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<<< < Предыдущая 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123124 / 130124 125 126 127 128 129 130 > Следующая >>>

upper mantle The region of rock in the Earth’s interior reaching roughly 5700 to 6330 km in radius.

upper neutral atmospheric regions These regions are classiﬁed on the basis of the distribution of temperatures and constituents with height. The lowest region closest to the ground is called troposphere, marked by a temperature decrease with height. At the tropopause, the temperature ceases to decrease and starts increasing in the stratosphere to the stratopause. The stratosphere includes the ozone layer, which shields the Earth from dangerous extreme ultraviolet rays. Above the stratopause the temperature again starts decreasing in the mesosphere until the mesopause is reached. Above the mesopause, the temperature starts increasing in the thermosphere until the thermospause is reached, above which it stays almost constant with height.

In terms of species, the region around 30 km where ozone is abundant is called ozonosphere. The region below 110 km is called homosphere, where species are well mixed by turbulence. The region above 110 km is called the heterosphere where various species are not well mixed as a result of prevalance of diffusive separation and many photochemical reactions. The region above about 500 km is known as the exosphere or geocorona, where atomic hydrogen predominates. In this region the atoms only occasionally collide and move in ballistic orbits and may escape. The critical level above which the region can be designated as the exosphere is deﬁned where the mean free path is in the order of the scale height of the atomic hydrogen.

Uranus The seventh planet from the sun. Named after the Greek elder god. It has a mass M = 8.683 × 1028 g and an equatorial radius of R = 25,559 km, giving it a mean density of 1.29 g cm−3 and a surface gravity of 0.91 that of Earth. Its rotational period is 17.24 h, and its rotation axis has an obliquity of 97.9◦. The planet’s oblateness is 0.023. Uranus’ orbit around the sun is characterized by a mean distance of 19.2 AU, an eccentricity of e = 0.047,

and an orbital inclination of i = 0.772◦. Its synodic period is 370 days, and its sidereal period is 84.01 years. It has an average albedo of 0.51, and a temperature at 1 bar of around 70 K. Unlike the other giant planets, it does not have a measurable internal heat source. The bulk of Uranus’ composition is probably a mixture of ice and rock, although about 20% of the mass is hydrogen and helium. The atmosphere also contains signiﬁcant amounts (2%) of methane, which is the cause of its blue color. Uranus has 15 known satellites, although a number of additional candidates have been reported. In addition, it has a system of 9 narrow rings. The larger named moons include Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, Puck, Miranda, Ariel, Umbriel, Titania, Oberon, Caliban, and Sycorax.

Urca process The high densities in the cores of massive stars can force energetic electrons to capture onto nuclei, emitting an electron neutrino ((Z, A) + e− → (Z − 1, A) + νe). This new nucleus then beta decays ((Z − 1, A) → (Z, A)+e− +ν¯e). The net result of this cycle is the emission of two neutrinos and the loss of energy. This cycle is known as the Urca process, named after the Casino d’Urca in Brazil where the net result of gambling has a similar effect on money as the Urca process has on energy. Cooling from the Urca process is the cause of the extremely short burning timescales of post main-sequence evolution of massive stars and triggers the collapse of the iron core which leads to supernovae explosions.

Urey ratio The heat ﬂow to the surface of the Earth from its interior has two contributions, (1) the heat generated within the Earth by the decay of the radioactive isotopes of uranium, thorium, and potassium, and (2) the secular cooling of the Earth. The Urey ratio, named after the eminent geochemist Harold Urey, is the fraction of the total heat ﬂow attributed to radioactive decay. The present estimate for the value of the Urey ratio is 70 ± 10%.

UT See Universal Time.

Valles Marineris

V471 Tauri stars Binary systems in which the more massive star has ended its life as a white dwarf, the companion is still on the main sequence, and no material is being transferred between the stars. Such systems are not very conspicuous, but a dozen or so have been identiﬁed. Further evolution of the main sequence star will transform them into cataclysmic variables.

vacuum In ﬁeld theory, and as applied to condensed matter physics and to cosmology, the lowest state of energy in which a system can be. In many cases the vacuum depends on external parameters, e.g., the temperature. Whether the vacuum is a particular conﬁguration, or whether a number of different conﬁgurations give the same lowest energy has a large inﬂuence on the physics associated with the ﬁeld.

vacuum ﬂuctuation The process, as a conse-

quence of the uncertainty principle, > ,

E t h in which pairs of particles and antiparticles spontaneously appear and disappear in space and time. From the uncertainty principle a ﬂuctuation E can last at most t = h/ E, whereE is the energy or mass ×c2 involved in the ﬂuctuation.

vacuum manifold In theoretical physics applied to condensed matter, and to cosmology, the result of spontaneous symmetry breaking. At high temperatures the system has a symmetry described by an invariance group, G. At lower temperatures, the lowest energy state (the vacuum) may be less symmetric (described by a subgroup H resulting in a structure for the vacuum which is not necessarily trivial). The vacuum is topologically equivalent (isomorphic) to the quotient space M G/H . The vacuum manifold M and its topological features (like connectedness) will unambiguously determine the kind of topological defect that will be produced during the phase transition. Such residual

defects (e.g., cosmic strings or monopoles) may have had a signiﬁcant effect on the early evolution of the universe. See cosmic topological defect, spontaneous symmetry breaking.

vacuum polarization If vacuum ﬂuctuations occur in the neighborhood of an electrically charged particle, the members of the created pairs with opposite charge to that particle will be attracted towards it, and members with identical charge will be repelled from it. This migration is called vacuum polarization. See vacuum ﬂuctuation.

Väisälä frequency See buoyancy frequency.

Valles Marineris A vast system of interconnected canyons that exists to the east of the Tharsis Province, just south of the equator. It extends from Noctis Labyrinthus, at 5◦S 100◦W, eastwards for 4000 km until it merges with the chaotic terrain. Along the entire length are multiple, parallel canyons, chains of craters, and graben. The canyons are better integrated towards Noctis Labyrinthus and more segmented towards the chaotic terrain. In most places individual canyons are over 3 km deep, and 100 km wide, although in the central section three parallel canyons merge to form a depression over 7 km deep and 600 km wide. Therefore, the canyons signiﬁcantly larger than the Grand Canyon, Arizona, U.S. on Earth, which is 450 km long, with a maximum depth of 2 km and a maximum width of 30 km.

The canyon walls are steep and gullied and in many places have collapsed in gigantic landslides. In contrast, the canyon ﬂoors are generally ﬂat and possess outcrops of layered terrain. The landslides, the layered terrain, and the debris, all imply a ﬂuidizing mechanism in their formation, but besides these features there is evidence only for slow erosion in the canyons, independent of ﬂuvial activity. It is generally agreed that the formation of Valles Marineris is “in some way” related to the evolution of the Tharsis Province, because canyon location correlates with the Tharsis circumferential extensional stress ﬁeld. The favored mechanism that is considered to be responsible for, or to have contributed to, the formation of Valles Marineris and Noctis Labyrinthus is incipient or aborted

valley networks

tectonic rifting (by analogy with Earth), with collapse along the crest of the bulge due to subsurface withdrawal of material.

valley networks The small dendritic channels found on Mars. These channels are found on the ﬂanks of volcanos and in the ancient terrain of the planet. Although similar in some respects to terrestrial channels formed by rainfall, the detailed morphologies of these channels suggest they were formed by sapping, whereby groundwater is removed and the overlying surface collapses along the path of the underground river, producing the channel shape. Those found on the ﬂanks of volcanos may have been produced by hydrothermal activity. Valley networks are typically about 1 km in width and tens to hundreds of kilometers in length.

Van Allen Belts Named after their discoverer, James Van Allen. First detected in 1958 by Explorer 1; James Van Allen correctly interpreted the results. Two distinct toroidal belts; the inner one, located between about 1.1 to 3.3 Earth radii, near the equatorial plane, contains primarily protons with energies exceeding 10 MeV. Flux maximum is at about 2 Earth radii. This population varies with 11-year solar cycle and is subject to occasional perturbations due to geomagnetic storms. The protons in this region are produced from cosmic rays striking the atmosphere. The outer belt (between 3 to 9 Earth radii, with a maximum around 4 Earth radii) contains mainly electrons with energies up to 10 MeV arising from solar wind electron injection and acceleration in geomagnetic storms. The population of this belt thus shows day/night variation and is sensitive to solar activity. The charged particles contained in the Van Allen belts remain trapped along ﬁeld lines of the Earth. The particles drift and spiral around the Earth’s magnetic ﬁeld lines. As the particles approach the converging ﬁeld lines near the poles, they are reﬂected back towards the opposite pole. “Horns” of especially the outer belt dip sharply in toward the polar caps.

VAN method A method of earthquake prediction proposed by three Greek researchers, Varotsos, Alexopouls, and Nomicos in the mid 1980s. According to VAN method, earthquakes

could be predicted based on the observed facts that abnormal signals of Earth current with a duration time of several minutes to hours (SES) and continuing more than several days appear weeks before generations of disastrous earthquakes. VAN claim that the magnitude of earthquakes, epicentral distance, and occurrence time of earthquakes are respectively predicted empirically from maximum amplitude of SES, station distribution where SES appears, and kinds of SES. VAN method was tested throughout Greece. Some researchers reported that prediction of disastrous earthquakes succeeded with substantially high probability, but others are critical owing to lack of objectivity of the method.

vapor A gas whose temperature is less than its critical temperature. In such a situation an increase of pressure at constant temperature will cause the gas to condense (liquify or solidify); an example is as water vapor in air at any temperature below 374◦C. (The corresponding pressure is 221 bar.) More colloquially, a dispersion of molecules of a substance that is a liquid or a solid in its normal state at the given temperature, through a substrate gas.

vapor concentration The vapor concentration or absolute humidity is the mass of vapor per unit of volume of moist air.

vapor pressure The partial pressure of the water vapor in moist air. It is a measurement of water vapor content in air. The pressure of moist air is equal to the partial pressure of the dry air in the moist air plus the vapor pressure, according to Dalton’s law (the additivity of partial pressures). Vapor pressure e is directly proportional to vapor density ρw and temperature T as

e = R ρwT mw

where R is the gas constant, R = 8.314 J/mol · K, and mw is the water vapor molecular weight. For a given temperature, a certain volume of moist air can only contain a limited content of water vapor. If vapor content reaches this limit, the air is said to be saturated, and its vapor pressure is the saturated vapor pressure or the maximum vapor pressure. As temperature increases, saturated vapor pressure increases.

, x¨i = d2xi dt2

variational principle

variable star A star that changes its ﬂux over time, which can be hours or years. Variable stars are classiﬁed as either extrinsic or intrinsic, depending on the cause for the variability being due to the environment or interior processes of the star, respectively. Three major groups of variable stars are: eclipsing, cataclysmic, and pulsating. An eclipsing variable is really two stars with different temperature types that, due to fortuitous line of sight with Earth, orbit in front of and in back of each other, thus changing the light the system puts out for observers on Earth. Pulsating variables alternately expand and contract their atmosphere and the pulsations are fueled by a driver under the photosphere that operates on the opacity changes of partially ionized H and He. The cataclysmic variables, which include novae, dwarf novae, recurrent novae, and ﬂare stars, can change their brightness by 10 magnitudes or more. The novae cataclysmic variables are binary stars whose increase in brightness arises because of the hydrogen-rich material of the companion star igniting (fusion) on the surface of the hot white dwarf. Other minor types of variables include the R CrB stars that are believed to puff out great clouds of dark carbon soot periodically, causing their light to dim.

variable stars [cataclysmic variables]	See
cataclysmic variables.
variable stars [geometric variables]	Binary

systems whose stars actually present constant luminosity, but are periodically eclipsing each other when seen from Earth, thus exhibiting apparent variability.

variable stars [peculiar variables] Stars that present an inhomogenous surface luminosity and, therefore, are observed as variables due to their rotation.

variable stars [pulsating variables] Stars that present variable luminosity due to instabilities in their internal structure.

variational principle A way of connecting an integral deﬁnition of some physical orbit, with the differential equations describing evolution along that orbit. Varying a path or vary-

ing the values of ﬁeld parameters around an extremizing conﬁguration in an action (which is deﬁned via an integral) leads to the condition that the ﬁrst-order variation vanishes (since we vary around an extremum). The vanishing of this ﬁrst-order variation is typically a differential equation, the equation deﬁning the ﬁeld conﬁguration which produces the extremum. In point mechanics, one may compute the action:

xf ,tf	L x,	dx d2x			· · · dt
I = xi ,ti C			,
		dt		dt2

(where x = xi , i = 1 · · · N, the dimension of the space). By demanding that the curve C extremize the quantity I, compared to other curves that pass through the given endpoints at the given times, one is led to differential conditions on the integrand

∂L		d ∂L			d2 ∂2L

∂xi			˙	+ dt2		¨
∂xi	− dt ∂xi			+ dt2			∂x

− · · · = 0, i = 1 · · · N .

Here the partial derivatives relate to the explicit

appearance of x˙i = dxdti , · · · . Newton’s laws for the motion of point in conservative

ﬁeld follow from the simple Lagrangian

L = T − V ,

where T = Tij (xl)x˙i x˙j is quadratic in the x˙i , and V is a function of the xi only. Then, one obtains

−	∂V	−	d	Til	dxl	= 0 .
	∂xi		dt		dt

For the simplest case of Til = 21 mδil, one ﬁnds:

mx¨i = −∂x∂Vi .

The quantity I = Ldt is called the action. For classical and quantum ﬁeld theories, one deﬁnes a Lagrangian density. The Lagrangian is then deﬁned as an integral of this density. For instance, the action for a free (real) scalar ﬁeld

satisfying a massless wave equation is

I = L |g|d4x ,

where

L = αφ β φgαβ