Добавил:

fench Опубликованный материал нарушает ваши авторские права? Сообщите нам.

Вуз:

Сумский государственный университет

Предмет:

Английский язык

Файл:

Dictionary of Geophysics, Astrophysic, and Astronomy.pdf

Скачиваний:

122

Добавлен:

10.08.2013

Размер:

5.66 Mб

Скачать

☆

►Содержание►

<<< < Предыдущая 105 106 107 108 109 110 111 112 113 114 115 116117 / 130117 118 119 120 121 122 123 124 125 126 127 128 129 > Следующая >>>

symmetry

( 1.5 × 105 km) and then either fade or return into the chromosphere along the trajectory of ascent.

suspended load The sediment that is transported in the water column in a river or ocean. See also bed load.

suspension The transport of small material by mixing of that material into a ﬂuid (air, water, etc.); or such a mixture itself. For suspension to occur, the gravitational force on the particle must be balanced by the uplifting force caused by turbulence in the ﬂuid. Suspension is most effective for the transport of silt and clay sized material (less than 60 µm in diameter).

suture The boundary between two colliding continents. When an ocean closes, the continents bounding the oceans collide. This is happening between India and China today resulting in the Himalayas.

Sv Sverdrup, the volume transport unit, 1 Sv = 106 m3/s.

swash zone The portion of a beach where the water rushes up and down due to wind wave action, alternately wet and dry.

Sweet–Parker reconnection In plasma physics, the slow stationary reconnection in an extended X-line (the length L of the diffusion region is large compared to its width d). The outﬂow speed of the plasma is equal to the Alfvén speed vA,in in the incoming plasma ﬂow, and the reconnection rate RSP equals the Mach number of the incident ﬂow:

RSP =

Lσ vA,in4π

with σ being the conductivity. The reconnection rate then depends on the conductivity. In space plasmas, the conductivity is high and, therefore, a low rate of reconnection results. Sweet– Parker reconnection thus, is a slow process during which about half of the incoming magnetic energy is converted into the kinetic energy of the outﬂowing plasma, leading to two high-speed plasma streams ﬂowing away from the recon- nection site.

Sweet–Parker reconnection seems to play an important role at the magnetopause where the high-speed streams ﬂowing away from the reconnection site can be detected in situ.

swell A broad region of elevated topography generally associated with seismic hot spots. A typical example is the Hawaiian swell which is a near circular, domal structure with a diameter of about 1000 km and an elevation of about 1 kilometer.

“Swiss cheese” model A model of the universe in which the background spacetime is the same as in the Friedmann–Lemaître dust-ﬁlled cosmological models, but there are holes in it, inside which the Schwarzschild solution applies. This model was used to discuss the inﬂuence of the expansion of the universe on the motion of planets; according to this model (Einstein and Straus, 1945) there is no such inﬂuence: since the planets remain in the region of the spacetime in which the Schwarzschild solution applies, they move as if the matter outside the hole does not exist. The mass-parameter in the Schwarzschild region and the active gravitational mass (counting binding) that was removed from the Friedmann background to make place for the planetary system are equal.

symbiotic star The category of cataclysmic variable in which the donor star is a red giant. The spectrum shows both absorption features characteristic of the cool, bright red giant and emission features from the accretion disk around the white dwarf.

symmetric instability A two-dimensional form of baroclinic instability in which the perturbations are independent of the coordinate parallel to the mean ﬂow.

symmetry An operation performed on a geometrical object after which the geometrical state of the object is indistinguishable from its initial state. Example: let the object be a sphere and the operation be any rotation around the center of the sphere. However, if the object is a cube, then only rotations by some deﬁnite angles around deﬁnite axes will be symmetries. (If the axis passes through the centers of op-

symmetry

posite walls of the cube, then only rotations by multiples of 90◦ are symmetries. If the axis passes through the centers of diagonally opposite edges of the cube, then only rotations by multiples of 180◦ are symmetries.) Continuous symmetries of an n-dimensional space can be labeled by up to 21 n(n + 1) parameters. For example, among two-dimensional surfaces some have no symmetry (think about the surface of a loaf of bread), some have one-parametric symmetry (think about the surface of a perfectly smooth and round doughnut, which is symmetric with respect to rotations around an axis that passes through its center: the parameter is the angle of rotation), some have a two-parametric symmetry (think about the surface of an inﬁnite cylinder, the parameters are the angle of rotation about the axis of the cylinder and the distance of displacement along the axis), and two have three-parametric symmetries. These latter are the plane and the surface of the sphere. For the plane, the symmetries are displacements along two perpendicular directions and rotations around an axis perpendicular to the plane. For the sphere, the symmetries are rotations around three mutually perpendicular axes. A characteristic property of each collection of symmetries is the difference between the following two operations: We take two of the basic symmetries (call them a and b), apply them to a point of the space in one order (ﬁrst a, then b), and then apply them in the reversed order (ﬁrst b, then a). The difference may be zero (like for the two translations on a plane) or nonzero (for any two rotations on a sphere or for a translation and a rotation on a plane). The fact that on a plane there exists a pair of symmetry transformations whose ﬁnal result does not depend on the order in which they are executed, while no such pair exists on a sphere, shows that these two collections of symmetries are inequivalent. Real objects usually have no symmetry in the strict sense but may be approximately symmetric. (Example: the surface of the Earth is nearly a sphere. The departures from spherical shape caused by rotational ﬂattening and by the mountains and other irregularities of the surface can be neglected for many purposes.) Symmetries are often assumed in physics in order to solve problems that would be too difﬁcult to consider by exact methods without such an assumption.

Small departures from symmetries can then be taken into account by approximate calculations. A typical example is the calculation of orbits of the planets around the sun. Both in Newtonian mechanics and in general relativity a ﬁrst approximation to the realistic solution was found under the assumption of spherical symmetry, which implies that the mass distribution inside the sun is perfectly spherical, and that the orbit of a given planet is not inﬂuenced by other planets. Then, small perturbations away from spherical symmetry caused by the other planets can be considered and corrections to the orbits calculated. Calculations without any symmetries assumed have been attempted only by numerical methods. A pronounced time dependent asymmetry in a gravitating system typically results in the emission of gravitational waves.

synchronous orbit For a small body orbiting a more massive one, the orbital radius for which the orbital period is equal to the rotation period of the more massive body. A satellite in a zero inclination synchronous orbit above the equator will always appear to be above a particular point on the more massive body.

synchronous rotation Rotation of a planet or other secondary body so that the same side always faces the primary; for instance, the moon rotates synchronously around the Earth. In the case of the moon, this is ensured by tidal locking; for commercial communications satellites, it is accomplished by active means, such as reaction wheels or thrusters.

synchrotron radiation The radiation produced by a charged particle as it gyrates around magnetic ﬁeld lines. The radiation is emitted at the gyrofrequency, ω = qB/γ mc, where q is the particle charge, B is the magnetic ﬁeld strength, m is the particle mass, γ is the Lorentz factor, and c is the speed of light. For an electron we have ω = 1.76 × 107Bγ . Synchrotron radiation from energetic electrons is an important production mechanism for microwave emission in the solar corona.

synchrotron self-Compton mechanism A mechanism suggested for the production of high energy photons in radio loud active galactic nu-

syzygy

clei. In the synchrotron self-Compton scheme, low-energy photons are produced in the radio domain by synchrotron emission by relativistic elections. If the source is very compact, the same relativistic electrons then turn the radio photons into higher energy photons by inverse Compton scattering. The synchrotron selfCompton mechanism suggests that the spectral shape of the seed synchrotron photons is maintained in the scattered spectrum. This prediction has been apparently conﬁrmed by observations of the blazar 3C 279; however, the general validity of the synchrotron self-Compton mechanism for active galactic nuclei is as yet controversial.

synodical month Synodic month.

synodic month The time from one full moon to the next (about 29.53 days).

synodic period In astronomy, the amount of time taken for a celestial body (usually the moon, but the concept can apply to other objects) to rotate once with respect to the Earth-sun line; the time between successive conjunctions of an orbiting body. Thus, the period of time between new moons (about 29.53 days) is the synodic month.

synoptic scale In the atmosphere, the synoptic scale is referred to as the scale of

moving weather systems associated with cyclones or anticyclones. This scale is larger than the mesoscale and less than the planetary scale.

synthetic aperture radar A radar imaging technique in which the angular resolution is improved by combining the returns from an image as seen when illuminated by the radar transmitter at different points as it moves past the target. The essential point allowing this combination is precise geometric reconstruction based on the Doppler shift associated with speciﬁc target features. These signals are reduced to zero Doppler shift (maintaining phase coherence) and combined to produce the effect of a large aperture radar (an aperture, d, as large as the maximum distance between the source transmitter locations illuminating a particular target). This process thus, gives enhanced angular resolution, θ, according to the standard formula:

θλ . d

syzygy An alignment of three celestial bodies in a straight line; in particular the position of the moon when it is new or full is a syzygy between the Earth, moon, and sun.

Taylor number

tachyon A hypothetical subatomic particle that travels faster than the speed of light. There is no experimental or observational evidence of the existence of tachyons.

TAI See International Atomic Time.

tail current Current system in the tail of the magnetosphere, consisting of the cross-tail current separating the northern and southern lobe and the Chapman–Ferraro currents in the magnetopause. Thus, one closed current encircles each lobe, with both currents running together in the cross-tail portion.

Tail current.

Talwani’s method An algorithm concerning analyses of magnetic anomaly and gravity anomaly proposed by M. Talwani (1965). For instance, the magnetic anomaly produced by a three-dimensional magnetic substance, such as a seamount, can be calculated, approximating the conﬁguration of the substance by a stack of polygonal thin plates with homogeneous magnetic susceptibility. Similarly, the free air anomaly produced by a two-dimensional substance with a density contrast can be calculated, making an approximation for a crosssection of the substance by a polygon.

tangential discontinuity See hydromagnetic wave.

tangential geostrophy A force balance believed to be appropriate for ﬂuid near the surface of the Earth’s core and which is useful as an assumption for constructing models of the ﬂow at the surface of the core from models of the magnetic ﬁeld at the core-mantle boundary, because it reduces the ambiguity inherent to such ﬂow modeling. Geostrophy is a balance between pressure and coriolis forces:

2ρ × u = − p

where ρ is the density, is the rotation vector, u is the velocity, and p the pressure. Tangential geostrophy assumes that this balance holds in the horizontal direction (but not necessarily in the radial direction), i.e., that the Lorentz and viscous forces, buoyancy, and inertia are only important in the radial direction, if at all. The justiﬁcation for this is that the Lorentz force may be reduced near the core-mantle boundary, since if the mantle is insulating, then the toroidal part of the magnetic ﬁeld should drop to zero at the boundary, that gravity is predominantly radial, and that viscosity and inertia are small. These assumptions are arguable. However, the assumptions lead to a constraint on the ﬂow:

H · (u cos θ) = 0

where θ is the co-latitude, and H is the horizontal portion of the divergence operator. As the inverse problem for the ﬂow is undetermined, this constraint can be useful for reducing ﬂow ambiguity. Also, any part of the ﬂow that represents torsional oscillations in the core will obey this constraint. See core ﬂow.

Taygeta Magnitude 4.4 type B7 star at RA 03h45m, dec +24.27’; one of the “seven sisters” of the Pleiades.

Taylor instability Formation of rolls generated in a column of ﬂuid bounded by differentially rotating cylindrical walls; governed by the dimensionless Taylor number: Ta = riω2h3/ ν2, where ri is the radius of the inner cylinder, ω is the rate of rotation, h is the space between cylinders, and ν is the kinematic viscosity.

Taylor number A dimensionless number measuring the inﬂuence of rotation on a convecting system. It is also called rotational Reynolds

<<< < Предыдущая 105 106 107 108 109 110 111 112 113 114 115 116117 / 130117 118 119 120 121 122 123 124 125 126 127 128 129 > Следующая >>>

Соседние файлы в предмете Английский язык

#
10.08.201313.07 Mб448Dictionary of Contemporary Slang, Third Edition; Tony Thorne (A & C Black, 2005).pdf
#
10.08.201313.14 Mб169Dictionary of Contemporary Slang.pdf
#
10.08.20132.5 Mб61Dictionary of DNA and Genome Technology.pdf
#
10.08.20135.81 Mб131Dictionary of Engineering, 2nd Edition.pdf
#
10.08.201310.44 Mб244Dictionary of Food Ingredients 4th Edition.pdf
#
10.08.20135.66 Mб122Dictionary of Geophysics, Astrophysic, and Astronomy.pdf
#
10.08.20133.03 Mб108Dictionary of Literary Influences.pdf
#
10.08.201314.56 Mб228Dictionary of Medical Terms 4th Ed..pdf
#
10.08.201310.83 Mб94Dictionary of Microbiology and Molecular Biology 3rd Ed.pdf
#
10.08.20137.45 Mб296Dictionary of Military Terms.pdf
#
10.08.201314.38 Mб64Dictionary of Mining, Mineral, & Related Terms.pdf