Libmonster ID: RU-17279
Author(s) of the publication: Boris KHRENOV

by Boris KHRENOV, Dr. Sc. (Phys. & Math.), leading research assistant of the Skobeltsyn Research Institute of Nuclear Physics. Lomonosov Moscow State University

The upper atmosphere (more than 100 km high) is a boundary layer between our planet and Cosmos, which has been attracting attention of researchers for a long time. Ordinary observations with the naked eye of "falling stars" were explained by scientists as luminescence during combustion of extra-terrestrial massive bodies (meteoroids and fireballs) in a night-time upper atmosphere. Luminescence in the polar regions was understood as a result of intrusion of cosmic protons and electrons into the Earth atmosphere. The more detailed observations have shown that in different wavelength ranges the upper atmosphere transmits external electromagnetic radiation in a different degree, which at the time of formation of life on the Earth determined the form of this life. Keen interest in the upper atmosphere properties arose due to the flights of rockets and satellites which crossed all its layers. For orbital satellites the upper atmosphere is a medium in which their braking takes place. In case of re-entry of the descending module through the upper atmosphere, it is prohibited to use parachute for small density of the substance, and the only way to protect it from burning is covering of the module with a heatproof protective layer of sufficient thickness. Perfect knowledge of density distribution throughout the altitude in the upper atmosphere was needed for calculation of such layers.

стр. 41

The atmospheric structure. Right scale-altitude in the atmosphere. Left scale--pressure in mbars.

ATMOSPHERIC STRUCTURE AND COMPOSITION

Recently the problem of coexistence of upper and lower atmospheres drew a special attention. Despite the protective role of the upper atmosphere, the main flux of solar radiation passes through the whole atmosphere and reaches the Earth surface. Here a gigantic quantity of solar energy is absorbed, and enormous atmospheric and ocean fluxes of substance and energy arise, which cannot be compared with a part of solar energy absorbed in the upper atmosphere. It is therefore expected that even a small part of energy transferred from the lower to upper atmosphere can play a significant role in the formation of its characteristics.

When discussing the atmospheric characteristics it is usual to divide it into smaller parts than lower and upper. Up to an altitude of about 100 km the atmospheric composition is rather stable. It is a mixture of nitrogen (78 percent) and oxygen (21 percent) with admixtures of other gases (mostly argon about 1 percent), water vapors and dust particles (aerosols). The troposphere (altitude--8-9 km) consists approximately of a half of the atmospheric mass. Present here are water vapors and ice particles and also most aerosols originating from soil weathering and human activities. Water vapors and ice particles form clouds and gigantic cloud formations, which play a key role in atmospheric electrical processes. In the troposphere the temperature decreases depending on the altitude (close to sea level and dry land it is determined by temperature of the Earth surface heated by the Sun). In the stratosphere (at an altitude of 10-50 km) the atmospheric pressure is 1-2 orders lower than in the troposphere. With the same main gas composition in the stratosphere, an ozone layer (oxygen molecule O3) is of paramount importance as it is responsible for absorption of solar ultraviolet radiation. The stratospheric temperature rises with altitude due to increase in solar radiation intensity in the ultraviolet range.

The ionosphere is above the stratosphere, and it is composed of a mixture of free electrons, positive ions of atmospheric atoms and molecules. It should be noted that ionospheric ions make only a small part of the mass of the upper neutral atmosphere. The ionosphere plays a key role in terrestrial electrical processes. At altitudes of above 100 km its composition changes substantially. Atomic oxygen is of paramount importance at an altitude above 200 km. Helium and hydrogen become main ele-

стр. 42

merits at altitudes above 800 km, where atmospheric density is comparable with cosmic (extra-terrestrial) density of the substance.

At a distance of above 800 km from the Earth surface, the number of charged particles (electrons and protons or, in other words, ionized atoms of hydrogen) is still enough, and the Earth's magnetic field is great (it decreases markedly only at distances much more than the Earth's radius, i.e. 6,000 km), that is why there form Earth's radiation belts.

The Earth's internal belt was discovered by the American physicist Van Allen in 1958, and it consists mainly of protons. The external radiation belt (further from the Earth) consisting mainly of electrons was discovered in the same year by Soviet physicists (Sergei Vernov, Alexander Chudakov, Pyotr Vakulov, Yevgeny Gorchakov and Yuri Logachev). The both belts are subjected to the pressure of solar wind, and the pattern of belt particle distribution is non-symmetrical as particles are pressed to the Earth on the part of the Sun (the subsolar point is located on average at a distance of 10 earth's radii), and in the Earth's shadow particles of belts move away so far (hundreds of earth's radii) that the tail of the magneto-sphere extends beyond the Lunar orbit. The charged particles distributed around the planet and tied into a single system with a geomagnetic field are called the Earth's magnetosphere. The particles, which get into the trap of such field fluctuate between the planet poles as they push off from the geomagnetic poles by an increasing magnetic field. The areas of belt particle penetration into the upper atmosphere where particles are absorbed in collisions with atmospheric molecules and atoms are defined by the structure of geomagnetic field and have latitudes and longitudes known from the experiment. In the magneto-spheric tail, at large distances from the Earth, magnetic field intensity decreases, and some particles of solar wind can merge with terrestrial magnetosphere. The solar wind is solar corona plasma outflow into interplanetary space. At the level of the earth's orbit an average velocity of solar wind particles (protons and electrons) is about 400 km/s, and the number of particles is several dozens per 1 cm2. The magnetospheric tail serves as a place for formation of "spilling" particle fluxes which return to the Earth's polar regions and cause aurora polaris.

Various atmospheric layers absorb solar radiation in a different way. The atmospheric absorption of solar UV, X-rays and a flux of charged solar particles spilling from the

Radiation belts of the Earth (magnetosphere) under action of solar wind.

Intensity of charged particles measured on one loop of the satellite with polar orbit as a function of measurement time (UT) on the loop (in 1.5 h satellite is on the same latitude), which corresponds to measurement of dependence on the Earth's latitude. Intensity peaks of different origin are seen: 1--polar zones where cosmic particles penetrate through geomagnetic field, 2--zone of aurora polaris, 3--particles of internal radiation belt, 4--particles of external radiation belt, 1*--particles of the South Atlantic magnetic anomaly. Information provided by the satellite TATYANA (Moscow State University).

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magnetosphere is the topmost effect in the upper atmosphere. The solar radiation not only heats the atmosphere but also ionizes atoms and molecules. Heating of the Earth's surface by solar radiation is the most important effect in the troposphere. Here, as was said, solar radiation releases a gigantic energy, which is responsible for dynamics of the Earth's surface, i.e. the ocean and atmosphere.

Ions and electrons formed in the upper atmosphere recombinate into neutral molecules and atoms but recombination velocity in the discharged atmosphere is not high, and there forms an ionospheric layer (objectively several layers of ions with different charges and mass).

The force of gravity and the Earth's rotation entrap ions to move across the Earth's magnetic field with a resultant electrical field perpendicular to magnetic lines of force. The electrical field is vertical to the planet at the equator where the magnetic lines offeree are parallel to the Earth's surface. The Earth's electrical field forms also outside the atmosphere in crossing of the Earth's magnetic field by a flux of charged particles of solar wind. The configuration of this electrical field is connected organically with the solar wind direction and the Sun's activity. The energy released by a global electrical field depends on an ion quantity in the ionosphere which is replenished not only on the Earth's dayside at the expense of solar radiation and solar wind but also on its nightside at the expense of cosmic ray particles of the extra-solar origin. But storm discharges are the most effective source of ions. Globally about 2,000 thunderstorm formations act continuously in the Earth's atmosphere, and the lightning frequency is about 30 per second. The geographical maximum of thunderstorm activity is in the equatorial area over Africa, South-East Asia and South America. The Earth's electrical field potential ranges from 200 kV to 400 kV. In cloudless zones of the planet, the atmosphere is low conductive, therefore the terrestrial electrical generator operates on average with a capacity of about 105 MW

The atmospheric electrical discharges are accompanied by electromagnetic radiation splashes in a broad wavelength range from UV and visible light (transient light phenomena) to radio emission (atmospherics), which originate during local heavy perturbations of the electrical field in the area of the very discharge. Radio waves penetrating into the magnetosphere meet resonators in its structure which are "tuned" to definite wavelengths (when a wavelength is equal to the size of the magnetosphere structure). The value of similar structural elements

стр. 44

of the magnetosphere is measured in thousands of kilometers, which corresponds to radiowave frequencies of an order effractions of Hz. This frequency band is called "very small frequencies" and is of considerable interest in view of interaction of electrical phenomena in the atmosphere and fluctuations of magnetic and electrical fields in the magnetosphere and structural changes of the magnetosphere.

Thus, our idea of the atmospheric structure involves action of not only extra-terrestrial factors (first of all, solar radiation and solar wind) but also terrestrial factors such as thunderstorms and winds (which virtually are also connected with the solar energy in the end).

METHODS OF UPPER ATMOSPHERIC STUDIES

The upper atmospheric altitude (above 50 km) is inaccessible for instruments located on aircraft or high-altitude balloons. Direct measurements of atmospheric density, a flux of charged particles and electromagnetic field intensity became accessible with emergence of geophysical rockets which reach the upper atmospheric altitude on vertical take-off and landing. The rocket flight lasts a few minutes, therefore measurements are of a "point" nature, i.e. at a given instant of time and in a specified region of the Earth. Global long-term measurements are possible by "indirect" methods when instruments on board the Earth's artificial satellite take regular measurements of parameters associated with the above-listed primary data in line with the physical theory and/or experimental calibration of indirect parameters.

The properties obtained by measurements of electromagnetic radiation generated by charged particles during atmospheric ionization are thoroughly studied by indirect parameters properties of the upper atmosphere. Development of the technology of near-earth satellite systems has provided another possibility of measuring atmospheric parameters, i.e. by measuring radiation absorption on the way through the upper atmosphere from the "reference" source operating on board the satellite to a radiation receiver located on the Earth's surface or on board another satellite. Both methods are widely used on micro-satellites.

The author of this article has an intimate knowledge of the measurements carried out on the micro-satellites of Moscow State University called Universitetsky-Tatyana and Universitetsky-Tatyana-2 (short names Tatyana-1 and 2).

Profile of aurora polaris (bold-face line) and fluxes of charged particles according to the data from the satellite TATYANA as of December 29,2005. The X-axis denotes observation time in hours and fractions of an hour, and the X-axis above denotes latitude of the observation place.

стр. 45

Time profiles of flashes in the upper atmosphere typical of the ELF discharges. The radiation wavelength is 240-400 nm.

Time profile of the SPRITE discharge.

Apart from particle detectors which measure particle intensity directly in orbit (altitude 900-1,000 km, which is much higher than the upper atmospheric altitude), these satellites are equipped with instruments which register radiation intensity in ultraviolet and red ranges in the upper atmosphere vertically downward. These devices allowed to make a global chart of such radiation intensity throughout the atmosphere. It turned out that distribution of this intensity over satellite position latitude correlates with distribution of particle intensity measured directly in orbit. The association between luminescence of aurora polaris in the upper atmosphere and latitude of particle flux spilling from the magnetosphere is most pronounced. It was managed to measure luminescence delay time in the upper atmosphere (at an altitude of 100-200 km) relative to the time of crossing by the satellite of magnetospheric particles at the orbit altitude of 900 km.

The luminescence concentration in the upper atmosphere at an altitude of 90-100 km is clearly observed by video camera installed on other satellites and directed at the Earth's horizon. This luminescence is described by the theory of electron-ion recombination on the Earth's nightside.

The abovementioned examples of atmospheric nighttime luminescence testify to a prominent role of forces acting "from top to bottom". Moreover, there are examples of events when the upper atmospheric luminescence takes place under action of forces directed "upwards". In recent years numerous data are obtained on the upper atmospheric luminescence under action of electromagnetic pulse generated by lightning in the lower atmosphere. Electromagnetic pulse of lightning creates an electrical field front spreading upward to an altitude of above 50 km where atmospheric density is small and the field intensity for starting of a cascade process of electrical discharge decreases to values which a lightning pulse has. The cascade multiplication of electrons and photons in the high-altitude category is supplemented also with acceleration of free electrons existing in the ionosphere. Action of the factors typical for the Earth's atmosphere leads to a fancy combination of discharges and luminescence in a range of altitudes of the upper atmosphere. The observed types of discharge were given the respective exotic names: elf, sprite, blue jets...

The video patterns of discharges were supplemented with a temporary pattern of discharge development in

стр. 46

Map of flash distribution with the number of photons Qa>1023.

Map of flash distribution with the number of photons Qa < 5 • 1021.

the upper atmosphere. The instruments providing a time profile of discharges were installed on satellites Tatyana-1 and 2. Short-time light pulses of 1 to 5 ms which repeat sometimes for 100 ms are typical of the elf discharges. For the sprite discharges the pulse duration is higher.

The photon inventory in each discharge (its intensity) can be obtained by summing up of the number of photons in a time profile. Based on evidence derived from the satellites Tatyana it was managed to construct distribution of discharges according to intensity for all positions of satellite above the Earth's surface (global distribution of discharges). As it turned out most bright flashes take place above continents in the equatorial zone, i.e. above Africa, South America and Asia. This distribution of flashes repeats global distribution of thunderstorms. Less bright discharges have a more uniform global distribution. These discharges occur above cloudless areas of the atmosphere which is indicative of their cosmic origin, for example, connected with short-term drop-out of magnetospheric particles to the upper atmosphere just as particles responsible for aurora borealis.

Actually the origin of flashes without thunderstorms is a subject for further research because we have no sufficient data to prove one or another hypothesis of their origin. It is necessary to conduct a simultaneous study on flashes in different wavelength ranges from ultraviolet to radio-waves. This task is being already performed by measurements on the already operating micro-satellite Chibis. The research will go on by measurements on the satellite RELEC, prepared for launching in 2014.


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Boris KHRENOV, UPPER ATMOSPHERE: RENDEZVOUS OF TERRESTRIAL AND COSMIC FORCES // Moscow: Russian Libmonster (LIBMONSTER.RU). Updated: 20.11.2021. URL: https://libmonster.ru/m/articles/view/UPPER-ATMOSPHERE-RENDEZVOUS-OF-TERRESTRIAL-AND-COSMIC-FORCES (date of access: 04.12.2021).

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