Файловый архив студентов. Файловый архив студентов.
1259 вузов, 4592 предметов.
/ Регистрация
FAQ Обратная связь Вопросы и предложения
Добавил:
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►

nebular hypothesis

nebular hypothesis The hypothesis that the solar system, and typical systems around other stars, formed by condensation of material in a molecular cloud in which the star first formed. Because there will be some initial angular momentum in such a cloud, cooling will lead to the formation of a disk. Planetary formation in the environment of a flattened disk gives an explanation for the general uniformity of rotational behavior among the planets, and for the fact that the planetary orbits all lie close to the same plane, the ecliptic. The composition of the planets, with rocky planets near the sun, and gas giants at larger distances can be explained by early strong radiation from the forming sun. However, several extrasolar planetary systems are now known, where this arrangement of planetary bodies does not seem to hold.

nebular lines Forbidden emission lines observed in optical spectra of gaseous nebulae, typically planetary nebulae, HI regions, and external galaxies. The strongest nebular lines are observed at 495.9, and 500.7 nm, at 654.8 and 658.4 nm, and at 372.7 nm. Nebular lines are forbidden lines, not observed in laboratory spectra. Nebular lines were shown (Bowen, 1928) to be due to forbidden transitions between the lowest terms of singly and doubly ionized abundant atomic species, such as oxygen and nitrogen. See forbidden lines.

needle-probe method A method for measuring thermal diffusivities of unconsolidated sediments or other soft materials. A needlelike probe containing a calibrated electric heat source and temperature sensors is inserted into the specimen. The diffusivity of the material is determined by analyzing the rate of the temperature rise after the heat source is turned on but within the time window when the line source of heat can be assumed to be infinitely long and the medium to be infinitely large.

neon (From the Greek neos, new.) Noble gas, Ne, discovered in 1898 by Ramsay and Travers. Atomic number 10, natural atomic weight 20.179. Colorless, odorless, inert gas. Present in the atmosphere as 15 ppm. Melting point 24.56 K, boiling point 27.07 K.

The dominant natural isotope is 20Ne, which constitutes more than 90% of the abundance.

The other two stable isotopes are 21Ne (0.25%) and 22Ne (9.25%).

neon burning The set of nuclear reactions that convert neon to heavier elements, including magnesium and silicon. It occurs in hydrostatic equilibrium in the evolution of stars of more than about 10 solar masses (between carbon burning and oxygen burning). It is not a major energy source even for them because neon is never very abundant.

Neptune Giant gas planet, eighth planet from the sun. (Occasionally, as at present, Pluto is actually inside the orbit of Neptune.) Outermost gas planet. Mass 1.02 × 1026 kg, diameter 49528 km. Average distance from sun 30.06 AU, orbital eccentricity 0.009. Rotation period 19.1 hours, orbital revolution period 164.8 years. Inclination to orbit 29.6◦. The visible surface has a composition closely matching primordial amounts: 74% hydrogen, 25% Helium. Its surface mean temperature is 48 K. Its surface albedo is of order 0.5.

Nereid Moon of Neptune, also designated NII. It was discovered by G. Kuiper in 1949. Its orbit has an eccentricity of 0.751, an inclination of 27.6◦, a precession of 0.39◦ yr−1, and a semimajor axis of 5.51 × 106 km. The orbit, which has the highest eccentricity of any satellite in the solar system, indicates that Nereid is a captured Kuiper Belt object. Its radius is 170 km, and its mass is not known, but has been estimated to be about 2 × 1019 g based on an assumed density of 1 g cm−3. It has a geometric albedo of 0.155, and orbits Neptune once every 360.1 Earth days.

net photosynthetic rate The total rate of photosynthetic CO2 fixation minus the rate of loss of CO2 in respiration.

network In solar physics, general name given to the distribution of photospheric magnetic field outside of sunspots and faculae. The network pattern consists of magnetic fields with characteristic cell dimensions of 20,000 to 40,000 km, covering the quiet photosphere. There is a cor-

© 2001 by CRC Press LLC

neutrino annihilation

responding chromospheric network defined by bright emission in Ca K spectroheliograms.

Neupert effect Statement that the temporal derivative of the observed soft X-ray emission during a solar flare reproduces the observed time development of the hard X-ray emission. This is found in many flares and indicates that a single energization process is responsible for the production of both the non-thermal and thermal radiation.

neutral equilibrium In mechanics, a configuration in which the system experiences no net force (equilibrium), and if the system is moved slightly from this state, no forces arise, so a nearby configuration is also an equilibrium. The paradigm is a ball on a smooth horizontal surface. See stable equilibrium, unstable equilibrium.

neutral point In electromagnetism, a point in a magnetic configuration in which the field intensity B drops to zero, and where therefore the direction of the magnetic field is undefined. Expanding the field B around an isolated neutral point gives B = B0 + r · B0 in the immediate neighborhood of the point, with subscript zero marking values at the point. Since B0 = 0, the constant dyadic B0 determines the character of the field. In an “x-type neutral point,” B0 has three real characteristic roots and three field lines cross at the point, giving a configuration like the letter “x” but usually an intricate smallscale geometry. In an “o-type neutral point”B0 has just one real root and the point is the limiting point of nested closed field lines.

Neutral lines of x-type and o-type form in two-dimensional geometries (extending unchanged in the third dimension), where the number of real roots is 2 or 0. The nearby field lines either form an x-shaped pattern outlined by the eigenvectors of B0, or form nested o-shaped loops.

Neutral points are of great interest in space plasma physics because theory associates them with magnetic reconnection. The polar cusps of the Earth magnetosphere are also evolved (x- type) neutral points.

In atmospheric physics, the points where the degree of polarization of sky diffuse radiation

equals zero. Due to aerosol scattering, multiple scattering and reflecting from surfaces, polarization of sky light is not consistent with the ideal status; there will be some abnormal neutral points. According to Rayleigh scattering theory, under ideal conditions the solar point (the direction of solar incident ray) and anti-solar point are neutral points. In the real atmosphere, in general, there are three neutral points: Arago neutral point, which locates about 15◦ to 25◦ above the anti-solar point; Babinet neutral point, which locates about 12◦ to 25◦ above the sun; and Brewster neutral point, which locates about 15◦ to 25◦ below the sun. All these angles and positions may change due to the variations of the position of sun, the atmospheric turbidity and the reflective properties of the Earth surface.

neutral stability In meteorology, the stratification status when the lapse rate of air temperature (γ ) equals its adiabatic lapse rate (γd ).

neutrino A stable elementary particle which carries zero electric charge, angular momentum of 21 h¯, little or no mass, but finite momentum and energy. Invented by Pauli in the 1930s to make energy, momentum, and angular momentum conservation hold in weak nuclear decays, the electron neutrino was identified in the laboratory by Cowan and Reines in 1953. Two additional types, called the µ and τ neutrinos (for the other particles co-produced with them), have been found since. Each has an antiparticle, leading to a total of six types. In astrophysics, electron neutrinos are produced in stellar nuclear reactions, including the protonproton chain and CNO cycle. All three types are produced by Type II Supernovae, and there should be a thermal sea of all types left from the early universe, corresponding to the cosmic microwave background radiation. Solar neutrinos and those from one supernova, 1987A, have been observed in laboratory detectors.

neutrino annihilation In core-collapse supernovae and gamma-ray burst models, the number density of neutrinos and anti-neutrinos can become large, and the rate of their annihilation and production of electron/positron pairs and photons can be an important part of the explosion energy. However, the annihilation

© 2001 by CRC Press LLC

neutrino viscosity

cross-section depends sensitively upon the incident angle of the neutrino and anti-neutrino, peaking for head-on collisions. For supernovae and spherically symmetric gamma-ray bursts, this angular dependence drastically diminishes the energy produced by neutrino annihilation. However, for asymmetric gamma-ray burst models, such as the class of black hole accretion disk models, neutrino annihilation can be quite high along the disk axis, producing a strongly beamed jet.

neutrino viscosity Stress transport and entropy generation carried out by neutrinos whose mean free time tc is comparable to the time scale τ of some system. In cosmology, neutrino viscosity could have been significant when the age of the universe τu H −1 was about 1 sec. Strictly “viscosity” applies when H tc 1, but extended simulations show that the entropy generation peaks when H tc 1. (Here H is the Hubble parameter at the time in question.)

neutron albedo Neutrons emitted outwards from the Earth’s atmosphere, following nuclear collisions by incoming cosmic ray ions, with typical energies of 10 to 50 MeV. Free neutrons decay with a lifetime of about 10 min, and such neutrons have a small but non-zero probability to decay in the region close to Earth where the magnetic field can trap the resulting proton. The inner radiation belt is believed to arise in this fashion.

neutron star A remnant after a core collapse (type II) supernova explosion. Chandrasekhar showed that a cold star of mass greater than1.4M cannot support itself by electron degeneracy pressure, but must undergo a collapse. A neutron star is a possible (much smaller) equilibrium configuration. In a neutron star, the gravitational compression has raised the Fermi level of electrons so high that inverse β-decay has occurred, and protons have been converted to neutrons, and the star is predominantly neutrons with relatively small amounts of protons, electrons, and other particles. Thus, the star is supported by neutron degeneracy pressure. Neutron stars which can be observed in binary pulsars have mass near 1.4M , and hence (from theoretical models) radii around 20 km. Be-

cause the exact constituents of the neutron star, and the exact behavior of the equation of state, are unknown, the maximum mass of a neutron star can only be estimated, but a wide range of models suggest 2.5M . Above such a mass the neutron star has no support and will collapse into a black hole. Known neutron stars are pulsars and typically rotate rapidly (observed periods of multiseconds to seconds) and possess high magnetic fields (up to 1012 Gauss equal to 108 Tesla). Radiation processes in magnetospheres of neutron stars — regions of highenergy plasma around the star — are responsible for radio-emission that makes them observable as radio pulsars. A neutron star forming a close binary system with a normal star can accrete matter from the normal companion due to its strong gravitational field; this may lead to strong emission in X-ray range. Such systems are observed as X-ray pulsars and bursters. Some theoretical models also link γ -ray bursts to neutron stars. See black hole.

neutron star kicks Observations of the pulsar velocity distribution and specific neutron star associations with supernova remnants require that at least some neutron stars receive large velocities ( 450 km s−1), or “kicks”, at or near the time of formation. A wide variety of mechanisms to produce these kicks exist, most of which assume some asymmetry in the ejecta or the neutrino emission in supernova explosions. Many proposed causes of these asymmetries exist: convection in the supernova, rotation, magnetic fields, asymmetric supernova collapse, asymmetric fallback, etc.

Newman–Penrose formalism

(1962) In

general relativity, a method that combines twocomponent SL(2, C) spinor calculus with complex null tetrads for the treatment of partial differential equation systems. Applications are mainly directed to the field equations of general relativity. Major results: perturbative treatment of black holes, asymptotic behavior of the gravitational field, exact solutions, new conserved quantities of isolated systems. See spin coefficients.

New Millennium Program (NMP) A National Aeronautics and Space Administration

© 2001 by CRC Press LLC

Newtonian gravity

(NASA) initiative to test and prove new technologies by flying them on missions similar to science missions of the future; thus, the missions are technologically driven. Included in them are Deep Space 1 (propelled by xenon ions), launched October 24, 1998; Deep Space 2 (which was to probe beneath the surface of Mars), launched January 3, 1999 and lost on Mars on December 3, 1999; Deep Space 3 (with telescopes flying in formation), scheduled for launch in 2005; Deep Space 4 (a voyage to the heart of a comet); Earth Orbiter 1 (with an advanced land imager); and Earth Orbiter 2 (with a laser wind instrument on the Space Shuttle). These missions will return results promptly to future users about whether or not the technologies work in space. The missions are high risk because they incorporate unproven technologies, probably without backup, that require flight validation.

Newtonian A type of reflecting telescope invented by Isaac Newton, with a small flat secondary mirror mounted in front of the primary mirror, to deflect rays approaching a focus out one side of the support tube, where they are viewed using a magnifying lens (eyepiece).

Newtonian cosmology A collection of models of the universe constructed by the rules of Newtonian mechanics and hydrodynamics. A Newtonian model describes the universes via the evolution of a portion of a fluid, assuming the fluid moves in 3-dimensional Euclidean space. The historically first such model (Milne and McCrea, 1934) demonstrated that some basic predictions of the Friedmann–Lemaître cosmological models could have been deduced much earlier from Newtonian physics. However, Newtonian models are of limited value only. Descriptions constructed in general relativity theory automatically imply the laws of propagation of light in the universe, and the geometry of space. The Newtonian models say nothing about geometry and cannot describe the influence of the gravitational field on the light rays. Moreover, they are unable to describe gravitational waves (which simply do not exist in Newtonian physics).

Newtonian fluid A viscous fluid in which shear stress depends linearly on the rate of shear strain. In tensor notation, the flow law is σ ij = 2µε˙ij , where σ ij is deviatoric stress, ε˙ij is the rate of strain, and constant µ is the viscosity. A solid such as a rock deforms like a Newtonian fluid as a result of diffusion creep at elevated temperatures.

Newtonian gravitational constant The constant G = 6.673 × 10−11m3kg−1s−2, which

enters the Newtonian force law:

F = Gm1m2rrˆ −2

where F is the attractive force, m1 and m2 are the masses, r is their separation, and rˆ is a unit vector between the masses.

Newtonian gravitational fields The region in which one massive body exerts a force of attraction over another massive body according to Newton’s law of gravity.

Newtonian gravity The description of gravity due to Newton: The attractive gravitational force of point mass m1 on point mass m2 separated by displacement r

F = −ˆrGm1m2/r2 ,

where G is Newton’s gravitational constant

G= 6.67 × 10−11m3/ kg sec2

= 6.67 × 10−8cm3/ g sec2 .

In Newtonian gravity forces superpose linearly, so the force between two extended bodies is found by summing (integrating) the vector forces between infinitesimally small masses in the two bodies. Newton proved that the force on a point mass due to a spherical extended mass is as if all the extended mass were concentrated at its center.

One can also show that the force on a point mass m1 due to a second mass is given by m1 times the gradient of the potential, defined in general by integration over the extended mass:

F = −m1 φ

with

φ = GρdV /r ,

© 2001 by CRC Press LLC

Соседние файлы в предмете Английский язык
Помощь Обратная связь Вопросы и предложения Пользовательское соглашение Политика конфиденциальности