Dark Lightning is a fascinating tale of discovery, obscuration and rediscovery that begins in the days when photographers used Photographic Plates.
Photographic plates preceded photographic film as a capture medium in photography.
The light-sensitive emulsion of silver salts was coated on a glass plate, typically thinner than common window glass, instead of a clear plastic film.
This form of photographic material largely faded from the consumer market in the early years of the 20th century, as more convenient and less fragile films were increasingly adopted.
However, photographic plates were reportedly still being used by one photography business in London until the 1970s, and they were in wide use by the professional astronomical community as late as the 1990s.
Glass plates were far superior to film for research-quality imaging because they were extremely stable and less likely to bend or distort, especially in large-format frames for wide-field imaging.
Early plates used the very inconvenient wet collodion process, which was replaced late in the 19th century by gelatin dry plates.
Dry plate, also known as gelatin process, is an improved type of photographic plate.
It was invented by Dr. Richard L. Maddox in 1871, and by 1879 it was so well introduced that the first dry plate factory had been established.
With much of the complex chemistry work centralized into a factory, the new process simplified the work of photographers, allowing them to expand their business.
One curious property associated early photographic plates [and films] is that they were “blackened by ionizing radiation” and were “usefully sensitive” to ultraviolet light.
Many solar system objects were discovered by using photographic plates, superseding earlier visual methods.
Discovery of minor planets using photographic plates was pioneered by Max Wolf beginning with his discovery of 323 Brucia in 1891.
The first natural satellite discovered using photographic plates was Phoebe in 1898.
Pluto was discovered using photographic plates in a blink comparator; its moon Charon was discovered 48 years later by carefully examining a bulge in Pluto’s image on a plate.
Photographic plates were also an important tool in early high-energy physics, as they get blackened by ionizing radiation.
For example, in the 1910s, Victor Franz Hess discovered cosmic radiation as it left traces on stacks of photographic plates, which he left for that purpose on high mountains or sent into the even higher atmosphere using balloons.
Early photographic plates and films were usefully sensitive only to blue, violet and ultraviolet light
Therefore, early photographers couldn’t capture true what you see is what you get images because [amongst other things] their photographs included artefacts caused by ultraviolet light [and “ionizing radiation”] which is not visible to the human eye.
These artefacts are clearly visible in a photograph from 1890 titled Dark Lighting.
The photographic study of lightning is a very interesting one, and the results obtained fully balance the outlay of labor and patience expended in the attempt to capture the evanescent image.
After many years experience in this branch of photography, the most satisfactory results have been obtained by the use of a little ten dollar camera with its Waterbury single lens.
The moon furnishes a good lightning focusing point, which is marked upon the camera bed.
” Ivory,” celluloid, or paper films, backed with black unglazed paper or cardboard, are more likely to give a true representation of the electric discharge, glass being apt to produce a double or halated image.
Of course, the absence of contrast between lightning and daylight, and the impracticability of using an instantaneous shutter renders the production of a photograph of lightning during day time out of the question.
An open window with a clear view of the horizon is desirable, otherwise it is well to have in mind several points from which clear views may be had looking north, south, east or west.
Upon the approach of a thunderstorm the camera, properly focused, is wrapped in a rubber cloth, and the operator is protected from the elements in a similar manner.
When the storm has reached its climax, the electric fire appearing to blaze up immediately in front of the camera, the lens is uncapped, and recapped immediately after a discharge of lightning.
The writer has often noted the fact that as soon as an electric pathway is opened up in space there immediately follows a succession of discharges along the same line, and the movement of the camera during a multiple discharge of this kind has given rise to the impression that lightning is not so quick in its action as is generally supposed.
The method of development arrived at after numerous experiments is to immerse the exposed plate or film in a weak soda solution for about five minutes, rocking the tray constantly followed by a dose of pyro developer containing about five drops of 10 per cent, bromide solution.
By this means more detail can be obtained than by the ordinary method of development.
The two photographs which illustrate this article were taken at midnight on June 11, 1890.
There was a ten minutes interval between the two discharges.
One occurred in a horizontal, the other in a vertical direction in the same portion of the sky.
The length of these sparks has been estimated to be in the neighborhood of five miles.
The writer has been hoping to obtain good stereoscopic lightning photographs which will help to designate the distance at which the discharges take place from the camera.
Page 129 – Photographs of Lightning – W N Jennings
The American Annual of Photography and Photographic Times Almanac – 1891
This Dark Lighting image caught the attention of R W Wood [the “father of both infrared and ultraviolet photography”] and in September 1899 he made a “very wild guess” that the dark flashes “might be produced by a preponderance of infra-red radiations”.
I have been greatly interested by some photographs showing the rare phenomena of dark lightning, which have recently been sent to me.
Mr. Jennings, of Philadelphia, who in 1890 secured a remarkable picture, reproduced in Photographic Times Annual for 1891, showing a very brilliant flash with countless dark flashes covering the sky around it, tells me that the appearance to the eye was a brilliant white discharge, with fainter rose colored ramifications, the latter developing in the negative, or rather positive, as dark flashes.
Some years ago it occurred to me that a dark flash might be produced by a preponderance of infra-red radiations, which, as Abney has shown, undo the work of ordinary light on the plate.
If we had a form of discharge capable of giving off very little actinic light, and an abundance of infra-red light, it might come out dark on a feebly illuminated background.
This is, of course, a very wild guess, with nothing to substantiate it, but the dark flash appears to be a reality, and a poor hypothesis is perhaps better than none at all.
My intention in writing this letter is not so much to advance theories accounting for the phenomenon of the dark-flash as to re awaken an interest in the subject and bring out ideas from persons better qualified than I to treat the matter.
Dark Lightning – R. W. Wood – Science – 8 September 1899
Robert Williams Wood (May 2, 1868 – August 11, 1955) was an American physicist and inventor.
He is often cited as being a pivotal contributor to the field of optics and a pioneer of infrared and ultraviolet photography.
Wood identified a very low ultraviolet albedo (reflectivity, that is most of the ultraviolet is absorbed) region in the Aristarchus Plateau region of the Moon which he suggested was due to high sulphur. The area continues to be called Wood’s Spot.
Wood has been described as the “father of both infrared and ultraviolet photography”.
Though the discovery of electromagnetic radiation beyond the visible spectrum and the development of photographic emulsions capable of recording them pre-date Wood, he was the first to intentionally produce photographs with both infrared and ultraviolet radiation.
In 1903 he developed a filter, Wood’s glass, that was opaque to visible light but transparent to both ultraviolet and infrared, and is used in modern-day black lights.
He was also the first person to photograph ultraviolet fluorescence.
He also developed an ultraviolet lamp, which is widely known as the Wood’s lamp in medicine.
The slightly surreal glowing appearance of foliage in infrared photographs is called the Wood effect.
In November 1899 R W Wood commented upon the mysterious Clayden Effect before speculating that it may be caused by electric sparks emitting some form of “peculiar radiation”.
I have been criticized in a letter which appeared recently in Nature for not alluding in my letter on dark lightning to the peculiar photographic reversal known as the Clayden Effect.
I must confess that at the time of writing my letter I was unaware of this effect, a description of which has only appeared, so for as I know, in one of the photographic journals.
Mr. Clayden has certainly explained dark lightning, and it only remains to explain his explanation.
Mr. Clayden showed that if a plate, which had received an impression of a lightning flash or electric spark, was subsequently slightly fogged, either by exposing it to feeble diffused light or by leaving the lens of the camera open, the flash on development came out darker than the background.
If, however, the plate was fogged before the image of the flash was impressed, it came out brighter than the background, as in the ordinary pictures of lightning.
I refer to the appearance in the positive print in each case.
This is quite different from ordinary reversal due to the action of a very intense light, for the order in which the lights are applied is a factor, and the phenomenon lies wholly in the region of under exposure.
I repeated Mr. Clayden’s experiment, and obtained dark flashes without any difficulty.
The effect cannot, however, be obtained by impressing an image of a filament of an incandescent lamp on a plate and subsequently fogging the plate.
Clearly there is something about the light of the electric spark which is essential to the production of the reversal.
It is not intensity, however, for I found it was impossible to obtain reversed images or bright sparks with the lens wide open.
Fig. 1 shows a series of spark images, some normal, some partly reversed and others wholly reversed.
The sparks are those of a large inductorium, with a good sized Leyden jar in circuit.
The sparks were all of equal intensity, but after each discharge the iris diaphragm of the lens was closed a little.
It will be seen that the borders of the bright sparks are reversed.
In some the image is reversed with the exception of a narrow thread down the core.
The first thing that occurs to one is that it may be some peculiar radiation which the spark emits, which is wanting in the light coming from other bodies.
On the Cause of Dark Lightning and the Clayden Effect
R. W. Wood – Science – 17 November 1899
In December 1899 [the eponymous] A W Clayden supported R W Wood when he [also] suggested that it was “just possible” that electrical sources of light “may differ from other kinds of light”.
Here Professor Wood has done better, my results were nil and I congratulate him on his success.
However I should like to suggest that it is just possible that light from a source whose excitement is electrical may differ from other kinds of light in some manner at present unknown and that it is not safe to regard it as proved that the time element is the only one involved until the phenomena have been repeated without employing electricity at all.
For ten years the facts have been before the world.
They were partly verified eight or nine years ago by Mr. Shelford Bidwell and it is highly satisfactory to find them verified again in so many particulars, by another physicist who has reached the same conclusions by means of somewhat different experiments.
Dark Lightning – A W Clayden – Science – 29 December 1899
However, as we move into the 20th century the story of Dark Lightning becomes obscure.
This is primarily because R W Wood developed a filter that was “opaque to visible light but transparent to both ultraviolet and infrared” [in 1903] which became classified military technology that was subsequently “used for secret communications during the War”.
In 1903 he developed a filter, Wood’s glass, that was opaque to visible light but transparent to both ultraviolet and infrared, and is used in modern-day black lights.
In a paper presented to the French Society of Physics in April 1919, Wood described how his filter made from a special barium-sodium-silicate glass incorporating about 9% oxidized nickel (later to become universally known as the Wood’s filter) had been used for secret communications during the War.
His ‘invisible radiation’ technique worked either in infrared, which he suggested was useful during the daytime, or in ultraviolet, which he used for night-time applications.
Towards the end of his address, he suggested that this might be useful not only for communication between aircraft and airfields but for many other applications and then went on to describe the remarkable effects of this kind of radiation on the human skin.
Pioneers of Invisible Radiation Photography – Prof Robert Williams Wood [1868 – 1955]
Prof. Robin Williams and Gigi Williams
R W Wood left Dark Lightning behind and moved on to greater things.
Illustrating Paper on the Selective Reflection of Light from the Moon’s Surface
Three views of the entire moon taken by yellow, violet and ultraviolet light.
Two enlargements of the region around the crate Aristarchus (indicated by an arrow) showing the black deposit which appears in the photograph made by ultraviolet light. (Probably sulphur.)
Three small photographs of a very light deposit of sulphur on a fragment of oche-colored rock taken by yellow, violet and ultraviolet light.
Researches in Physical Optics – Part I
With Especial Reference to the Radiation of Electrons – R. W. Wood – 1913
Four photographs of the phenomenon are reproduced on Plate VII, Figure 3.
A vacuum-tube was put in circuit with the tube leading to the pump, to serve as an auxiliary manometer, and it was found that the secondary radiation did not appear at its maximum intensity until the green fluorescence due to cathode rays appeared in the vacuum-tube.
It is clear from the photographs that when the mercury vapour is in the highest possible vacuum, the light which it emits is capable of exciting a secondary radiation in the surrounding vapor which lies wholly outside of the path of the primary exciting beam.
The presence of air at 4 or 5 mm pressure, while it materially decreases the intensity of the primary resonance radiation, causes the secondary radiation to disappear entirely.
Researches in Physical Optics – Part II
Resonance Radiation and Resonance Spectra – R. W. Wood – 1919
And academia entered a new twilight world where classified military science was obscured [for public consumption] by introducing misdirection, misinformation and Settled Science.
This twilight world is painfully evident in the 1920 edition of Physics of the Air where clear photographic evidence of Dark Lightning is presented whilst the text boldly asserts “there is, of course, no such thing as dark lightning” before misdirecting the reader towards the [still scientifically unexplained] Clayden Effect.
When a photographic plate is exposed to a succession of lightning flashes it occasionally happens that one or more of the earlier streak images, on development, exhibits the “Clayden Effect” – that is, appears completely reversed – while the others show no such tendency.
Obviously, then, on prints from such a negative the reversed streaks must appear as dark lines (Fig. 120), and for that reason the lightning flashes that produced them have been called ” dark lightning.”
There is, of course, no such thing as dark lightning, since the only invisible radiation to which the ordinary photographic plate is sensitive is the ultra-violet, which cannot be excited by electric discharges in the atmosphere without at the same time producing visible radiation.
Nevertheless, the photographic phenomenon that gives rise to the name “dark lightning” is real, interesting, and reproducible at will in the laboratory.
Physics of the Air – William Jackson Humphreys – 1920
The same bizarre game was played by Ferdinand Ellerman [Mount Wilson Observatory] in Popular Astronomy during 1921 where he produced photographic evidence of Dark Lightning whilst claiming “there is no such thing in reality”.
That there is no such thing in reality as “dark lightning” we are sure, and the four illustrations are reproduced to show how differently the plates are affected at different times.
A Few Remarks on “Dark Lightning” – Ferdinand Ellerman
Popular Astronomy – No: 283 – Volume 29 – March 1921
Having successfully misdirected the general public towards the Clayden Effect [in the 1920s] the topic of Dark Lightning quietly slid into forgetful oblivion.
On the other hand, the [still scientifically unexplained] Clayden Effect develops a surreal life of its own and by 1983 its dictionary definition had morphed into a masterpiece of misdirection wrapped around a well camouflaged admission that the Clayden Effect [aka Dark Lightning] can be produced by x-rays.
If a photographic emulsion is given a very brief exposure to light of high intensity, it is desensitized towards a subsequent longer exposure to light of moderate intensity.
That is, the second exposure produces less effect that if the pre-exposure had not been given.
Observed originally by A. W. Clayden in 1899 when photographing lightning flashes.
It can be produced equally well by any type of light source, provided the intensity is sufficiently high and the duration short enough, e.g. 1/1000 sec or less, or by x-rays.
The pre-exposure may be of longer duration if it is given at the temperature of liquid air, because all exposures made at this temperature behave as though they had been made at very high intensity.
The density produced by the pre-exposure is not reduced by the subsequent low-intensity exposure.
Thus, the normal Clayden effect is not a reversal but a desensitization caused by first exposure.
Elsevier’s Dictionary of the Printing and Allied Industries – Second Edition – 1983
F.J.M. Wijnekus and E.F.P.H. Wijnekus
However, when Settled Science entered the satellite age the stage was set for the top-down rediscovery of Dark Lightning [aka Terrestrial Gamma-Ray Flashes – TGFs] by a new generation of Space Cadets.
Terrestrial gamma-ray flashes were first discovered in 1991 by BATSE, or Burst and Transient Source Experiment, on the Compton Gamma-Ray Observatory, a NASA spacecraft.
A subsequent study from Stanford University in 1996 linked a TGF to an individual lightning strike occurring within a few ms of the TGF.
BATSE detected only a small number of TGF events in nine years (76), due to its having been constructed to study gamma rays from outer space, which last much longer.
The red dots show some of the ~500 terrestrial gamma-ray flashes daily detected by the Fermi Gamma-ray Space Telescope through 2010.
Terrestrial gamma-ray flashes (TGFs) are very short blasts of gamma rays, lasting about one millisecond, that are emitted into space from Earth’s upper atmosphere.
TGFs have been correlated with lightning strikes and may be related to visible phenomena that occur in the upper atmosphere over thunderstorms, such as red sprites and blue jets.
The duration of TGFs recorded by RHESSI ranged from 0.2 to 3.5 milliseconds.
The most energetic TGF photons detected by RHESSI were in the range of 10 to 20 million electron volts (10-20 MeV), or about 300 times as energetic as medical x-rays.
The electrons that emitted these gamma rays would have been traveling at 99.99 percent of the speed of light, with energies on the order of 35 MeV.
New satellite observations of terrestrial gamma-ray flashes reveal surprising features of mysterious blasts from Earth – Tim Stephens – 21 February 2005
University of California Santa Cruz
Scientists using NASA’s Fermi Gamma-ray Space Telescope have detected beams of antimatter produced above thunderstorms on Earth, a phenomenon never seen before.
Scientists think the antimatter particles were formed in a terrestrial gamma-ray flash (TGF), a brief burst produced inside thunderstorms and shown to be associated with lightning.
It is estimated that about 500 TGFs occur daily worldwide, but most go undetected.
“These signals are the first direct evidence that thunderstorms make antimatter particle beams,” said Michael Briggs, a member of Fermi’s Gamma-ray Burst Monitor (GBM) team at the University of Alabama in Huntsville (UAH).
NASA’s Fermi Catches Thunderstorms Hurling Antimatter into Space – 01.10.11
Now we have a whole new generation of explanations for Dark Lightning.
Dark lightning is a process similar to ordinary lightning that creates pairs of electrons and antielectrons (or positrons), and that produces gamma rays and relatively little (visible) light. Since the amount of light created is so small, dark lightning is practically invisible to the human eye.
Though the details of the mechanism are uncertain, there is a consensus forming about the physical requirements.
It is presumed that TGF photons are emitted by electrons traveling at speeds very close to the speed of light that collide with the nuclei of atoms in the air and release their energy in the form of gamma rays (bremsstrahlung ).
Large populations of energetic electrons can form by avalanche growth driven by electric fields, a phenomenon called relativistic runaway electron avalanche (RREA).
The electric field is likely provided by lightning, as most TGFs have been shown to occur within a few milliseconds of a lightning event (Inan et al. 1996).
Beyond this basic picture the details are uncertain.
Recent research has shown that electron-electron (Bremsstrahlung) leads first to an enrichment of high-energy electrons and subsequently enlarges the number of high-energy photons.
Researchers have identified a burst of high-energy radiation known as ‘dark lightning” immediately preceding a flash of ordinary lightning.
The new finding provides observational evidence that the two phenomena are connected, although the exact nature of the relationship between ordinary bright lightning and the dark variety is still unclear, the scientists said.
“Our results indicate that both these phenomena, dark and bright lightning, are intrinsic processes in the discharge of lightning,” said Nikolai Østgaard, who is a space scientist at the University of Bergen in Norway and led the research team.
Dark lightning is the most energetic radiation produced naturally on Earth, but was unknown before 1991
(left) formation within the cloud of a small channel, or ‘leader,’ of electrical conductivity (yellow line) with weak emission of radio signals (ripples), to
(middle) a burst of both dark lightning (pink) and radio waves (larger ripples), to
(right) a discharge of bright lightning and more radio waves.).
Scientists detect dark lightning linked to visible lightning – 24 April 2013
Plus a whole new generation of pioneers blindly bumping into Dark Lightning.
A terrifying few moments flying into the top of an active thunderstorm in a research aircraft has led to an unexpected discovery that could help explain the longstanding mystery of how lightning gets initiated inside a thunderstorm.
University of New Hampshire physicist Joseph Dwyer and lightning science colleagues from the University of California at Santa Cruz and Florida Tech describe the turbulent encounter and discovery in a paper to be published in the Journal of Plasma Physics.
In August 2009, Dwyer and colleagues were aboard a National Center for Atmospheric Research Gulfstream V when it inadvertently flew into the extremely violent thunderstorm – and, it turned out, through a large cloud of positrons, the antimatter opposite of electrons, that should not have been there.
Physicist finds mysterious anti-electron clouds inside thunderstorm
David Sims – 13 May 2015
Thankfully, these pioneers haven’t resorted to the Clayden Effect so far.