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aurora(ô-ror -ă, -roh -ră) A display of diffuse changing colored light seen high in the Earth's atmosphere mainly in polar regions. Aurorae show varying colors from whitish green to deep red and often have spectacular formations of streamers or drapery. They occur principally at altitudes of approximately 100 km, forming around two irregular and changing auroral ovals that are centered on the Earth's magnetic poles; the ovals enlarge at times of high solar activity. Aurorae are caused by charged particles from the solar wind and solar flares that become trapped in the Earth's magnetosphere. Formed into beams, they are focused by the magnetic fields into the upper atmosphere, spiraling along the Earth's magnetic field lines toward the poles. Their interactions with atmospheric atoms and molecules produce the auroral emissions.
a glowing of the upper, rarefied layers of the atmosphere, caused by the interaction of atoms and molecules at altitudes of 90 to 1,000 km with high-energy charged particles, that is, electrons and protons, penetrating into the earth’s atmosphere from space. The collisions of particles with components of the upper atmosphere, such as oxygen and nitrogen, lead to the excitation of these components, that is, to a transition to a higher energy state. The return to the initial, ground state occurs by means of the emission of light quanta of the characteristic wavelengths, that is, through an aurora.
References to auroras can be found even in classical Greek and Roman literature. M. V. Lomonosov was the first to propose the electrical nature of the glow. The first charts of isochasms, curved lines of equal frequency of auroral occurrence, were compiled between 1860 and 1873 by E. Loomis of the USA and H. Fritz of Austria for the northern hemisphere and in 1939 by F. White and M. Geddes of New Zealand for the southern hemisphere. These charts indicate the existence of regions on the earth’s surface where auroras appear most often. The isochasms in each hemisphere are somewhat deformed, concentric circles with centers near the geomagnetic pole. The auroral zone is located 23° from the pole. Observations in the last decade, such as those of Ia. I. Fel’dshtein and O. V. Khorosheva from 1960 to 1963, have shown that the glow usually appears along an auroral oval, whose center is displaced 3° from the pole along the midnight meridian. The radius of the oval is about 20°, so that at approximately midnight the oval coincides with the auroral zone, while at other hours it is located at higher latitudes.
In the late 19th and early 20th centuries the Norwegian scientists K. Birkeland and C. Størmer advanced and developed ideas to the effect that the particles responsible for auroras are of solar origin. Subsequent investigations showed that both the frequency of occurrence and the intensity of auroras, especially in the middle latitudes, clearly correlate with solar activity. Auroras have surprisingly diverse forms of luminescence and arrangement. However, the arrangement at any instant may be considered to consist of various superimposed elementary forms of luminescence, which may be subdivided in a first approximation into homogeneous arcs and bands extending throughout the entire sky in the form of a straight or bent line, rays with a sizable vertical extension, diffuse and irregular patches, and large, homogeneous, diffuse surfaces. In many cases, auroras are located along lines of geomagnetic force. The average thickness of ray forms is approximately 200 m and decreases with increasing brightness.
The investigation of the auroral spectrum was begun by A. Ångstrom in 1869. In 1924, J. McLennan and G. Shrum of Canada showed that the green line with wavelength λ = 5,577 angstroms (Å) is radiated by atomic oxygen. Atomic oxygen also forms the lines of the red doublet 6,300-6,364 Å at an altitude of 200 to 400 km (type A aurora). The states corresponding to the radiation are metastable, and the lifetimes of the excited atoms are 0.74 and 110 sec. Beginning in the 1950’s, the auroral spectrum was investigated in the infrared and ultraviolet regions. In addition to the atomic lines, the spectrum of an aurora consists of systems of bands of neutral and ionized molecular nitrogen and oxygen. The radiation of ionized nitrogen with λ = 3,914 Å, together with λ = 5,577 Å, is the brightest in the visible part of the spectrum from 3,800 to 7,000 Å. Since the human eye has maximum spectral sensitivity at λ ~ 5,550 Å, in most cases auroras appear to be pale green. Some auroras are characterized by a purplish red boundary as a result of the radiation of bands of neutral molecular nitrogen. Auroras with developed systems of molecular bands are classified as type B.
Incursions of protons with energies of 10 to 100 kiloelectron volts lead to the appearance of lines of the Balmer series in the auroral spectrum, as demonstrated by L. Vegard of Norway in 1939 and by A. B. Meinel of the USA in 1950. The line Hα with λ = 6,563 Å is the most intense. The hydrogen lines differ from the others in that they are significantly broader and, in observations in the direction of the zenith, prove to be shifted toward the region of the spectrum with shorter wavelengths. This Doppler shift of hydrogen lines was the first proof that auroral radiation is due, albeit partially, to the entry of beams of charged particles into the earth’s atmosphere. The luminescence associated with protons has the form of a weak band extending several hundred km in latitude and several thousand km in longitude. The spectral lines of helium are sometimes observed in auroras.
The auroral spectrum varies with latitude. Red auroras of type A usually predominate in the middle latitudes, type B auroras predominate in the latitudes of the auroral zone, and type A auroras predominate at the polar caps. In the circumpolar region the uniform polar cap aurora, with λ = 3,914 Å, appears after intense chromospheric flares on the sun; this auroral form is due to the direct entry of solar protons with energies from 1 to 100 megaelectron volts penetrating to altitudes of 20 to 100 km. The intensity of the aurora is measured in international brightness coefficients (IBC) or by intensity classes. Four intensities have been established, each differing in brightness by an order of magnitude. An aurora of intensity I is equivalent to the brightness of the Milky Way and corresponds to the radiation of 109 quanta/cm2-sec with λ = 5,577 Å, or to 1 kilorayleigh. Intensity corresponds to the brightness of the full moon, that is, to the radiation of 1012 quanta/cm2-sec with λ = 5,577 Å, or to 1,000 kilorayleighs.
The entry into the atmosphere of particles causing auroras is a result of the complex interaction of the solar wind with the geomagnetic field. Under the influence of the solar wind, the magnetosphere becomes asymmetric and stretches away from the sun. Auroras on the night side of the earth are related to processes in the plasma sheet of the magnetosphere. During magnetic storms a ring-shaped current of protons is formed within the magnetosphere at a distance of 3 to 5 earth radii. The magnetic field of this current deforms the lines of force of the magnetosphere, and auroras are observed much closer to the equator than usual. On the sunlit side of the earth, the plasma of the solar wind reaches the upper layers of the atmosphere through a funnel formed by diverging lines of force (the daytime cusp). The sequence of the forms and movements of auroras is closely related to specific phenomena occurring in the magnetosphere, that is, to magnetospheric substorms, during which the magnetosphere enters an unstable state. The return to a state of lower energy is explosive in character and is accompanied by the release of approximately 1022 ergs of energy in 1 hour. This causes the glowing of the atmosphere, which is the auroral substorm.
During the interaction of high-speed electrons with atoms and molecules of the atmosphere, X rays are formed as the bremsstrahlung of the electrons. The bremsstrahlung is much more penetrating than the particles and therefore reaches altitudes of 30 to 40 km. Auroras emit infrasonic waves with periods ranging from 10 to 100 sec, which are accompanied by fluctuations of atmospheric pressure of 1 to 10 dynes/cm2.
The study of auroras has two essentially different aspects. First, since optical radiation is one of the final results of processes occurring in space between the earth and the sun, it can serve as a source of information about processes in circumterrestrial space, particularly in investigations of the magnetosphere. Second, the effect of the primary flux of particles on the ionosphere can be assessed from data pertaining to optical radiation. Such investigations are necessary in connection with the problem of radio-wave propagation and other phenomena in the radio-frequency band, such as the appearance of sporadic E layers, the scattering of radio waves, and the occurrence of radiation and radio-frequency interference in the very low frequency range (3-30 kilohertz). Observations of auroras by means of television equipment have made it possible to establish the interlinkage of auroras in the two hemispheres and to investigate rapid changes in and the fine structure of auroras. Not all problems associated with the aurora can be solved by ground facilities or by observations of natural auroras.
The development of satellites and rockets has made it possible to study auroras in close connection with investigations of circumterrestrial space and to conduct direct experiments in the earth’s outer atmosphere and in interplanetary space. Thus, in 1969 the USA, in 1973 the USSR, and in 1975 the USSR and France conducted experiments to create artificial auroras. In these experiments a beam of high-energy electrons was injected into the atmosphere from a rocket at an altitude of several hundred km. Conducting controlled experiments together with ground observations opens up new ways of investigating auroras and other processes in the upper atmosphere. Since 1971–72 scientists have measured the intensity of individual emissions and photographed auroras from satellites in polar orbits. This has made it possible to determine the distribution of auroras throughout the entire region of the high latitudes in a few minutes.
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Krasovskii, V. I. “Nekotornye rezul’taty issledovanii poliarnykh siianii i izlucheniia nochnogo neba vo vremia MGG i MGS.” Uspekhi fizicheskikh nauk, 1961, vol. 75, issue 3.
Chamberlain, J. Fizika poliarnykh siianii i izlucheniia atmosfery. Moscow, 1963. (Translated from English.)
Akasofu, S.I. Poliarnye i magnitosfernye subburi. Moscow, 1971. (Translated from English.)
Osaev, S. I., and M. I. Pudovkin. Poliarnye siianiia i protsessy v magnitosfere Zemli. Leningrad, 1972.
Omholt, A. Poliarnye siianiia. Moscow, 1974. (Translated from English.)
Störmer, C. The Polar Aurora. Oxford, 1955.
International Auroral Atlas. Edinburgh, 1963.
IA. I. FEL’DSHTEIN