Silver clouds. Astronomy for everyone
noctilucent clouds, being formed almost on the border of the earth's atmosphere and space, which greatly complicates their study, they still keep many secrets about their nature and origin.
The first documented evidence of the observation of noctilucent clouds can be found in the astronomical works of scientists from the Old World. These records date back to the middle of the 17th century and are characterized by extreme scarcity, unsystematic and inconsistent facts. Only in the summer of 1885 did this strange phenomenon attract the attention of several astronomers from different countries of the Northern Hemisphere at once. The honor of discovering unusual clouds based on the results of independent observations was shared between the Russian scientist V.K. Tserasky and the German scientist T.W. Backhouse. It was the domestic astronomer who most responsibly approached the study of a phenomenon new to science. He was able to determine the approximate distance to the boundaries of the manifestation of a unique atmospheric process (about 80 km) and the negligible optical density of these formations. Over the next three years, noctilucent clouds were studied by another German scientist, Otto Jesse. He confirmed the data obtained by Tserasky and gave the newly discovered phenomenon its current name.
General information
Noctilucent (night luminous, polar mesomorphic) clouds are the champions of the earth's atmosphere, the height of formation of which varies within 70-95 km. The formation of phenomena of this kind is possible only in areas of the stratosphere with minimal temperature regimes ranging from -70 to -120°C. The time of occurrence of noctilucent clouds is evening and predawn twilight. The zonal features in which the processes of their formation take place have for many years made it practically impossible to obtain objective information about this amazing atmospheric phenomenon. Additional negative factors included the proximity of space, penetrating particles of meteor matter and interstellar dust, the action of magnetic fields, various physical and chemical reactions, and the dependence of observations on the position of the Earth and the time of day. In addition, the height of noctilucent clouds in the mesosphere has proven difficult to reach for many modern aircraft (too high for airplanes, low for satellites). Today, representatives of geophysical and astronomical directions in science dominate in the study and research of a unique phenomenon.
Properties and types
Online image of noctilucent clouds from the AIM satellite
The basis of noctilucent clouds are crystals of frozen moisture, which condenses, and then forms an ice shell around microscopic particles (0.1-0.7 microns) of terrestrial or cosmic origin. This explains the maximum transparency of such formations, which retain only a thousandth of the light flux.
The stars are perfectly visible through the silvery clouds. The core of crystals can be fragments of meteoric or cometary matter invisible to the eye, volcanic or interplanetary dust, frozen particles of water vapor. Since the discovery of this phenomenon, scientists have put forward various assumptions about its causes and origin. Hypotheses have evolved as follows: volcanic (since 1887), meteoric (since 1926), condensational (since 1950). Periodically, other theories appeared that tried to explain the atmospheric phenomenon with the help of various geophysical phenomena, but they did not win support in scientific circles.
Noctilucent clouds have a diverse structure, on the basis of which they are classified according to these criteria into several types:
- fleur- the most primitive form, characterized by a blurry structure and a dull whitish glow.
- stripes- line up in small parallel or intertwining lines, resembling jets. They are sharply defined or blurry.
- Waves- visually very similar to the water surface distorted by small ripples. They are divided into 3 subspecies.
- Whirlwinds- represent twisted annular swirls with a dark central part. According to the radius and complexity of the structure, 3 subgroups are distinguished, the last of which belongs to the rarest phenomenon - clouds, resembling a luminous substance flying apart from an explosion.
Today, noctilucent clouds are unique and one-of-a-kind formations that carry important information for science about the processes occurring in the mesopause. Studies of this phenomenon are carried out by rocket, laser and radar sounding methods, providing ever new information about atmospheric wave motions, high-altitude winds and the processes that affect their temporal changes.
Image gallery
Conditions and time of observation
During the daylight hours, it is hardly possible to find and examine silvery clouds in the sky. Their time is a dark clear sky in deep evening or predawn twilight, when the earthly luminary falls 6-12 ° below the horizon line. During this period, the sun's rays cease to illuminate the lower atmospheric masses, continuing their impact on the rarefied upper regions: the stratosphere and mesosphere. The background created under such conditions is optimal for observing the beauty of noctilucent clouds. Despite the significant force of the wind at high altitudes, the objects formed are quite static, which makes them easy to study and photograph, creating an excellent opportunity to consider all the details of a rare phenomenon. Residents of both the Southern and Northern hemispheres can enjoy fantastic shapes and colors of noctilucent clouds. For the former, this is possible in January-February at 40°-65° latitude, for the latter - June-July, 45°-70°. The most possible place for the appearance of objects is the northern part of the sky at a height above the horizon from 3 to 15 degrees.
Traveling noctilucent clouds in the sky over Belarus in the summer of 2013!
The first high-quality images of noctilucent clouds were obtained by the German scientist Otto Jesse back in 1887.
Unique atmospheric formations of this type are very difficult to distinguish from their cirrus counterparts, so from time to time there is confusion in this matter among lovers of celestial light shows.
For residents of Russia, the optimal area for observing an interesting phenomenon will be latitudes from 55 ° to 58 °.
In our hemisphere, the study and study of noctilucent clouds is available only to astronomers and meteorologists from the Russian Federation, Canada and Northern Europe. Moreover, the maximum contribution of discoveries in this area belongs not to professional scientists, but to amateurs.
The altitudinal range, in which the processes of formation of the phenomenon take place, is inexplicably capable of shrinking to 80-85 km, expanding after that to 60-120 km.
The main reason for the colorful glow of noctilucent clouds is the effect of scattering of the ultraviolet spectrum of sunlight.
By 2007, NASA specialists developed and launched the AIM project. The mission was made up of a satellite whose equipment captures the main processes occurring in the mesosphere of our planet. High-precision instruments have expanded the field of knowledge about chemical composition noctilucent clouds by analyzing and measuring ice crystals, gas molecules and cosmic dust particles.
Lecture by O.S. Ugolnikova about silvery clouds
The content of the article
SILVER CLOUDS, the highest cloud formations in the earth's atmosphere, formed at altitudes of 70–95 km. They are also called polar mesospheric clouds (PMC) or noctilucent clouds (NLC). It is the last name that most closely matches their appearance and the conditions of their observation, is accepted as standard in international practice.
You can observe noctilucent clouds only in the summer months: in the Northern Hemisphere in June-July, usually from mid-June to mid-July, and only at geographical latitudes from 45 ° to 70 °, and in most cases from 55 ° to 65 °. In the Southern Hemisphere at the end of December and in January at latitudes from 40° to 65°. At this time of the year and at these latitudes, the Sun does not sink very deep below the horizon even at midnight, and its gliding rays illuminate the stratosphere, where noctilucent clouds appear at an average altitude of about 83 km. As a rule, they are visible low above the horizon, at an altitude of 3° to 15° degrees in the northern part of the sky (for observers of the Northern Hemisphere). With careful observation, they are noticed annually, but they do not reach high brightness every year.
During the day, even against the background of a clear blue sky, these clouds are not visible: they are very thin, "ethereal". Only deep twilight and night darkness make them visible to a ground observer. True, with the help of equipment raised to great heights, these clouds can also be recorded in the daytime. It is easy to be convinced of the amazing transparency of the silvery clouds: the stars are perfectly visible through them.
For geophysicists and astronomers, noctilucent clouds are of great interest. After all, these clouds are born in the region of the temperature minimum, where the atmosphere is cooled to -70 ° C, and sometimes even to -100 ° C. Altitudes from 50 to 150 km have been poorly studied, since airplanes and balloons cannot rise there, and artificial satellites of the Earth cannot able to go down there for a long time. Therefore, scientists are still arguing both about the conditions at these heights and about the nature of the noctilucent clouds themselves, which, unlike low tropospheric clouds, are in the zone of active interaction between the Earth's atmosphere and outer space. Interplanetary dust, meteoric matter, charged particles of solar and cosmic origin, magnetic fields are constantly involved in physical and chemical processes occurring in the upper atmosphere. The results of this interaction are observed in the form of auroras, airglows, meteor events, color changes and the duration of twilight. It remains to be seen what role these phenomena play in the development of noctilucent clouds.
At present, noctilucent clouds are the only natural source of data on winds at high altitudes, on wave motions in the mesopause, which significantly complements the study of its dynamics by other methods, such as meteor trail radar, rocket and laser sounding. The vast areas and considerable time of existence of such cloud fields provide a unique opportunity to directly determine the parameters of atmospheric waves of various types and their temporal evolution.
Due to the geographical features of this phenomenon, noctilucent clouds are mainly studied in Northern Europe, Russia and Canada. Russian scientists have made and are making a very significant contribution to this work, and qualified observations made by science lovers play a significant role.
Opening of silvery clouds.
Some references to glowing clouds at night are found in the works of European scientists of the 17th and 18th centuries, but they are fragmentary and indistinct. The time of discovery of noctilucent clouds is considered to be June 1885, when they were immediately noticed by dozens of observers in different countries. The discoverers of this phenomenon are T. Backhouse (T.W. Backhouse), who observed them on June 8 in Kissingen (Germany), and the astronomer of Moscow University Vitold Karlovich Tserasky, who discovered them independently and observed them for the first time on the evening of June 12 (according to a new style). In the following days, Tserasky, together with the famous Pulkovo astrophysicist A.A. Belopolsky, who then worked at the Moscow Observatory, studied noctilucent clouds in detail and for the first time determined their height, obtaining values from 73 to 83 km, confirmed after 3 years by the German meteorologist Otto Jesse (O. Jesse).
Night luminous clouds produced on Tserasky great impression“These clouds shone brightly in the night sky with pure, white, silvery rays, with a slight bluish tint, taking on a yellow, golden hue in the immediate vicinity of the horizon. There were cases when light was made from them, the walls of buildings were very noticeably lit up and obscurely visible objects protruded sharply. Sometimes the clouds formed layers or layers, sometimes they looked like rows of waves or resembled a sandbar covered with ripples or undulating irregularities ... This is such a brilliant phenomenon that it is absolutely impossible to imagine it without drawings and detailed description. Some long, dazzling silver bands, crossing or parallel to the horizon, change rather slowly and are so sharp that they can be kept in the field of view of the telescope.
Observation of noctilucent clouds.
It should be remembered that noctilucent clouds can be observed from the Earth's surface only during deep twilight, against an almost black sky and, of course, in the absence of lower, tropospheric clouds. It is necessary to distinguish the twilight sky from the dawn sky. Dawns are observed during early civil twilight, when the center of the solar disk descends below the observer's horizon to a depth of 0° to 6°. In this case, the sun's rays illuminate the entire thickness of the layers of the lower atmosphere and the lower edge of the tropospheric clouds. Dawn is characterized by a rich variety of bright colors.
In the second half of civil twilight (the depth of the Sun is 3–6°), the western part of the sky still has quite bright dawn illumination, but in neighboring areas the sky is already acquiring deep dark blue and blue-green hues. The region of the greatest brightness of the sky during this period is called the twilight segment.
The most favorable conditions for the detection of noctilucent clouds are created during navigational twilight, when the Sun sinks below the horizon by 6–12 ° (at the end of June in middle latitudes this happens 1.5–2 hours after true midnight). At this time, the earth's shadow covers the lower, most dense, dusty layers of the atmosphere, and only rarefied layers are illuminated, starting from the mesosphere. Sunlight scattered in the mesosphere forms a faint glow of the twilight sky; against this background, the glow of silvery clouds is easily detected, which attract the attention of even casual witnesses. Various observers define their color as pearly silver with a bluish tint or white-blue.
At dusk, the color of the noctilucent clouds seems unusual. Sometimes the clouds appear to be phosphorescent. Barely visible shadows move across them. Separate parts of the cloud field become much brighter than others. After a few minutes, neighboring areas may become brighter.
Despite the fact that the wind speed in the stratosphere is 100–300 m/s, the high altitude of noctilucent clouds makes them almost motionless in the field of view of a telescope or camera. Therefore, the first photographs of these clouds were taken by Jesse as early as 1887. Several groups of researchers around the world systematically study noctilucent clouds in both the Northern and Southern hemispheres. The study of noctilucent clouds, as well as other difficult-to-predict natural phenomena, involves the widespread involvement of science lovers. Every naturalist, regardless of his main profession, can contribute to the collection of facts about this remarkable atmospheric phenomenon. A high-quality photograph of noctilucent clouds can be obtained using a simple amateur camera. For example, you can use a Zenith camera with a standard Helios-44 lens; with an aperture of 2.8–3.5 and a film with a sensitivity of 100–200 units. GOST recommended shutter speeds from 2–3 to 10–15 seconds. It is very important that the camera does not shake during the exposure; for this, it is advisable to use a reliable tripod, but in extreme cases, it is enough to press the camera with your hand against the window jamb, tree or stone; When releasing the shutter, be sure to use the cable.
In order for the resulting images to be of not only aesthetic interest, but also to have scientific meaning and provide material for subsequent analysis, it is necessary to accurately record the circumstances of the shooting (time, parameters of equipment and photographic materials), as well as use the simplest devices: light filters, polarizing filters, a mirror to determine the speed of movement of the contrasting details of the clouds.
In appearance, noctilucent clouds bear some resemblance to tall cirrus clouds. To describe the structural forms of noctilucent clouds during their visual observation, an international morphological classification has been developed:
Type I. Fleur, the simplest, even form, filling the space between more complex, contrasting details and having a foggy structure and a faint pale white glow with a bluish tint.
Type II. Stripes resembling narrow streams, as if carried away by air currents. Often arranged in groups of several pieces, parallel to each other or intertwined at a slight angle. The stripes are divided into two groups - blurry (II-a) and sharply defined (II-b).
Type III. Waves are divided into three groups. Scallops (III-a) - areas with a frequent arrangement of narrow, sharply defined parallel stripes, like light ripples on the surface of the water with a slight gust of wind. Ridges (III-b) have more noticeable signs of a wave nature; the distance between adjacent ridges is 10–20 times greater than that of scallops. Wavy bends (III-c) are formed as a result of the curvature of the cloud surface occupied by other forms (bands, scallops).
Type IV. Vortices are also divided into three groups. Small radius swirls (IV-a): 0.1° to 0.5°, i.e. no larger than the lunar disk. They bend or completely twist stripes, scallops, and sometimes fleur, forming a ring with a dark space in the middle, reminiscent of a lunar crater. Swirls in the form of a simple bend of one or more strips away from the main direction (IV-b). Powerful vortex ejections of "luminous" matter away from the main cloud (IV-c); this rare formation is characterized by rapid variability of its form.
The zone of maximum frequency of observation of noctilucent clouds in the Northern Hemisphere runs along a latitude of 55–58°. Many large cities of Russia fall into this band: Moscow, Yekaterinburg, Izhevsk, Kazan, Krasnoyarsk, Nizhny Novgorod, Novosibirsk, Chelyabinsk, etc., and only a few cities of Northern Europe and Canada.
Properties and nature of noctilucent clouds.
The altitude range at which noctilucent clouds form is generally very stable (73–95 km), but in some years narrows to 81–85 km, and sometimes expands to 60–118 km. A cloud field often consists of several rather narrow layers. The main reason for the glow of clouds is the scattering of sunlight by them, but it is possible that the effect of luminescence under the influence of ultraviolet rays of the Sun also plays a certain role.
The transparency of noctilucent clouds is extremely high: a typical cloud field blocks only about 0.001% of the light passing through it. It was the nature of the scattering of sunlight by noctilucent clouds that made it possible to establish that they are clusters of particles with a size of 0.1–0.7 microns. A variety of hypotheses were expressed about the nature of these particles: it was assumed that they could be ice crystals, small particles of volcanic dust, salt crystals in an ice coat, cosmic dust, particles of meteoric or cometary origin.
Bright noctilucent clouds, first observed in 1885-1892 and, apparently, not noticed before, suggested that their appearance is associated with some kind of powerful catastrophic process. Such a phenomenon was the eruption of the Krakatau volcano in Indonesia on August 27, 1883. In fact, it was a colossal explosion with an energy equal to the explosion of twenty hydrogen bombs (20 Mt TNT). About 35 million tons of volcanic dust was thrown into the atmosphere, rising to a height of up to 30 km, and a huge mass of water vapor. After the explosion of Krakatau, optical anomalies were noticed: bright dawns, a decrease in the transparency of the atmosphere, polarization anomalies, the Bishop ring (a brown-red crown around the Sun with an outer angular radius of about 22 ° and a width of 10 °; the sky inside the ring is light with a bluish tint). These anomalies continued for about two years, gradually weakening, and noctilucent clouds appeared only towards the end of this period.
The hypothesis of the volcanic nature of noctilucent clouds was first expressed by the German researcher W. Kohlrausch in 1887; he considered them to be condensed water vapor ejected during an eruption. Jesse in 1888-1890 developed this idea, believing that it was not water, but some unknown gas (possibly hydrogen) was ejected by a volcano and froze in the form of small crystals. Opinions have been expressed that volcanic dust also plays a role in the formation of noctilucent clouds, since it serves as centers of water vapor crystallization.
The gradual accumulation of observational data produced facts that spoke clearly against the volcanic hypothesis. An analysis of light anomalies after the largest volcanic eruptions (Mont Pele, 1902; Katmai, 1912; Cordillera, 1932) showed that only in rare cases they were accompanied by the appearance of noctilucent clouds; most likely it was a coincidence. At present, the volcanic hypothesis, which at the beginning of the 20th century. considered generally accepted and even penetrated the textbooks of meteorology, has only historical significance.
The emergence of the meteor hypothesis of the origin of noctilucent clouds is also associated with a grandiose natural phenomenon - the Tunguska catastrophe on June 30, 1908. From the point of view of observers, among whom were very experienced astronomers and meteorologists (W. Denning, F. Bush, E. Esklangon, M. Wolf, F Arkhengold, D.O. Svyatsky and others), this phenomenon manifested itself mainly as various optical anomalies observed in many European countries, in the European part of Russia and Western Siberia, up to Krasnoyarsk. Along with bright dawns and "white nights" that came where they usually do not exist even at the end of June, many observers noted the appearance of noctilucent clouds. However, in 1908, none of the eyewitnesses of optical anomalies and luminous clouds knew anything about the Tunguska meteorite. Information about him appeared in print only about 15 years later.
In 1926, the idea of a connection between these two phenomena was independently expressed by L.A. Kulik, the first researcher of the site of the Tunguska catastrophe, and L. Apostolov, a meteorologist. Leonid Alekseevich Kulik developed his hypothesis in detail, proposing a well-defined mechanism for the formation of noctilucent clouds. He believed that not only large meteorites, but also ordinary meteors, which are completely destroyed just at altitudes of 80-100 km, supply products of their sublimation to the mesosphere, which then condense into particles of the finest dust that form clouds.
In 1930, the famous American astronomer H. Shapley, and in 1934, independently of him, the English meteorologist F. J. Whipple (not to be confused with the American astronomer F. L. Whipple) hypothesized that the Tunguska meteorite was the nucleus of a small comet with a dusty tail. The penetration of the tail material into the earth's atmosphere led, in their opinion, to the appearance of optical anomalies and to the appearance of noctilucent clouds. However, the idea that the reason for the optical anomalies in 1908 was the passage of the Earth through a cloud of cosmic dust was expressed as early as 1908 by one of the eyewitnesses of the “bright nights” of that period, F. de Roy, who, of course, knew nothing about the Tunguska meteorite.
In subsequent years, the meteor hypothesis was supported and developed by many astronomers, trying to explain with its help the observed features of noctilucent clouds - their morphology, latitudinal and temporal distribution, optical properties, etc. But the meteor hypothesis in its pure form did not cope with this task, and since 1960 its development has practically ceased. But the role of meteor particles as nuclei of condensation and growth of ice crystals that make up noctilucent clouds still remains indisputable.
The condensation (ice) hypothesis itself has been developed independently since 1917, but for a long time did not have sufficient experimental grounds. In 1925, the German geophysicist A. Wegener, based on this hypothesis, calculated that for steam to condense into ice crystals at an altitude of 80 km, the air temperature should be about –100 ° C; as it turned out during rocket experiments 30 years later, Wegener was not far from the truth. Starting from 1950, in the works of V.A. Bronshten, I.A. Khvostikov and others, the meteor-condensation hypothesis of noctilucent clouds was developed; in it, meteor particles play the role of condensation nuclei, without which the formation of droplets and crystals from vapor in the atmosphere is extremely difficult. This hypothesis is partly based on the results of rocket experiments, during which microscopic solid particles were collected at altitudes of 80–100 km with an ice “coat” frozen on them; when rockets were launched into the zone of observed noctilucent clouds, the number of such particles turned out to be a hundred times greater than in the absence of clouds.
In addition to the mentioned "classical" hypotheses, other, less traditional ones have been put forward; the connection of noctilucent clouds with solar activity, with polar lights, and with other geophysical phenomena was considered. For example, the reaction of atmospheric oxygen with solar wind protons (the “solar rain” hypothesis) was considered to be the source of water vapor in the mesosphere. One of the latest hypotheses links noctilucent clouds to the occurrence of ozone holes in the stratosphere. The area of formation of these clouds is being studied more and more actively in connection with space and stratospheric transport: on the one hand, launches of powerful rockets with hydrogen-oxygen engines serve as an important source of water vapor in the mesosphere and stimulate the formation of clouds, and on the other hand, the appearance of clouds in this area creates problems during the return of spacecraft to Earth. It is necessary to create a reliable theory of noctilucent clouds, which makes it possible to predict and even control this natural phenomenon. But so far, many facts in this area are incomplete and contradictory.
Vladimir Surdin
Just a few hundred years ago, the Earth was full of the unknown, and in order to paint over the blank spots, hypothetical natives with dog heads and human faces on their stomachs were drawn on geographical maps. Since then, mysteries on our planet have diminished. The more interesting are those who modern science still can't figure it out...
Sergey Sysoev
Polarization of Light Light is an electromagnetic wave. Polarization for electromagnetic waves is a phenomenon of directional oscillation of the vectors of electric and magnetic fields. Linear polarization is a special case of polarization, when the oscillations of the electric field strength vector lie in the same plane
Today, lidar installations (LIDAR, Light Identification, Detection and Ranging), in which a laser serves as a light beam source, are widely used to study the atmosphere. A small part of its radiation, having scattered in the atmosphere, returns back and is captured by the receiver. This makes it possible to calculate the distance from the setup to the region of the atmosphere that scattered the signal from the time of arrival of the reflected signal. The picture shows the lidar of the Pierre Auger observatory (Argentina)
The diagram clearly shows the principle of operation of the lidar installation. Unfortunately, the method has an insurmountable limitation: it requires a clear sky - in dense clouds, the laser beam is lost almost completely
Noctilucent clouds form at an altitude of about 80 km, in the region bordering between the meso- and thermosphere, the so-called mesopause. The mesosphere is cold - the temperature in it drops to -150°C. The thermosphere is characterized by high temperatures- air (if this monstrously rarefied substance can be called that) under the influence of solar radiation sometimes heats up to 1500 K. The concentration of gas molecules in the thermosphere is so low that the mechanisms of thermal energy transfer familiar to us practically do not work, and the only way to cool down is to radiate energy. In such difficult conditions, noctilucent clouds "live"
The reason why noctilucent clouds are observed at night and not during the day is clear from the above diagram. While the observer is still in "night territory", noctilucent clouds fall into the sunlit zone; Noctilucent clouds "love" not just the night, but the summer night. The reason is simple. Oddly enough, the upper mesosphere cools most of all in summer: the dynamics of air flows in the atmosphere are to blame for this. There are also no problems with crystallization centers - after all, microparticles of meteoric origin are really present in the mesosphere
In June 1885, with an interval of several days, an unusual phenomenon was noticed by several European astronomers: strange clouds of a structure not seen before, glowing in the evening or early morning twilight, when the Sun was below the horizon. In Germany, this phenomenon was observed by astronomers Otto Jesse and Thomas William Backhouse, in Austria-Hungary by Vaclav Laska, in Russia by Vitold Karlovich Tserasky. Since all the first observations were made independently of each other, it would be unfair to consider someone alone to be the discoverer. Jesse and Tserasky paid the most serious attention to the new phenomenon. The latter managed to establish with acceptable accuracy the height of new clouds above the Earth's surface - about 75 versts. He was the first to establish the negligible optical density of clouds - the brilliance of the stars "closed" by them almost did not lose strength! Jesse also made corresponding measurements, but with somewhat less accuracy. But it was he who came up with the name that has been common since then - “silver clouds”. In English-language literature, this phenomenon is usually called noctilucent clouds or (especially in NASA materials) polar mesospheric clouds - PMC.
Conditions of existence
By the end of the 19th century, there were many astronomers in Europe who regularly observed the sky. None of them described anything resembling noctilucent clouds until the summer of 1885. Maybe observations of clouds were not recorded in scientific history due to triviality? But the same Witold Tserasky by 1885 had been engaged in photometry of the twilight sky for about ten years. This painstaking task required close attention to any cloud that could distort the data. Tserasky wrote: “It would be quite difficult for me not to notice a phenomenon that sometimes covers no more no less than the entire firmament.” Otto Jesse was of the same opinion. Therefore, we will proceed from the fact that noctilucent clouds were not really observed before the summer of 1885 and probably did not exist. Of course, attempts to explain the novelty of nature were made very soon. The most logical explanation at that moment seemed to be the catastrophic eruption of the Krakatoa volcano on the territory of modern Indonesia, which led to a powerful explosion that literally lifted the whole island into the air. There were other theories - we will consider them below. But before talking about the noctilucent clouds themselves, it is worth paying attention to the conditions in which they exist.
The Earth's atmosphere is a complex object characterized by various conditions. According to its height, it is customary to subdivide it into the troposphere (up to 10 km), the stratosphere (10–50 km), the mesosphere (50–85 km), the thermosphere, and the exosphere. Noctilucent clouds form in the region bordering between the meso- and thermosphere - the so-called mesopause.
The physical conditions above and below the mesopause are different. The mesosphere is cold - the temperature in it drops to -150°C. The thermosphere, on the contrary, is characterized by very high temperatures - the air under the influence of solar radiation is sometimes heated up to 1500K. The concentration of gas molecules in the thermosphere is so low that the mechanisms of thermal energy transfer that we are used to do not work, and the only way to cool down is to radiate energy.
Now imagine what kind of clouds can appear in such "hard" conditions? Ordinary cirrocumulus clouds "live" in the troposphere, at an altitude of 5-6 km, and are something like water fog. A cloud that can form at an altitude of 70 km can be compared with a person who has adapted to existence without protective equipment, for example, on Jupiter ...
Where did they come from?
Above, we mentioned the volcanic hypothesis of the formation of noctilucent clouds, proposed by the German physicist Friedrich Kohlrausch at the end of the 19th century. Alas, subsequent studies have shown that the properties of clouds and the properties of volcanic aerosols suspended in the atmosphere are very different.
In the 1920s, meteorite researcher Leonid Kulik proposed the hypothesis of the meteorite origin of noctilucent clouds - according to it, they consist of the smallest particles of meteorite substance dispersed in the upper atmosphere. Indeed, studies of the mesosphere by meteorological rockets as early as the 1960s showed that there is a certain amount of material of obviously meteorite origin in noctilucent clouds. But by that time, the scientific mainstream was already another theory - condensation, which was initiated by the Soviet physicist Ivan Andreevich Khvostikov.
Important feature noctilucent clouds lies in the fact that they are observed from year to year at the same heights (about 80 km), the same latitudes (50-70 degrees) and only in summer, and all these rules are fulfilled both in the Northern and in the southern hemispheres. Neither the volcanic nor the meteor hypothesis could explain these facts. The condensation version suggests that noctilucent clouds consist of tiny ice crystals frozen on aerosol particles. The zone of occurrence of these ice floes is located at an altitude of about 90 km, from there they gradually drift downwards under the influence of gravity, increasing in size. At an altitude of about 85 km, their clusters become visible at dusk with solar illumination from below - clouds appear. For the formation of such ice floes, at least three conditions are needed: low temperature, sufficient humidity, and the presence of crystallization centers.
Biggest Problem consists of air humidity. The upper kilometers of the mesosphere are drier than the Sahara - water is negligible there and it comes there mainly from two sources. This is, firstly, water vapor from below, and secondly, the destruction of methane molecules under the influence of solar ultraviolet radiation, after which water is formed with the participation of atmospheric oxygen. The difficulty is that water molecules also break up under the action of solar radiation - the average time of their life in the mesopause is calculated in several days. While there is no complete clarity as to under what conditions and at what time a sufficient amount of water can accumulate in the mesopause, therefore, with all the plausibility of the condensation version, the issue is far from being closed.
Learning Tools
The study of noctilucent clouds is not an easy task. The air above the stratosphere is so rarefied that neither an airplane nor a balloon can stay in it; the only aircraft capable of reaching such heights is a rocket. This creates considerable inconvenience for researchers: a rocket flying at high speed stays in the area under study for a few seconds and has very limited contact with the environment. Its launch is not possible from everywhere and is quite expensive.
In the first half of the 20th century, it was proposed to use optical sounding to study the atmosphere. At first, a powerful searchlight was used for this. The observed scattering of the light beam provided information on the composition and state of the air masses. In the United States, searchlight sounding was mainly used to determine air density and temperature; in the USSR, the study of atmospheric aerosols was also considered an important task, for which the searchlight beam was polarized and then the distribution of polarization with height was studied. Of course, a searchlight as a light source was not very convenient - the sounding ceiling never exceeded 70 km.
Since the 1960s, so-called lidar installations have been increasingly used to study the atmosphere, in which a laser serves as a light beam source. A small part of its radiation, having scattered in the atmosphere, returns back and is captured by the receiver. Laser radiation is coherent, its wavelength and polarization can be determined with great accuracy. The laser beam can be emitted for a period of time determined with high precision. This sets the length of the light beam. This makes it possible to calculate the distance from the setup to the region of the atmosphere that scattered the signal from the time of arrival of the reflected signal with an accuracy of several meters. Well, the characteristics of the reflected (scattered) radiation carry information about the medium from which it was reflected.
The second important tool is the study of the polarization of light. The fact that the sunlight we see is polarized was discovered by Francois Arago back in 1809, he also established that the polarization maximum is at an angular distance of 90 degrees from the Sun. The degree of polarization of light is affected by the properties of the medium on which it is scattered. This is the basis of the method. It is especially remarkable that at dusk, when the Sun below the horizon illuminates the earth's atmosphere from below, polarimetry provides information about the properties of a particular layer of air, which is most brightly illuminated at that very moment. Thus, by measuring the polarization during twilight, one can obtain the distribution of properties over height.
With the beginning of the space age, the question arose that it was possible to observe noctilucent clouds from space as well. The first device designed specifically for the study of the mesosphere and noctilucent clouds was the American satellite AIM (The Aeronomy of Ice in the Mesosphere), launched in 2007 and still operating in orbit.
…and the Tunguska meteorite
The most famous case of mass observation of noctilucent clouds occurred in the summer of 1908, immediately after the fall of the Tunguska meteorite and, logically, in connection with it. Almost all over Europe, because of the luminous clouds, "white nights" have come - even where no one has ever heard of them. Eyewitnesses recalled that there was enough light in the middle of the night to read a newspaper. Unfortunately, almost no reliable instrumental measurements have been carried out, and contemporary estimates strongly diverge - the illumination of those nights is estimated as exceeding the natural background by 10-8000 times.
Contemporaries, as a rule, did not associate unusual clouds with the Tunguska meteorite, since they did not know about its existence. The very fact of the fall of some celestial body somewhere in the Yenisei province was known - they even tried to look for it, but scientists were able to assess the true scale of what happened only two decades later. In addition, just in those places, atmospheric anomalies, at least obvious ones, were not observed. Night illumination was explained by volcanism, which at that time sounded plausible.
From the point of view of today's ideas, the noctilucent clouds of the summer of 1908 are still more likely to be connected with Tunguska - but how? Although there are about a hundred versions of what happened in 1908, two scientists enjoyed the greatest confidence: meteorite and comet. Meteoritic stumbles upon a fundamental problem - where did the pebble go? The cometary seems to be better in all respects, but the appearance of noctilucent clouds within its framework looks difficult to explain. The substance dispersed in the atmosphere should have flown east from Vanavara, and noctilucent clouds would have been visible in Vladivostok and Tokyo - but nothing of the kind happened. In addition, the size of the comet "aura" reaches hundreds of thousands, and sometimes millions of kilometers. Approaching the Earth approximately from the side of the Sun, the tailed guest should have sprayed in the atmosphere a couple of days before the fall, and the rotation of the Earth in a completely natural way would distribute all the matter evenly around the circumference.
So it turns out that the mysterious Tunguska phenomenon greatly increases the number of questions to noctilucent clouds. 125 years after Privatdozent Vitold Karlovich Tserasky saw unusual clouds in the sky in the morning, we still cannot say with certainty that we understand where and how they came from.
Silver clouds - what is it.
General information about noctilucent clouds.
Noctilucent clouds were first seen in 1885. Prior to this, there was no information about noctilucent clouds. The discoverer of silvery clouds is considered to be V.K. Tserasky, Privatdozent of Moscow University. He observed noctilucent clouds on June 12, 1885, when he noticed unusually bright clouds in the predawn sky filling the twilight segment. The scientist called them night luminous clouds. The scientist was especially surprised that the clouds stood out brightly against the background of the twilight segment, and completely disappeared, going beyond it. He was very worried about this, since they, not being visible, can absorb the light of stars and distort the results of photometric measurements. But the very first measurements of luminous clouds showed that these clouds are very transparent and do not noticeably weaken the light of stars. Noctilucent clouds form at an altitude of 73 to 97 km, with a maximum of their distribution of 83-85 km, when the temperature drops to 150-165 K. Although this is an atmospheric phenomenon, historically its research is considered astronomical, since a number of phenomena in our atmosphere are so or otherwise associated with the processes occurring on the Sun, with meteor showers. In addition, the study of the atmospheres of other planets is inextricably linked with the study of our own atmosphere. In addition, noctilucent clouds, unlike other clouds, are observed at night, and their observation and registration of their appearance can be carried out simultaneously with the observation of other astronomical phenomena or objects.
Noctilucent clouds can be observed from March to October in the northern hemisphere and from November to April in the southern hemisphere. But most often in the northern hemisphere they are observed from late May to mid-August (with a maximum peak in June-July), in the southern hemisphere during the winter months.
The range of observations is limited to latitudes from 50 to 65 degrees. But known rare cases their observations at lower latitudes - up to 45 degrees. In the book by V.A. Bronshten "Noctilucent Clouds and Their Observation" presents data from a catalog of noctilucent clouds compiled by N.P. Fast on the basis of 2000 observations for 1885-1964. This catalog gives the following distribution of observation points by latitude:
Latitude......................... 50...... 50-55..... 55-60..... 60 Number of observations (%)......... ..3.8 ......28.1 ......57.4 ......10.8
What is the reason for this? At this time, it is in these latitudes that favorable conditions are created for their visibility, since it is at these latitudes that at this time the Sun sinks shallowly below the horizon even at midnight, and beautiful silvery formations are observed against the background of the twilight sky, resembling light cirrus clouds in structure. This happens because they glow mainly by the reflected light of the Sun, although some of the rays they send out may be born in the process of fluorescence - the re-emission of energy received from the Sun at other wavelengths. In order for this to happen, it is necessary that the rays of the Sun illuminate the noctilucent clouds. Knowing their average height above the earth's surface, we can calculate that the Sun's immersion should not exceed 19.5 degrees. At the same time, if the Sun has sunk less than 6 degrees, it is still too light (civil twilight) and clouds may not be visible in a bright sky. Thus, the most favorable conditions for the observation of noctilucent clouds correspond to the time of the so-called navigational and astronomical twilight, and their probability is the greater, the longer this twilight is. Such conditions are created in summer at middle latitudes. It is at mid-latitudes from late May to mid-August that noctilucent clouds are most often observed. True, this coincidence is purely coincidental. In fact, noctilucent clouds form precisely in the summer period and precisely at middle latitudes, because at this time at these latitudes there is a significant cooling in the mesopause, and the necessary conditions are created for the formation of ice crystals.
The first assumptions about the nature of noctilucent clouds were associated with the eruption of the Krakatau volcano on August 27, 1883. In the twenties of the 20th century, L.A. Kulik, a researcher of the famous Tunguska meteorite, put forward a meteor-meteorite hypothesis of the formation of noctilucent clouds. Kulik also suggested that not only giant meteorites, but also ordinary meteors are the source of the formation of noctilucent clouds. The meteor hypothesis was popular for a long time, but could not answer a number of questions:
- Why do they appear in a narrow range of heights with an average value of 82-83 kilometers?
- Why are they observed only in summer and only in middle latitudes?
- Why do they have a characteristic fine structure, very similar to that of cirrus clouds?
The answer to all these questions was given by the condensation (or ice) hypothesis. This hypothesis was seriously substantiated in 1952 in the work of I.A. Khvostikov, who drew attention to the external similarity of noctilucent and cirrus clouds. Cirrus clouds are made up of ice crystals. I.A. Khvostikov suggested that noctilucent clouds have the same structure. But in order for water vapor to condense into ice, certain conditions are needed. In 1958 V.A. Bronshten explained the seasonal and latitudinal effects of the appearance of noctilucent clouds by the fact that it is at mid-latitudes in the summer in the mesopause that the temperature drops to extremely low values of 150-165 K. Thus, the hypothesis of I.A. Khvostikov about the possibility of formation in this area the atmosphere of noctilucent clouds has been confirmed.
True, the researchers faced one more question: does water vapor exist at such a high altitude in an amount sufficient to form noctilucent clouds? The work of scientists in this direction gave an unexpected result. A distinct maximum of water vapor content was established in July-August and a minimum in January-February (in the northern hemisphere). That is, the fact of an increase in humidity in those seasons, over those latitudes and at the level where noctilucent clouds are formed, has been established. This fact has a simple explanation: above 25-30 kilometers at middle latitudes, ascending air currents are observed in summer, which carry water vapor to the mesopause region. There, water vapor freezes out, forming silvery clouds. Its disadvantage is compensated by a new influx of steam from below. At other latitudes and in other seasons, ascending air currents either do not occur or are suppressed by the absence of freezing. There is another explanation. It consists in the fact that water vapor at high altitudes is formed by the interaction of hydrogen atoms flying to the Earth from the Sun with oxygen atoms of the upper layers of the earth's atmosphere. This idea was expressed by the Norwegian scientist L. Vegard in 1933 and received a quantitative justification in 1961 in the work of the French scientist C. de Tourville. True, this "sunshine" hypothesis has weaknesses and cannot fully explain the increased humidity in the mesopause. In recent years, some researchers have put forward another source of water vapor supply to the mesopause. Such a hypothesis is supported, for example, by L. Frank, a professor at the University of Iowa, a Russian scientist, V.N. Lebedinets, and some other scientists. They believe that the mesopause region is supplied with water vapor in sufficient quantities for the formation of noctilucent clouds of a mini-comet. What particles serve as condensation nuclei in the formation of noctilucent clouds? Various assumptions have been made: particles of volcanic dust, crystals of sea salt, meteor particles. The hypothesis that it is meteor particles that serve as condensation nuclei was put forward by L.A. Kulik in 1926 in his meteor-meteorite hypothesis of the origin of noctilucent clouds. In 1950, this hypothesis was again independently put forward by V.A. Bronshten.
The hypothesis of the cosmic origin of condensation nuclei is now preferred. In fact, the destruction of meteoroids that penetrate the earth's atmosphere and are observed in the form of meteors occurs mainly just above the mesopause, at altitudes of 120-80 km. Studies show that up to 100 tons of matter “falls” to the Earth every day, and the number of particles with a mass of 10 grams, suitable as condensation nuclei, is quite enough to ensure the formation of noctilucent clouds. Attempts were made to find a connection between the appearance of noctilucent clouds and the intensity of meteor showers.
Structure of noctilucent clouds.
In 1955 N.I. Grishin proposed a morphological classification of forms of noctilucent clouds. Later she became international classification. The combination of various forms of noctilucent clouds formed the following main types:
Type I. Fleur, the simplest, even form, filling the space between more complex, contrasting details and having a foggy structure and a weak pale white glow with a bluish tint.
Type II. Stripes resembling narrow streams, as if carried away by air currents. Often arranged in groups of several pieces, parallel to each other or intertwined at a slight angle. The stripes are divided into two groups - blurry (II-a) and sharply defined (II-b).
Type III. Waves are divided into three groups. Scallops (III-a) - areas with a frequent arrangement of narrow, sharply defined parallel stripes, like light ripples on the surface of the water with a slight gust of wind. Ridges (III-b) have more noticeable signs of a wave nature; the distance between adjacent ridges is 10–20 times greater than that of scallops. Wavy bends (III-c) are formed as a result of the curvature of the cloud surface occupied by other forms (bands, scallops).
Type IV. Vortices are also divided into three groups. Small radius swirls (IV-a): 0.1° to 0.5°, i.e. no larger than the lunar disk. They bend or completely twist stripes, scallops, and sometimes fleur, forming a ring with a dark space in the middle, reminiscent of a lunar crater. Swirls in the form of a simple bend of one or more strips away from the main direction (IV-b). Powerful vortex ejections of "luminous" matter away from the main cloud (IV-c); this rare formation is characterized by rapid variability of its form.
But even within the type of noctilucent clouds are different. Therefore, in each type of clouds, groups are distinguished that indicate a specific structure of clouds (blurry stripes, sharply defined stripes, scallops, crests, undulating bends, etc.). Details on this classification of noctilucent cloud forms can be found in the book by V.A. Bronshten "Noctilucent Clouds and Their Observations". Usually, when observing noctilucent clouds, you can see several of their forms of different types and groups at once.
Types and methods of observations of noctilucent clouds.
Studies of noctilucent clouds are necessary for a deeper understanding of the circulation of the Earth's atmosphere, as well as many processes occurring outside the Earth, on the Sun. It is possible that the weather on Earth depends not only on the conditions in the troposphere, but also on the state of the higher layers of the atmosphere. Observations of noctilucent clouds are different, their organization, methodology and conduct depend on the tasks set. The following types of observations of noctilucent clouds can be distinguished:
- 1. Synoptic observations are systematic observations of the twilight segment in order to establish the fact of the presence or absence of noctilucent clouds, and in case of their visibility, the registration of some characteristic features.
- 2. Study of the structure. It can be done by visual observation, photographing or slow motion filming.
- 3. Studying the movements of noctilucent clouds. Produced by their successive photographing or slow motion filming. Here you may need a theodolite.
- 4. Determination of heights. To solve this problem, it is necessary to photograph noctilucent clouds at pre-agreed moments from two points separated by a distance of 20-0 km. Cameras in both cases should be the same. Need accurate clock. To process observations, you need a special palette.
- 5. Photometry and polarimetry. Produced from photographs. But to perform these tasks, special devices are needed.
These are the main types of observations. Some of the above tasks can be performed on the same observations. The same photographs can be used to study the structure, movements, determination of heights and photometry of noctilucent clouds. An observer-forecaster can photograph noctilucent clouds between recordings. The synoptic method is most acceptable for amateur observations of noctilucent clouds. It involves patrolling the twilight segment, statistics of noctilucent clouds, description of their structure and brightness. In my work, I mainly used the synoptic method of observing noctilucent clouds. The photographic method was used to study the structure of noctilucent clouds. The azimuth and height of noctilucent clouds above the horizon were also measured.