The cities of light hidden from view

Elves, fairies and blue jets are not just part of a fairy tale. These are spectacular electrical discharges that break out of the winter clouds * The writer was a member of the MEIDEX experiment which examined, among other things, the elf phenomenon in the Columbia shuttle mission, Ilan Ramon's last mission, which in about a month will be six years since the disaster

Professor Yoav Yair Galileo

Imagine that, while a lightning strikes the night sky in a winter storm over Tel Aviv, the entire dome of the sky from Kiryat Shmona to Beer Sheva is illuminated for a brief moment by the red light of a ring spreading over the location of the lightning at a speed that seems to us to be higher than the speed of light, and a few thousandths of a second later Huge flashes of red and blue appear in shapes reminiscent of carrots, sticks or luminous jellyfish of several tens of kilometers in size, which flicker and disappear in the blink of an eye. A whole city of lights resides on high, and is all hidden from view.

This is not hyperbole or the wild imagination of a failed Hollywood special effects specialist. These are physical phenomena that occur whenever a particularly strong lightning strikes the ground. The brightness of the phenomena is as high as the aurora borealis, but they last only milliseconds, so that not enough photons reach the eye and they cannot be seen, except with CCD cameras that can turn the photons into an electric current and produce a digital image.

Please meet: Mesosphere

These phenomena occur between the altitudes of 40 and 95 km above the surface of the ground in the layer of the atmosphere called the mesosphere, which does not usually receive much attention, probably because it lies far high above the layers where the weather phenomena, clouds and rains occur, and also above the ozone layer which is in the stratosphere. A scientist who studies the upper atmosphere called the region the "Ignorosphere" because, according to him, "Until now we were completely ignorant about it, but we also ignored it and its possible importance."
The elves are flashes of light that split the sky in the blink of an eye, the fairies are rings of light that are shaped like a candle, and the blue jets are cones of bluish light that emerge from the cloud

Indeed, it seems that in recent years a considerable research effort has been devoted to reducing the ignorance regarding the properties and processes that occur in the mesosphere, that cold and thin layer between the altitudes of 50 and 90 km, where these spectacular optical phenomena appear.

All the fast-lighting phenomena that were discovered were given names taken from Shakespeare's play "A Midsummer Night's Dream", and this is because of their whimsical, agile and playful nature like the elves and fairies appearing in the play.

The sprites are flashes of light that pierce the sky in the blink of an eye: the fairies (Elves) are rings of light that are shaped like a candle; And the blue jets) are cones of bluish light emanating from the cloud. In order to gather all the phenomena in one family, a general technical name was determined for them and it is "Transient Luminous Events" or for short TLE - Transient Luminous Events.

What happens inside the cloud?

To understand what happens above lightning storm clouds and how these bright and beautiful phenomena are created, one must first understand the processes that take place inside the clouds when lightning breaks up. The physics of lightning and its extensive effects on the atmosphere are currently a very active area of ​​research. Observations from satellites, from remote sensing systems in different areas of the electromagnetic spectrum, laboratory experiments and numerical simulations on supercomputers, create a fairly clear picture of what is happening inside these clouds.

Inside thunderstorm clouds, known as cumulonimbus, there is a huge concentration of water droplets and ice crystals, which are carried by the winds that rise and fall in the cloud in an endless sequence of collisions. Some of the collisions result in the coalescence of the particles that become raindrops or hailstones, but there are quite a few collisions that result in the colliding particles colliding and scattering tiny fragments.

From laboratory studies and computer simulations it became clear that such collisions lead to a separation of electric charges so that eventually the small fragments are carried upwards with an excess of positive electric charge, while the large ones fall down due to their higher valve, and they carry a negative charge with them. This spatial separation between opposite electric charges causes the formation of a very strong electric field within the cloud, the value of which can reach hundreds of thousands of volts per meter. At this stage, the cumulonimbus cloud can be seen as a kind of huge electric battery, in the upper part of which there is a positive charge and in the lower part (which is in the temperature range of zero to minus twenty degrees Celsius) a negative charge.

About 20 to 30 minutes after crystals and droplets have already formed in the cloud in a concentration and size that can be detected by cloud radar, the first lightning is received. From the observations it became clear that initially most of the lightning activity occurs between the centers of positive and negative charge within the cloud, but within a few minutes - at the same time as precipitation particles arrive at the base of the cloud - a pocket of positive electric charge appears near the lower part (base) of the cloud: the precipitation is formed in several areas of the cloud and falls down, When they carry with them a positive charge to a lower altitude near the base of the cloud, then lightning is formed from the cloud towards the ground.

These lightnings are the most dangerous and deadly for people and animals, and every year hundreds of people around the world are killed by their impact and huge damages are caused to buildings, technological infrastructures and power grids. Most natural forest fires in the world occur as a result of ignition by lightning striking the ground.

A physical explanation for the lightning phenomenon

The total lightning rate on Earth is between 40 and 60 lightnings per second, which are formed within about 1800 to 2000 active storm clouds at any given moment. Most of the activity occurs over the continents in the tropical regions, while the oceans have weaker electrical activity on average. This enormous electrical activity has a fairly regular diurnal cycle, reflecting the peak hours of solar heating over Africa, Southeast Asia, and South America, which creates the atmospheric instability essential to the development of storm clouds.

Until the end of the 90s of the last century there were very few reports of lightning bursting from the tops of the clouds towards the upper atmosphere and flashes of light in these layers. Most of the reports were not accurate or detailed enough.

the operation of Lightning storms It causes the preservation of an almost constant potential difference of 300 thousand volts between the surface of the earth and the ionosphere. At a conference held in 1988 in the city of Uppsala in Sweden, the physicist Bernard Vonnegut (brother of the author Kurt Vonnegut) presented a study in which he compiled dozens of reports from airliner pilots who observed electrical breakdowns above lightning storms during normal commercial flights, which are usually conducted at heights of 10-11 km M. According to the pilots, these flashes were surprising in their appearance, lasted in the blink of an eye and clearly erupted from the bottom up, and there was no doubt that their origin was natural and not from the plane itself. The reports were met with great skepticism and might have remained a curiosity, had it not been for a chance observation made in 1989 by astronomers from the University of Minnesota.

Jack Winckler and his team were engaged in calibrating sensitive cameras for measuring light intensities from stars and pointed their camera towards the horizon. As luck would have it, a strong lightning storm occurred along their line of sight. The scientists detected clear flashes of light within the field of vision and were able to place them at heights of 90-40 km according to the position of the stars that appeared in the image. The report of the strange sighting caused great excitement in the scientific community, as it could not be dismissed as eyewitness testimony by a person without a scientific background, and a physical explanation was required.

The explanation is found in early writings from 1925 by the Scottish physicist Charles T. Wilson (Wilson, winner of the Nobel Prize in Physics. In an article from the same year that dealt with the electrical structure of lightning storms, Wilson determined that there would be an altitude at which the strength of the electric field induced in the atmosphere as a result of the cloud would pass the electrical breakdown threshold.

A fairly simple theoretical calculation shows that for acceptable values ​​of electric charge inside the cloud, the strength of the field above the top of the cloud will lead to an electrical breakthrough at a height of approximately 80 km. Wilson even determined that the action of lightning storms is what causes the preservation of an almost constant potential difference of 300 thousand volts between the surface of the earth and the ionosphere, that ionized and conductive layer that is between the altitudes of 70 km and 300 km.

The conditions for the appearance of the elves

The observation of Winkler and his team led to a series of optical studies from the ground, in which they watched from ranges of 200 to 300 kilometers with the help of sensitive cameras above large storms, to the heights where these flashes are supposed to appear, which were named "lightning sprites" or sprites, according to the line from "A Midsummer Night's Dream" ” (Act 5, Scene 1). Only extremely strong lightning bolts, whose peak current strength is over 75 thousand amperes, which drop a positive charge from the top of the cloud to the ground, create sprites

Thanks to the observations, the researchers were able to discover dozens of sprites during the nights of lightning storms and determined the conditions for their appearance in the mesosphere, helped in mapping their dimensions, brightness and different shapes, and also identified the sprites' properties and the type of lightning that creates them.

A 1995 study by Dennis Bosipio and colleagues showed that only extremely strong lightning, with a peak current of over 75 amperes, drop a positive charge from the top of the cloud to the ground (or more precisely, cause electrons to flow from the surface up towards the center of the positive electric charge at the top of the cloud ) create elves. These lightnings are called positive lightnings, and they are relatively rare compared to the "normal" lightnings that drop a negative charge to the ground.

These positive lightnings are characterized by a large amount of electric charge, a long continuous current and a clear electromagnetic signature in the Schumann resonance frequency range, low frequency frequencies of 8-20 Hz. itself, and are therefore absorbed at huge ranges from the lightning itself).

The blue fairies and jets

At the same time as these observations, researchers at NASA and Stanford University began to conduct calculations using physical models in order to try to determine whether this phenomenon poses a danger to spacecraft and satellites passing through the mesosphere, and in particular to the one expected for the space shuttles, which, before landing in Cape Canaveral in Florida, perform a long descent route that often passes Above active lightning storms. NASA even approved taking pictures from the space shuttle of the night sky above lightning storms to try to determine the frequency of the phenomenon.

Indeed, from a long and tedious manual analysis (frame-by-frame is an analysis in which one examines image after image) of the photo films from the space shuttles, it became clear that sprites appear above lightning storms almost all over the world wherever a strong storm occurs, especially in the tropics. The same study also showed that there is a strong brightening of the airglow layer at an altitude of 100 km, which appears as a sort of horizontal light band right above the lightning.

Theoretical calculations confirmed that this is another phenomenon that received the nickname "fairies" (Elves. In 1996, a first observation from the ground by a group of Japanese researchers confirmed the measurement from the shuttle, and then it was nicknamed "fairy". The phenomenon occurs at an altitude of 95-100 km, and is shaped like rings Light with diameters of 250 to 400 km, with a minimum brightness in the center, which creates a shape like a donut or a donut (in the center of the ring there is a "hole" of a non-illuminating area).

A third type of bright flashes was discovered from an aerial observation carried out by the research group of Davis Sentman (Sentman) from the University of Alaska at Fairbanks, which detected cones of bluish light emanating from the top of the cloud up to a height of 40 km right near the lightning breakup. These flashes were almost never observed from the ground due to a strong dispersion in the wavelengths in which the light emission is the main, but from an airplane they are clearly visible, and they were named "blue jets".

The mechanisms responsible for the creation of elves and fairies

In 2002, a research group from Penn State University engaged in an optical observation from Puerto Rico towards a lightning storm in the Caribbean region. Before their eyes, a huge blue jet erupted that started from the top of the cloud and accelerated upward, but unusually, it did not stop at an altitude of 40 km, but split and became a giant carrot-like elf that extended up to a height of 70 km. Victor Pasko reported the observation in an article in Nature, called it a "Gigantic Jet", and stated that the jet created an electrical "short" that connected the lightning storm directly to the base of the ionosphere.

As far as we know, the mechanism responsible for the formation of the carrot-like elves and jellyfish (sprites) is related to the appearance of an electrostatic electric field between two opposite charges (as in Coulomb's law) between the ionosphere and the top of the cloud. This field exists in the mesosphere for tens of milliseconds only, immediately after the positive charge is removed from the top of the cloud in the lightning strike that hits the ground, but it is enough to accelerate free electrons present in it downward so that they hit nitrogen molecules, excite and ionize them. The excitation results in the emission of light in red wavelengths around 665 nm and blue around 470 nm. The entire phenomenon lasts a few hundredths of a second and at the end it fades and disappears.
French scientists combined optical photographs with acoustic measurements, and it turned out that elves have a characteristic signal that sounds like a kind of "chirp" in the subsonic frequency range

The lightning-like fairies are formed in a different way, as a result of the electromagnetic signal created by the lightning, which spreads in a three-dimensional apple-like shape at the speed of light around the place where the huge lightning current passed. The EMP (Electro-Magnetic Pulse) hits the base of the ionosphere and gives an energetic "blow" to the electrons in it, and these stimulate the nitrogen molecules to emit very fast light in red wavelengths. The duration of the phenomenon does not exceed 50 millionths of a second, and it is much more common than the carrot gnomes, because it does not depend at all on the polarity of the lightning (positive or negative).

From observations using a telescope with a 1-meter diameter mirror conducted by Elizabeth Gerken and Gerken (Inan, Gerken) from Stanford University, it becomes clear that the disintegration has a fractal structure similar to branches and roots, which appear as tiny canals extending downward from an altitude of 75-80 kilometers, with each "branch" being hundreds of meters long meters. The strength of the branches and their brightness are probably related to the lightning current that preceded the elf.

French scientists combined optical photographs with acoustic measurements, and it turned out that elves have a characteristic signal that sounds like a sort of "chirp" in the infrasound range. In other words, just as the sound of thunder accompanies lightning, there is a kind of silent chirping hidden from our ears that accompanies the elves (Fargas et al., 2004).

How do you photograph an elf?

On the flight of the space shuttle Columbia in January 2003, the shuttle crew also performed Night observations with a sensitive and calibrated camera towards lightning storms, as part of the Israeli experiment "Madex", whose main task was to observe the effect of desert dust on the Earth's climate. The crew managed to photograph dozens of luminous events from ranges of 1500 to 1800 km from the shuttle, manually pointing the camera towards the horizon above the twinkling lightning they saw with their eyes. Among others, the late Ilan Ramon succeeded To photograph elves in infrared light (at a wavelength of 860 nm), a field in which no measurement has ever been performed.

The complete calibration of the camera in this experiment allowed the scientists to accurately calculate the intensity of the light emitted by the phenomena and to show that the values ​​correspond to the theoretical prediction. The measurements made by the astronauts also indicated a possible connection between meteors penetrating the atmosphere and sprites occurring at the same altitudes. Another observation from space was made a year later by European astronauts from the International Space Station who photographed vertically above the lightning storms, and by means of a special optical filter separated the light emitted by the sprites from the light of the lightning that created them, which is about a million times more powerful.

In recent years, significant progress has been made in the study of the elf phenomenon and in understanding the form of its development, through rapid photography at a rate of 10,000 images per second. In videos presented by Steve Cummer of Duke University in North Carolina, it is clearly seen that the phenomenon begins with a pale initial glow that looks like a faint halo, below which bright points of light hundreds of meters in size emerge, moving downward at high speed - 1000 to 10,000 kilometers per second - splitting and creating fractal branches and a jellyfish-like structure with coiled arms. In some cases there is also an upward movement, which creates the inverted tangle of roots emerging from the upper part of the elf and which looks like a carrot head.

As a matter of fact, the colorful descriptions of these structures are only illusions visible to us because a slow-paced photograph of such a fast phenomenon inevitably performs an integration-in-time on the entire process and "freezes" it in a single image. As a result, the speed and complex dynamics of the propagation of decay channels in the elf remain hidden from view when shooting at a normal video rate of between 25 and 30 frames per second, and are revealed in their full beauty only at much higher speeds.

The elves' influence on the mesosphere

The importance of the gnomes' effects on the chemistry of the mesosphere is still in question, because despite the enormous volume that the gnomes occupy, the short duration in which they exist probably causes a relatively small effect, which is manifested in the appearance of nitrogen oxides at these altitudes. At the same time, in the measurements carried out in the summer of 2006 above a terrible lightning storm in Brazil, more than 400 sprites were detected in one night, an extremely high rate that implies that in some cases there will likely be a significant local effect that may also manifest itself in damage to the ozone layer (nitrogen oxides are known as "ozone sinks" ).

Research groups from many countries continue to study the phenomenon of elves and fairies. Taiwan's Formosat satellite photographs and maps the global distribution of the ephemeris, in order to obtain a more complete picture of their climatic distribution. In the winter of 2009 the Japanese satellite SpriteSat will join it, and in 2012 the Danish ASIM experiment will begin on the International Space Station, which will be joined by the French satellite Taranis (named after the god of lightning in Gaelic mythology.

Joint studies from the ground and from space will make it possible to better understand the effects of the phenomena and their importance, because precisely the light emanating from them opens a window for us to that area of ​​the atmosphere that is almost completely hidden from our eyes.

And what about Israel? In the last three winters, researchers and students from the Open University and Tel Aviv University watched storms over lightning that occurred off the coast of Israel. The observations were made from the Wiz Observatory of Tel Aviv University near Mitzpe Ramon or from the university campus in Ramat Aviv, and they saw that the phenomenon also occurs in the atmosphere above us.

Winter elves are smaller and their common form is like glowing sticks several tens of kilometers long. Last winter (2007-8) a research group from the Hebrew University of Jerusalem joined, and with the help of simultaneous measurements from two sites it became possible to decipher the three-dimensional spatial structure of the phenomenon. It turns out that in some cases the glowing elves line up in an almost symmetrical circle, located above or at a certain horizontal offset to where the lightning was.

Today, the cameras and antennas of the groups of researchers in Israel are used as ground verification points for all international observation missions from space, in particular in the storms that occur over the Mediterranean Sea and over the deserts and in the east in Sinai, Saudi Arabia and Jordan. Although Israel is not prone to lightning storms, its location allows long-range observations of storms occurring in Cyprus, Turkey, Saudi Arabia or Iraq. This winter, spectral observations with calibrated cameras are planned, with the aim of better tying the properties of the elves with the lightning and clouds that created them. We can only hope for a rainy and blessed winter.

The author is a faculty member in the Department of Natural and Life Sciences at the Open University and director of the University of Jerusalem. Writes, researches and teaches atmospheric sciences and specializes in lightning storms on Earth and other planets.

Looking for elves

Site "tree" In memory of Ilan Ramon, the sightings of lightning elves from Israel are famous as part of the studies of Tel Aviv University, Hebrew and Open.

the site of "Sonotco” – a Japanese astronomy enthusiast who photographs elves, lightning, meteors and everything that sparkles in the upper atmosphere