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Сумский государственный университет

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Английский язык

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Dictionary of Geophysics, Astrophysic, and Astronomy.pdf

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aurora

ning signal was heard as a sharp, short duration crackle on a radio receiver. This bursty crackle of interference was called an atmospheric, to distinguish it from the internal and local site interference. The sum of many atmospherics from remote lightning strokes all over the world produces a steady background noise limit at these radio frequencies called atmospheric noise. Atmospherics were observed at lower frequencies and used as a measure of thunderstorm activity. Early receivers for this application were sometimes caller spheric receivers.

atmospheric tide Oscillations in any atmospheric ﬁeld with periods that are simple integer fractions of either a lunar or a solar day. In addition to being somewhat excited by the gravitational potential of the sun and moon, atmospheric tides are strongly forced by daily variations in solar heating. The response of these forcings is by internal gravity waves. Unlike ocean tides, atmospheric tides are not bound by coastlines but are oscillations of a spherical shell.

atomic mass The mass of an isotope of an element measured in atomic mass units. The atomic mass unit was deﬁned in 1961, by the International Union of Pure and Applied Physics and the International Union of Pure and Applied Chemistry, as 1/12 of the mass of the carbon isotope counting 6 neutrons (and 6 protons) in its nucleus.

atomic number The number of protons in the nucleus of a given element.

atomic structure calculations — one-electron models The calculation of possible states of an electron in the presence of an atomic nucleus. The calculations consist in obtaining the electron distribution or wave function about the nucleus for each state. This is achieved by solving the Schrödinger equation for the electron wave function in a ﬁxed Coulomb potential generated by the nucleus of the atom. The quantiﬁed nature of the possible solutions or states appear naturally when the conditions of continuity and integrability are applied to the wave functions. An important characteristic of the one-electron models is that they can be solved

exactly; the wave functions may be expressed in terms of spherical harmonics and associated Laguerre polynomials. Relativistic treatment is done through Dirac’s equation. Dirac’s equation leads to the ﬁne structure as a relativistic correction to Schrödinger’s solution. Another important result of Dirac’s equations is that even for non-relativistic cases one ﬁnds that the electron has two possible states, generally interpreted as two possible states of intrinsic angular momentum or spin.

atomic time Time as measured by one or more atomic clocks, usually a cesium-beam atomic clock or a hydrogen maser. Measured since January 1, 1958, it is the most uniform measure of time available and has, therefore, replaced Universal Time as the standard.

attenuation coefﬁcient In propogation of a signal, beam, or wave through a medium, with absorption of energy and scattering out of the path to the detector, the attenuation coefﬁcient α is

α = d−1 ln (S/S0) ,

where this is the natural logarithm, and S and S0 are the current intensity and the initial intensity. Since α is an inverse length, it is often expressed in terms of decibel per meter, or per kilometer. See beam attenuation coefﬁcient, diffuse atten- uation coefﬁcient.

attenuation efﬁciency factor The sum of the absorption plus scattering efﬁciency factors.

aulacogen Mantle plumes create regions of elevated topography which typically have three rift valleys at about 120◦ apart; these are aulacogen. These are also known as triple junctions, and they participate in the formation of new ocean basins. An example is the southern end of the Red Sea. Typically two arms participate in the opening of an ocean, and the third is known as a failed arm. The St. Lawrence river valley is a failed arm associated with the opening of the Atlantic Ocean.

aurora Polar lights. The aurora borealis (northern lights) and aurora australis (southern lights). Energetic electrons are trapped from the solar wind and spiral around the ﬁeld lines of the

aurora australis

Earth’s magnetic ﬁeld. They enter the Earth’s upper atmosphere where the ﬁeld lines intersect the atmosphere, i.e., in the polar regions. There they excite atoms in the high thin atmosphere at altitudes of 95 to 300 km. The red and green colors are predominantly produced by excitations of oxygen and nitrogen. The polar lights are typically seen within 5000 km of the poles, but during times of intense solar activity (which increases the electron population), they can become visible at midlatitudes as well. Any body that possesses both a magnetic ﬁeld and an atmosphere can produce aurorae. Aurorae are commonly seen not only on Earth but also the Jovian planets of Jupiter and Saturn.

aurora australis Southern light, aurora in the southern hemisphere. See aurora.

aurora borealis Northern light, aurora in the northern hemisphere. See aurora.

auroral cavity A region on magnetic ﬁeld lines which guides the aurora, typically within 10,000 km or so of Earth, where abnormally low ion densities are observed at times of strong aurora, presumably caused by it.

auroral electrojet A powerful electric current, ﬂowing in the auroral oval in the ionospheric E-layer, along two branches that meet near midnight. The branches are known as the eastward and westward auroral electrojets, respectively, and the region in which they meet, around 2200 magnetic local time, is the Harang discontinuity.

The electrojets are believed to be Hall currents in the ionospheric E-layer and to be a secondary effect of the currents linking Birkeland currents of region 1 with those of region 2. Because of Fukushima’s theorem, the magnetic disturbance due to the Birkeland current sheets on the ground is very weak, and the main signature of their circuit — which can be quite strong

—comes from the electrojets. The usual way of estimating the current ﬂowing in that circuit

—which is a major signature of substorms — is therefore by means of the AE, AL, and AU indices which gauge the strength of the electrojets.

auroral oval Circular region several degrees wide around the geomagnetic pole at a geomagnetic latitude of about ±70◦, its center shifted by about 200 km towards the nightside; the region in which aurora is observed at any instant, covering the region of the diffuse aurora, which is also where the discrete aurora can be seen. The auroral oval can be seen in satellite images in UV as a closed circle. From Earth, in visible light, in the auroral oval aurora can be seen nearly each night, during polar night for a full 24 hours. Shapes and structure of the aurora vary with local time: with a rather diffuse auroral brightening between local noon and midnight, quiet arcs during the evening hours up to around 21 local time, followed by homogeneous or rayed bands or draperies, which after about 3 local time, are complemented by patches at the southern rim of the auroral oval. These patches, together with short arcs, dominate the appearance of the aurora during the morning hours. The size of the auroral oval varies greatly; it grows during magnetic storms and may sometimes extend well beyond the region where aurora is ordinarily seen (auroral zone). At magnetically quiet times the oval shrinks and may assume a non-circular “horsecollar” shape, narrower near noon. Physically, the auroral oval is related to upward ﬂowing Birkeland currents coupling the ionosphere and magnetosphere. See Birkeland current.

auroral zone The region where auroras are ordinarily seen, centered at the magnetic pole and extending between magnetic latitudes 66◦ and 71◦. The auroral zone is generally derived from ground observations of discrete aurora, but it also approximates the statistical average of the auroral oval, averaged over many nights.

autumnal equinox The epoch at the end of Northern hemisphere summer on which the sun is located at the intersection of the celestial equator and the ecliptic; on this day, about September 21, the night and day are of equal length throughout the Earth. The date of autumnal equinox is the beginning of the Southern hemisphere spring. Autumnal equinox also refers to a direction of the celestial sphere: 12h RA, 0◦ declination, antipodal to the direction of the vernal equinox. See vernal equinox. After autumnal

AXAF

equinox, in the Northern hemisphere, the period of daylight becomes shorter and the nights longer, until the winter solstice.

available potential energy (Lorenz, 1955) The energy that could be obtained by some welldeﬁned process. Such process is usually an adiabatic (or isentropic) redistribution of mass without phase changes to a statically stable state of rest. The estimate of mean available potential energy is about 11.1 × 105 J m−2 in the Earth atmosphere and is of order of 105 J m−2 in a typical mid-latitude ocean gyre.

avalanche In Earth science, the sudden slumping of earth or snow down a steep slope.

average cosine Mean cosine of radiance or scattering.

average matter-density The mean amount of mass in a unit of volume of space. The relativity theory taken to the extreme would require that the distributions of matter density and of velocities of matter are speciﬁed down to the size of single stars, and then a cosmological model is obtained by solving Einstein’s equations with such a detailed description of matter. This approach would be mathematically intractable; moreover, sufﬁciently precise observational data are not available except for a small neighborhood of the solar system in our galaxy. Hence, for the purposes of cosmology, average values of physical quantities over large volumes of space must be given. Average matter density ρ¯ must also include the rest mass equivalent to radiation. In cosmology, the averaging volume is taken to be of the size of several galaxies at least, possibly of several clusters of galaxies. If the universe, represented in this way, is spatially homogeneous (see homogeneity), then ρ¯ does not depend on which volume is used to evaluate it and so it is well deﬁned at least in the mathematical sense. If the universe is inhomogeneous, then the value of ρ¯ depends on the averaging volume, and choosing the right volume becomes a problem that has not yet been solved in a general way.

averaging The mathematical procedure of calculating an average value of a given quantity.

In cosmology, average values of various quantities with respect to the volume of space are used in order to avoid introducing too detailed mathematical models of the real universe — they would be too difﬁcult to handle. Averaging is straightforward only for scalars (such as matterdensity, pressure, or rate of volume expansion; see average matter-density). For vectors (such as the velocity of matter-ﬂow) and tensors (see tidal forces for an example of a tensor) this simple procedure does not work; for example, the sum of two vectors attached to different points of a curved space does not transform like a vector under a change of the coordinate system. In particular cases, a suitable concept of averaging of such objects can be found by careful consideration of the physical processes being described.

Avogadro’s number The number of atoms or molecules in an amount of substance whose total mass, when expressed in grams, equals its atomic mass: NA = N/n = 6.02214199(47)× 1023 molecules/gm-mole, a fundamental constant of nature. N is the total number of molecules and n is the number of gram-moles. Named after Amadeo Avogadro (1776–1856).

away polarity One of two possible polarities of the interplanetary magnetic ﬁeld, corresponding to magnetic ﬁeld lines which, at the points where they are anchored in the sun, point away from it. In interplanetary magnetic sectors with away polarity, magnetic ﬁeld lines linked to the northern polar cap of the Earth come from the sun and contain polar rain, whereas those linked to the southern polar cap extend into the outer solar system and contain none.

AXAF Acronym of Advanced X-ray Astrophysics facility, a space-borne astronomical observatory launched in July 1999, devoted to the observation of soft and medium energy X- rays, and renamed “Chandra” to honor Subrahmanyan Chandrasekhar. Imaging resolution is 0.5 to 1 sec of arc (comparable to that of groundbased telescopes without adaptive optics), over the photon energy range of 0.2 to 10 keV. The ﬁeld of view is 31 x 31 square arcmin. Two grating spectrometers yield a maximum spectral resolving power (E/*E) 1000 over the energy range from 0.09 to 10 KeV. Chandra provides

axial dipole principle

an order of magnitude improvement in resolution and two orders of magnitude improvement in sensitivity over the imaging performances of the Einstein observatory (HEAO-2). The improvement in spectral resolving power is also very signiﬁcant: for comparison, the spectrometers on board the Japanese X-ray observatory ASCA, operating since 1993, had maximum energy resolving power E/*E ≈ 50 between 0.5 and 12 KeV. Chandra is expected to detect supernova remnants in M31, to resolve single galaxies in the Virgo Cluster, and distant quasars that may contribute to the diffuse X-ray background. The Chandra spectrometers are, in principle, able to resolve emission lines and absorption edges from hot plasmas, such as the intra-cluster medium in clusters of galaxies, making feasible a study of their physical properties and of their chemical composition, and to resolve the proﬁle of the prominent iron K lines, which, in active galactic nuclei, are thought to be produced in the innermost regions of an accretion disk.

axial dipole principle A fundamental principle established in paleomagnetic studies, which states that the axis of the geomagnetic dipole nearly coincides with Earth’s rotational axis at all geological times. The principle makes it possible to use paleomagnetic data to constrain the position of continents in the geological past relative to Earth’s rotational axis.

axionic string Axions are scalar-like ﬁelds which have been proposed to solve the strong CP-problem in QCD. They are present in many grand uniﬁed models and also in superstring the-

ories. Axions behave essentially as the phase of a scalar ﬁeld and would be expected to have very small values in vacuum in order to solve the CPproblem. However, in some instances, such as phase transitions, they play the role of the phase of a Higgs ﬁeld (although there is no Higgs ﬁeld in most models), and thus could be forced to undergo a variation by an amount of 2π, just like the phase of an ordinary Higgs ﬁeld would, thereby being responsible for the appearance of axionic cosmic strings. During the evolution of a network of these strings, they would radiate some energy in the form of axion particles, whose remnant density is calculable given a speciﬁc model. This is one means of constraining the actual mass of the axion particle. See cosmic string, CP problem, global topological defect.

azimuth The azimuth of a line is the angle from a vertical plane passing through North to that line, measured positive eastwards. Thus, the points North, East, South, and West on the compass are, in turn, at 0◦ , 90◦ , 180◦ , and 270◦ azimuth. Alternatively, it is possible to use the range from −180◦ to + 180◦, in which case West is −90◦. Azimuth is part of the topocentric system of coordinates. See also altitude. In critical applications, it is necessary to distinguish true North (as deﬁned by the Earth’s rotation axis) from magnetic North. Azimuth can also be deﬁned relative to another marker besides North, such as the direction of motion of an aircraft. In that case, for example, 0◦ is “dead ahead” and “to the right” is 90◦. In this extended usage, one does not refer to the topocentric system.

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