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ionosphere(īŏn`əsfēr), series of concentric ionized layers forming part of the upper atmosphereatmosphere
[Gr.,=sphere of air], the mixture of gases surrounding a celestial body with sufficient gravity to maintain it. Although some details about the atmospheres of other planets and satellites are known, only the earth's atmosphere has been well studied, the science of
..... Click the link for more information. of the earth from around 30 to 50 mi (50 to 80 km) to 250 to 370 mi (400 to 600 km) where it merges with the magnetosphere, the region of the Van Allen radiation beltsVan Allen radiation belts,
belts of radiation outside the earth's atmosphere, extending from c.400 to c.40,000 mi (c.650–c.65,000 km) above the earth. The existence of two belts, sometimes considered as a single belt of varying intensity, was confirmed from information
..... Click the link for more information. . The degree of ionization and the heights of the ionized layers fluctuate on a daily and a seasonal basis and show latitudinal variations as well. Causes for other variations in characteristics may include changes in the amount of ultraviolet radiation received from the sun and effects of the earth's magnetic field. Ionization of nitrogen and oxygen molecules from X-rays and ultraviolet radiation from the sun produces a layer of charged particles which allows radio waves to be reflected around the world. Such activity makes possible long-distance wireless communication. The layers comprising the ionosphere are the D layer, E layer, and F layer (divided into F-1 and F-2). The lower layers have the lowest concentration of charged particles and reflect low frequency waves. The middle layers are called the Kennelly-Heaviside layers (named after Oliver Heaviside in England and A. E. Kennelly in the United States who independently discovered the existence and effects of the ionosphere); while the Appleton, or highest layer, has the highest concentration of charged particles due to the low density of gases.
ionosphere(ÿ-on -ŏ-sfeer) A region in the atmosphere of a planet, satellite, or comet that contains a high concentration of free electrons and ions produced by the ionizing action of solar X-ray and ultraviolet radiation on atmospheric constituents. The Earth's ionosphere extends from about 60 km to more than 1000 km in altitude. It disturbs the propagation of radio waves through it by reflecting or refracting them and attenuating them. The degree of ionization varies with time of day, season, latitude, and state of solar activity. The charged particles are found in four distinct layers: the D-layer lies below about 90 km, the E-layer lies between about 90 and 120 km, the F1 -layer (a daytime feature) is centered on about 150 km, and the F2 -layer is centered on about 300 km. Radio waves of sufficiently low frequency are reflected by the E- and F-layers. The D-layer, being lower in the atmosphere where collisions between molecules and ions are more frequent, is more an absorber than a reflector. The total dispersion measure of the ionosphere is about 5 × 1017 m–2 by day and 5 × 1016 m–2 by night.
The ionosphere is important in radio communications, allowing long-distance propagation at frequencies up to about 30 megahertz by successive reflections between it and the ground. In radio astronomy, however, it makes ground-based observations almost impossible below 10 megahertz, causes phase and amplitude scintillations (see scattering) on radio sources viewed through it, and alters the observed polarization of radio waves by Faraday rotation. Other bodies, including Mars, Venus, and the giant planets, the moons Titan and Triton, and comets Halley and Giacobini-Zinner, are now known to possess ionospheres.
the ionized part of the upper atmosphere, lying above 50 km. The upper boundary of the ionosphere is the outer part of the earth’s magnetosphere. The ionosphere is a natural formation of rarefied, weakly ionized plasma, which is located in the earth’s magnetic field and has, by virtue of its high electric conductivity, specific properties that determine the character of the propagation within it of radio waves and various disturbances. Radio transmission, a simple and convenient means of communication over long distances, is possible only because of the ionosphere.
The first hypotheses concerning the existence of a conducting layer high above the earth were advanced in connection with the investigation of the earth’s magnetic field and atmospheric electricity by K. Gauss (1839), W. Thomson (1860), and B. Stewart (1878). Soon after A. S. Popov’s invention of the radio (1895), A. Kennelly in the United States and O. Heaviside in Great Britain suggested almost simultaneously (in 1902) that the propagation of radio waves beyond the limits of direct visibility is due to their reflection from a conducting layer lying at altitudes of 100–300 km. Scientific research on the ionosphere was begun in the 1920’s, using ionospheric sounding stations; reflections from the corresponding regions of the ionosphere were observed of short radio signals of different wavelengths sent from earth. In 1912 the English scientist W. Eccles proposed a mechanism by which charged particles affect radio waves. In 1923 the Soviet scientist M. V. Shuleikin reached the conclusion that there are at least two layers in the ionosphere. In 1931 the English scientist S. Chapman constructed a theory of a simple layer which, in rough approximation, describes the ionosphere. Major contributions were also made by the Soviet scientists D. A. Rozhanskii, M. A. Bonch-Bruevich, A. N. Shchukin, and S. I. Kriuchkov and by the English scientists J. Larmor and E. Appleton.
Observations throughout a worldwide network of stations made it possible to produce a global picture of variation in the ionosphere. It was established that the concentration of ions and electrons in the ionosphere is distributed nonuniformly with respect to altitude: there are regions, or layers, where this concentration reaches a maximum (see Figure 1). There are several such layers in the ionosphere; they do not have clearcut boundaries, and their location and intensity change regularly throughout the day, season, and the 11-year solar cycle. The upper, F, layer corresponds to the principal maximum ionization of the ionosphere. At night it rises to altitudes of 300–400 km, and during the day (chiefly in summer) it bifurcates into the F] and F 2layers, with maximums at altitudes of 160–200 km and 220–320 km, respectively. The E region is found at altitudes of 90–150 km; the D region, below 90 km. The stratified nature of the ionosphere is due to the abrupt variation, with respect to altitude, in the conditions of its formation (see below).
The use first of rockets and later also of satellites has made it possible to obtain more reliable information concerning the upper atmosphere, to measure directly with rockets the ionic composition (by means of the mass spectrometer) and principal physical characteristics of the ionosphere (temperature, the concentration of ions and electrons) at all altitudes, and to investigate the sources of ionization—the intensity and spectrum of the shortwave ionizing radiation of the sun and of various corpuscular beams. This has made it possible to explain the regular changes in the ionosphere. By using satellites that carry an ionospheric station and sounding the ionosphere from above, it has been possible to investigate the upper part of the ionosphere, which lies above the maximum of the F layer and which therefore is inaccessible for study by ground stations.
It has been established that the temperature and the electron density nein the ionosphere increase sharply up to the F region (see Table 1 and Figure 2); in the upper part of the ionosphere the increase in temperature slows down, and above the F region n, declines with altitude, first gradually to altitudes of 15,00020,000 km (the so-called plasmopause) and then more abruptly, changing to low values of nein the interplanetary medium.
In addition to rockets and satellites, new ground methods of investigation, which are especially important for studying the lower part of the ionosphere in the D region, have been successfully developed. These include the methods of partial reflection and cross modulation; measurement, by means of rheometers, of the absorption of cosmic radiation at various frequencies; investigation of the field of long and superlong radio waves; and the method of oblique and reverse-oblique sounding. Of great importance is the method of incoherent back (Thomson) scattering, which is based on the principle of radar, in which a short powerful radio pulse is sent, and then the weak scattered signal, expanded in time as a function of the distance to the point of scattering, is received. This method not only makes it possible to measure the nedistribution to very high altitudes (1,000 km or more), but it also shows the temperature of electrons and ions, the ionic composition, regular and irregular motions, and other parameters of the ionosphere.
The processes of ionization and recombination take place continuously in the ionosphere. The concentrations of ions and electrons observed in the ionosphere are a result of the balance between the rate of their formation in the process of ionization and the rate of their destruction through recombination and other processes. The sources of ionization and the processes of recombination are different in different regions of the ionosphere.
During the day the shortwave radiation of the sun with a wavelength X shorter than 1,038 A is the primary source of ionospheric ionization, but corpuscular beams, galactic and solar cosmic rays, and other factors are also important. Each type of ionizing radiation has a maximum effect on the atmosphere only within the particular range of altitudes that corresponds to its penetrating power. For example, the soft shortwave radiation of the sun, with X = 85–911 A, forms ions mostly within the 120–200 km range (but also at greater altitudes), whereas longer radiation, with X = 911–1,038 A, causes ionization at altitudes of 95–115 km, that is, in the E region, and X-radiation, with X shorter than 85 A, in the upper part of the D region, at altitudes of 85–100 km. In the lower part of the D region, below 60–70 km during the day and below 80–90 km at night, ionization is caused by so-called galactic cosmic rays. At altitudes of about 80 km, corpuscular beams (for example, electrons with an energy of S 30–0 kilo electron volts) and solar radiation from the first line of the Lyman series (La) of hydrogen, with X = 1, 215.7 A, make a significant contribution to the ionization in the D region.
The discussion so far has dealt with the ordinary conditions of ionization. During solar flares a burst of X-radiation causes an abrupt disturbance in the lower part of the ionosphere. Several hours after the solar flares, solar cosmic rays, which bring about increased ionization at altitudes of 50–100 km that is especially strong at the polar caps (the regions near the magnetic poles), also penetrate the earth’s atmosphere. In the auroral zones, fluxes of protons and electrons, which cause not only ionization but also noticeable atmospheric luminosity (the auroras) at altitudes of 100–120 km are active at certain periods of time. They are also active in the lower, D region. During magnetic storms, these corpuscular beams are intensified and their zone of activity expands to lower latitudes (sometimes these so-called low-latitude red auroras can be seen at the latitude of Moscow and to its south).
The reverse of ionization is the process of neutralization, or recombination. The rate of disappearance of ions in the ionosphere is characterized by the effective recombination coefficient a’, which determines the magnitude of neand its change in time. For example, when the source of ionization is known, that is, when the rate of formation of ions per cu cm in 1 sec is q, then The values of a’ vary for the various regions of the ionosphere (see Table 1 and Figure 3).
Complex physico-chemical processes take place under the influence of ionizing radiation in the ionosphere. These may be subdivided into three types: ionization, ion-molecular reactions, and recombination, which correspond to the three stages of the life of ions: their formation, transformations, and destruction. In different regions of the ionosphere each of these processes is manifested uniquely, leading to a difference in ion composition according to altitude. For example, in daytime the positive molecular ions NO+ and 02+ predominate at altitudes of 85200 km, the atomic ions O+ predominate above 200 km in the F region, and H+ protons prevail above 600–1,000 km. In the lower part of the D region (below 70–80 km) there is considerable formation of complex ion hydrates of the (H20)„H+ type and of negative ions of which the most stable are N02 and NO 3. Negative ions are found only in the D region.
The ionosphere is continually changing. A distinction is made between regular changes and disturbance. Insofar as shortwave solar radiation is the primary source of ionization, many regular changes in the ionosphere are a result of a change either in the sun’s height above the horizon (daily, seasonal, and latitudinal changes) or in the level of solar activity (11-year and 27-day cycles).
So-called sudden ionospheric disturbances occur after solar flares, when ionizing irradiation intensifies dramatically. Disturbed states of the ionosphere are frequently connected with magnetic storms as well. Many of the phenomena in the earth’s upper atmosphere and magnetosphere are closely connected, because of the simultaneous influence of solar activity on all of them. When a solar corpuscular beam that is captured by the magnetosphere grows in the interplanetary space near the earth, there follows not only a disturbance of the geomagnetic field (a magnetic storm) but also a change in the radiation belts of the earth and an intensification of the corpuscular beams in the auroral zones. Additional heating of the upper atmosphere takes place as well, and the conditions for ionization of the ionosphere change. The changes and motions in the ionosphere in turn have an effect on variations of the geomagnetic field and other phenomena in the upper atmosphere.
The regular patterns of change in the ionospheric parameters—the degree of ionization or the ne, the ionic composition, and the effective recombination coefficient—are different in different regions of the ionosphere; this is due above all to the significant change with altitude in the concentration and composition of neutral particles in the upper atmosphere.
The lowest ne< 103 cm’1. These are observed in the D region (see Figure 2). In this region the strongest absorption of radio waves takes place because of the high concentration of molecules and the consequent high frequency of their collision with electrons. Occasionally this leads to the interruption of radio communications. Here, as in a waveguide, long and superlong radio waves are propagated. The D region differs from the rest of the ionosphere in that in addition to positive ions, negative ions, which determine many properties of the D region, may be observed in it. Negative ions are formed as a result of the triple collisions between electrons and neutral O2 molecules. Below 70–80 km the concentration of molecules and the number of such collisions increase so much that there are more negative ions than electrons. Negative ions are destroyed by mutual neutralization with positive ions. Since this process is very rapid, it explains the rather high effective recombination coefficient observed in the D region.
As night approaches, the electron density nein the D region declines sharply and, accordingly, so does the absorption of radio waves. Therefore, it was believed previously that the D layer disappears at night. During solar flares the intensity of X-radiation, which increases the ionization of the D region, increases sharply on the surface of the earth illuminated by the sun. This leads to an increase in the absorption of radio waves and sometimes even to total cessation of radio communications—a sudden ionospheric disturbance (the Dellinger effect). The duration of such disturbances is usually 0.3–1.5 hours. Longer and more significant absorption occurs at high latitudes (polar cap absorption, or blackout). Increased ionization here is caused by solar cosmic rays (primarily protons with an energy of several mega electron volts), which are able to penetrate the atmosphere only in the area of the geomagnetic poles (the polar caps), that is, where the magnetic lines of force are not closed. Polar absorption phenomena sometimes last several days.
The ionospheric region at altitudes of 100–200 km, which includes the E and Fi layers, is distinguished by highly regular changes, since it is here that most of the shortwave ionizing radiation of the sun is absorbed. The photochemical theory, which refines the theory of the simple layer of ionization, explains all regular changes in the neand ionic composition during the course of the day as a function of the level of solar activity. At night, because of the absence of ionizing sources in the 125–160-km range, the magnitude of nedeclines sharply; however, in the E region, at altitudes of 100–120 km, a rather high ne— — (3–30) X 103 cm-3—is usually retained. Opinions diverge concerning the nature of the source of nocturnal ionization in the E region.
Brief and unusually narrow layers of heightened ionization (sporadic Es layers), which consist mainly of the ions of metals such as Mg+, Fe+, and Ca+, are often found at D and E altitudes. Long-range transmission of television broadcasts is possible by virtue of Es. The accepted theory of the formation of Es layers is that of wind shear, according to which under the conditions of a magnetic field the motions of the gas in the atmosphere “drive” the ions to a region of zero wind speed, where the Es layer is formed.
The concentration of O+ ions becomes greater than 50 percent above the level of 170–180 km at the height of the day, and above 215–230 km in the morning, evening, and night. The conditions for the formation of the ionosphere are totally different above and below this level. For example, the F1 layer forms during the day, when the region of maximum ionization by shortwave solar radiation lies below this level. Therefore, the F1 layer is observed regularly on ionograms only when the sun is high above the horizon, chiefly in the summer and when there is low solar activity, but it is not generally observed during maximum activity in the winter. Favorable conditions for the formation of the F2 region are created above this level.
The behavior of the main ionization maximum, or the F region, is very complex and differs fundamentally from the behavior of the E and F1 regions. Thus, although the electron density in the F2 layer is a function on the average of solar activity, it changes sharply from day to day. The maximum ne in a 24-hour period may be strongly shifted with respect to midday, depending on the latitude, season, and even longitude. The unusual increase in ne in winter in comparison to summer is called seasonal anomaly. In the equatorial region there is one maximum nebefore midday and two maximums, located at the geomagnetic latitudes ±15° (the equatorial or geomagnetic anomaly), occur after midday and at night. During the ascension of the sun both maximums begin to diverge, shifting to higher latitudes and rapidly disappearing, while a new maximum is forming at the equator. Unusual behavior of the F region and, in particular, the formation of a narrow zone of reduced ionization running parallel to the auroral zone, where increased ionization is observed, have been detected at high latitudes. All this indicates that the changes in nein the F region are determined by a number of geophysical factors in addition to solar radiation.
The altitude of the principal maximum of ionization (hmaxF) in the middle latitudes of the northern hemisphere changes throughout the day in a complex manner (see Figure 4), dipping deeply in the morning and reaching a maximum near midnight. The altitude of the F layer is lower in the winter (curve I) than in the summer (curve II), and is higher when solar activity is great (curve III) than when it is low (curves I and II).
A new theory of the formation of the F region has recently been developed that takes into account the effect of ambipolar diffusion and that has explained many of the peculiar features of the F region, including, in particular, the principal anomaly— the formation of the maximum ne much higher than the ion-formation maximum, which lies in the 150-km range. The theory links the variations in the altitude of the F layer described above to the variation during the course of the day in the intensity of ionization and in the atmospheric temperature. The existence of the F layer at night is explained in terms of the influx of ions from above, from the protonosphere, where they accumulate during daylight. Because of the difference in the formative mechanism the altitude of the layer is higher at night than during the day.
Many of the features of the change in the upper part of the ionosphere, which lies above the maximum of the F region, duplicate the 24-hour progression and global distribution of the nein the maximum of the layer. This indicates a close connection between these regions of the ionosphere. Above the maximum of the F region the decline in the concentration of ions with altitude takes place according to the barometric formula. The share of lighter ions increases with altitude. Therefore, the predominance of O+ ions in the F region is replaced above 1,000 km during the day by a predominance of H+ ions (the protonosphere). At night the protonosphere drops to altitudes of about 600 km because of the lower temperature. In the upper part of the ionosphere an increase is found toward higher latitudes in the share of heavy ions at a given altitude. There is, analogously, a relation to the observed rise in temperature. However, the behavior of the ionosphere in the polar regions has not yet been fully explained.
The motions of fluxes of charged particles in the ionosphere lead to the appearance of turbulent inhomogeneities of electron concentration. The reasons for their appearance are the fluctuation in the ionizing radiation and the continual intrusion into the atmosphere of meteors, which form ionized tracks. The motion of ionized masses and the turbulence of the ionosphere affect the propagation of radio waves, causing fading.
The study of the ionosphere continues to develop in two directions—from the standpoint of its influence on the propagation of radio waves and in the investigation of the physicochemical processes occurring in it. The latter has given rise to the birth of the new science of aeronomy. Modern theory has made it possible to explain both the distribution of ions by altitude and the effective coefficient of recombination. The task is now to construct a unified global dynamic model of the ionosphere, which demands the combination of theoretical and laboratory investigations with the methods of direct measurement by rockets and satellites and systematic observation of the ionosphere by the network of ground stations.
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