimetres across, although only when the microwave was switched on. The spheres often floated to the top of the chamber, which is not in agreement with the majority of ball lightning observations, however it was thought that the drill-bit played the role of the lightning strike in the model.



FIG 2: orbs of light created whilst passing a current through silicon[i 2].

Another team in Brazil studied this model for ball lightning, and came up with an experiment where they managed to create luminous balls which lasted up to eight seconds[14] (see FIG 2). The team placed thin wafers of silicon between two electrodes. Passing a current through the silicon, the electrodes were moved, creating an electrical arc which vaporised the silicon, analogous to the lightning strike in the proposed model. The arc emitted luminous white and orange orbs the size of ping-pong balls, with a lifetime of up to eight seconds. The orbs appeared to move in random directions, often bouncing off surfaces, and were observed to burn plastic.


Although this experiment has been considered an early success in recreating ball lightning in the lab, the model still does not explain the observation that ball lightning has been observed occasionally to occur away from electrical storm events, and to penetrate solid materials.
Lastly, an exotic theory has been proposed to suggest a reason for this and other ‘extreme’ ball lightning. One scientist has proposed that ball lightning exhibiting some of the extreme behaviour mentioned above is not created by a lightning strike, but rather by a mini black hole left over from the big bang[15].
After visiting the aforementioned peat bog in Donegal, Ireland, where Michael Fitzgerald witnessed his famous ball lightning event, and ruling out all other proposed models in this instance, Dr Pace VanDevender proposed that the phenomenon observed was actually ball lightning with a mini black hole as its power source. The theory suggests that as the object would have a minute sub-atomic core, it would have the ability to exhibit some of the extreme behaviour sometimes observed, such as the ability to penetrate walls and glass.

VanDevander describes a model where the black hole is so tiny, that particles attracted to its gravitational pull are kept in orbit without falling through the ‘event horizon’. The floating mechanism is achieved by orbiting charged particles travelling at such a velocity that they create an oscillating magnetic field, which would induce a current on the ground. The induced current and oscillating magnetic field would repel, levitating the ball above the ground. The luminosity associated with ball lightning is suggested to come from stimulated emission of the surrounding air by radio waves. These would theoretically be generated by plasma created by atoms becoming ionised as they orbit the black hole.

FIG 3: image from New Scientist magazine, of how ball lightning powered by a black hole would interact with its environment

Although the black hole theory may sound far-fetched, it does suggest that there may not be one theoretical model to describe all the observed characteristics of ball lightning. Therefore the possibility is also raised that the wildly varying events described under the term ‘ball lightning’, are actually completely separate phenomena requiring their own independent research. However, the recent research success in Brazil is an encouraging sign that scientists are finally getting to grips with one type of ball lightning, and the future surely holds a greater understanding of the phenomenon as a whole.


by David Johnson

Y2 Physics

Personal tutor: Dr Dan Browne





References

[1]- Stenhoff, Mark “Ball Lightning- an unsolved problem in atmospheric physics”, 13, ISBN: 0-306-46150-1, 1999.

[2]- Stenhoff, 13, 75.

[3]- Stenhoff, 3

[4]- Muir, Hazel “Black holes in your backyard”, 48-51, New Scientist magazine, issue 2583, 03/01/07.

[6]- Stenhoff, 113.

[7]- http://en.wikipedia.org/wiki/St_Elmos_Fire

[8]- Stenhoff, 46

[9]- http://www.sciencenews.org/articles/20020209/bob8.asp

[10]- Abrahamson & Dinniss, “Ball lightning caused by oxidation of nanoparticle networks from normal lightning strikes on soil”, 519-521, Nature magazine, issue 403, 03/02/2000.

[11]- http://www.sciencenews.org/articles/20020209/bob8.asp

[12]- Abrahamson & Dinniss

[13]- Merali, Zeeya, “The source of ball lightning?”, 16, New Scientist magazine, issue 2538, 11/02/06

[14]- Muir, Hazel “lightning balls created in the lab”, 12, New Scientist magazine, issue 2586, 10/01/07

[15]- Muir, Hazel “Black holes in your backyard”
Images

[i 1]- from http://en.wikipedia.org/wiki/Georg_Richmann

[i 2]- image from http://www.newscientist.com.libproxy.ucl.ac.uk/channel/fundamentals/mg19325863.500-lightning-balls-created-in-the-lab.html

[i 3]- image from http://www.newscientist.com.libproxy.ucl.ac.uk/channel/fundamentals/mg19225831.700-blackholes-in-your-backyard.html






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