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psychrometer A wet-bulb/dry-bulb hygrometer. Depending on the ambient humidity, the wet bulb cools by evaporation to a lower temperature than the dry bulb. The difference in temperature is calibrated to indicate relative humidity.

psychrometric constant The psychrometric constant is often used in the calculation of evapotranspiration:

γ = caP/0.622λv = −0.066 kPa/K

at 293.2 K (20C) and 100 kPa, where ca is the heat capacity of the atmosphere, P is the atmospheric pressure, and λv is the latent heat of vaporization.

Puck Moon of Uranus also designated UXV. Discovered by Voyager 2 in 1986, it is a small, roughly spherical body, approximately 77 km in radius. Its orbit has an eccentricity of 0, an inclination of 0.3, a precession of 81yr1, and a semimajor axis of 8.60×104 km. Its surface is very dark, with a geometric albedo of less than 0.1. Its mass has not been measured. It orbits Uranus once every 0.762 Earth days.

pulsar A neutron star in which a combination of rapid rotation and strong magnetic field result in radio waves or other forms of electromagnetic radiation being emitted primarily along narrow beams or cones, so that an observer sees pulses of radiation at the rate of one or two per rotation period. Young, single pulsars are sometimes found in supernova remnants, for instance PSR 0531+21 in the Crab Nebula (formed in 1054 CE, rotation period 0.033 sec) and a 0.016 sec period pulsar in the Large Magellanic Cloud: SNR N157B. These fade in about 107 years as their rotation slows. Neutron stars in close binaries can appear as X-ray sources (X-ray binaries), radiating energy that comes from material transferred from their companions. These are sometimes called accretion powered pulsars. The mass transfer can spin them up to rotation periods as short as 0.0015 sec and, when accretion stops, they will appear as millisecond pulsars, either in binary systems or single if they are liberated by the explosion of the second star.

pulsar velocities Measurements of the transverse velocities of radio pulsars, most reliably made by proper motion measurements, indicate that these pulsars are moving through the galaxy at a mean velocity of 450 km s1. These high velocities require that neutron stars, which become pulsars, receive a 450 km s1 kick at, or near, the time of formation in a supernova explosion (for kick mechanisms, see neutron star kicks). The actual velocity depends sensitively upon the model of electron density distribution in the galaxy, from which the distance to the pulsars, and hence velocities, are calculated.

pycnocline The region of large gradient of potential density in oceans.

pycnostad A layer of the ocean in which density varies little with depth. Such regions are usually well-mixed parts of the ocean with little variation in temperature.

pyconuclear reaction Nuclear reaction at zero temperature in condensed matter arising because zero point fluctuations of nearby ions result in the penetration of the Coulomb barrier. The rate of pyconuclear reactions increases steeply with density, and this is presumably the limiting factor in white dwarf masses; pyconuclear reactions induce explosive burning if the central density exceeds about 1010 gm/cm3, though the exact value is uncertain.

pyroclastic Involving explosive volcanic activity. A pyroclastic rock is one that is formed by the accumulation of volcanic rock fragments that were scattered by explosive activity. Pyroclastic deposits are usually composed of ash and cinders that resulted from the eruption of gas-rich viscous magma; such deposits are also called tuffs and ignimbrites.

pyroxene A silicate mineral common in igneous and high-temperature metamorphic rocks such as basalt and gabbro; a mixture among FeSiO3, MgSiO3, and CaSiO3. Abundant in lunar rocks.

© 2001 by CRC Press LLC

quanta meter


Q “Quality”; a measure of attenuation in an oscillating system. It is conventionally defined in terms of the energy in the oscillation:


Q = 2π


where E is the energy stored in the oscillation, and E is the energy dissipated over the period of one cycle. If there is no other process at work, then Q can be expressed in terms of the real and imaginary parts of the complex frequency:

Q = ωr


where the amplitude of the oscillation is governed by

A = A0eiωt = A0ei(ωr +i )t = A0er t eωi t .

Q may also be written in terms of (or measured from) the width of the peak associated with the oscillation in a power spectrum (that might be obtained from data through a fourier transform):

Q = ω


ω in this case is real and is the frequency of the center of the peak, while ω is the width between the half power points on each side of the peak. Q may be a function of frequency. There are many different geophysical oscillations for which Q may be defined, such as the free seismic oscillations of the Earth, torsional oscillations of the core, and gravity waves in the atmosphere.

QCD Quantum Chromo–Dynamics, the quantum theory of the strong interactions.

Q factor Quality factor, Q.

QSO Acronym for QuasiStellarObject. QSOs are high luminosity active galactic nuclei which show optical appearance almost undistinguished from stars, and a spectrum with strong

and broad emission lines invariably shifted to the red. The optical emission line spectrum closely resembles that of Seyfert-1 galaxies. Searches for quasars have been carried out in several regions of the electromagnetic spectrum, from the infrared to the X-ray. The final identification of a quasar is, however, made on the identification of redshifted spectral emission lines. Many QSOs are strong radio sources, but a large class is radio-quiet. The terms QSO and radio-quiet QSO are a synonym of radio-quiet high luminosity active galactic nucleus. While in typical Seyfert galaxies the luminosity of the nucleus is comparable to the luminosity of the host galaxy, in QSOs the nucleus can be hundreds of times more luminous. It is customary to define as QSOs all radio-quiet active galactic nuclii (AGN) above the luminosity of 1011L . This subdivision is somewhat arbitrary, since at this limit there is no break in continuity of the luminosity distribution.

Other defining properties of QSOs are large UV flux, broad emission lines, large redshift, and time-variable continuum flux. Some of the brightest radio sources are associated with QSOs whose host galaxy is not clearly visible. The most distant QSOs are now being observed at redshift < 5. The 8th Edition of A Catalogue of Quasars and Active Nuclei by M.- P. Vé-Cétty and P. Véron lists more than 11,000 QSOs (both radio-quiet and radio-loud) known as of early 1998. See Seyfert galaxies.

quadrupole formula In general relativity the radiated power of a slowly varying source of gravitational waves is given by the quadrupole formula

P = 45c5




















where the quadrupole moment of a point mass m at xµ is

Qµν = 3m xµxν (1/3µν xρ 2 .

quanta meter An instrument to measure the number of photons (e.g., photosynthetically available radiation), as opposed to energy.

© 2001 by CRC Press LLC

quantization of redshift

quantization of redshift A controversial systemic trend in the radial velocity difference between pairs of galaxies, deduced from redshift between pair members, preferentially an integer multiple of 72 km/s, i.e., cz = n72 km/s.

quantum efficiency The ratio of the number of photons incident on a detector to the number of electrons produced in the first stage of the detector; the inverse of the number of photons needed on average to produce one such photoelectron.

quantum field theory in curved spacetime

Quantum field theory which is extended from its original definitions in flat (gravity-free) spacetimes, to situations where the curvature of spacetime (i.e., the gravitational field) affects the quantum field, but gravity itself is completely classical and is not quantized.

quantum gravity A yet-to-be-defined description of gravity as a quantum theory. Because of the tensor nature of general relativity, it is not renormalizable as a field theory in perturbation from flat space. Various attempts to quantize general relativity have thus far been unsuccessful. String theory contains general relativity in some limit, and gravity may eventually be quantized in the context of the quantum theory of strings.

quantum yield The number of CO2 molecules fixed in biomass per quantum of light absorbed by a plant; it is linearly related to energy conversion efficiency.

quark An elementary entity that has not been directly observed but is considered a constituent of protons, neutrons, and other hadrons.

quasar Acronym of quasi stellar radio source, also QSO; an active galactic nucleus that is of high luminosity, radio loud, and that does not show, in visual images, any clear evidence of an underlying galaxy. It is customary to define as quasars all radio-loud active galactic nuclei above the luminosity 1011L , although this subdivision is somewhat arbitrary. Other defining properties of quasars are large UV flux, broad emission lines, large redshift, and time-

variable continuum flux. Some of the brightest radio sources are associated with quasars. The term “quasars” designates often, albeit improperly, the broader class of both distant radio quiet and radio loud active galactic nuclei whose image is nearly stellar, i.e., whose host galaxy is not clearly visible.

quasar-galaxy association The observation of an unusually large number of quasars surrounding bright, nearby galaxies. For example, more than 40 quasars have been found in a field by 3×3 square degrees centered on the spiral galaxy NGC 1097. This excess of quasars has been explained as due to gravitational lensing of the light of very far, background quasars by the matter associated with a nearby galaxy. H. Arp and co-workers, on the contrary, have argued that the quasars surrounding the nearby galaxy must be physically associated with it, and that they are, therefore, not very far background objects as implied by Hubble’s law and by their redshift. This is a distinctly non-standard interpretation, at variance with the opinion of most of the astronomical community. It is complementary to the usual interpretation that quasars are in fact the extremely active nuclei of very distant galaxies, and in this sense there is an intimate quasar-galaxy association.

quasi-biennial oscillation The alternation of easterly and westerly winds in the equatorial stratosphere with an interval between successive corresponding maxima of 20 to 36 months. The oscillation begins at altitudes of about 30 km and propagates downward at a rate of about 1 km per month.

quasi-hydrostatic approximation An approximate use of hydrostatic equilibrium for large scale weather systems. Hydrostatic equilibrium is such that in an ideal atmosphere, for an air parcel, the upward vertical pressure gradient force is equal to the gravitation force:

∂p = −ρg ∂z

where p is pressure, z is altitude, ρ is density, and g is gravitational acceleration. The negative sign shows pressure decreases as altitude increases. Thus, for an atmosphere in hydro-

© 2001 by CRC Press LLC

quasi-viscous force

static equilibrium, air parcels have zero vertical acceleration. There is no absolutely zero vertical acceleration in real atmosphere, especially in small scale weather systems. However, for large scale weather systems, comparing to the gravitational acceleration, the vertical acceleration is very small and can be neglected, and thus the quasi-hydrostatic approximation is appropriate.

quasi-linear theory In plasma physics, a theory based on perturbation theory; interactions between waves and particles are considered to be first order only. As in perturbation theory, all terms of second order in the fluctuating quantities should be small enough to be ignored. Only weakly turbulent wave-particle interactions can be treated this way: The particle distribution is only weakly affected by the self-excited waves in a random-phase uncorrelated way. This requirement not only corresponds to small disturbances in perturbation theory but even directly results from it as the waves are described in the framework of perturbation theory. The waves generated by the particles will affect the particles in a way that will tend to reduce the waves. Thus, the plasma is assumed to be a self-stabilizing system: Neither indefinite wave growth happens nor can the particles be trapped in a wave well.

The basic equation of quasi-linear theory is the Vlasov equation. All quantities then are split into a slowly evolving part and a fluctuating part. The long-term averages of the fluctuating part vanish. In quasi-linear theory, the slowly evolving quantities give the evolution of the system under study, while in case of the plasma waves the fluctuating quantities are of interest because they describe the wave.

With the index ‘o’ describing the slowly evolving quantities and the index ‘1’ giving the fluctuating quantities, the average Vlasov equation as the basic equation of quasi-linear theory can be written as



q v × Bo




∂t + v · fo









= −m E1





· v




























with f being the phase space density, v the plasma speed, q the charge, m the mass of the charged particles, E the electric, and B the magnetic field. The term on the right-hand side con-

tains the non-vanishing averages of the fluctuations and describes the interaction between the fluctuating fields and the fluctuating part of the particle distribution. These interactions, combined with the slowly evolving fields on the lefthand side lead to the evolution of the slowly evolving part of the phase space density. Note that no assumption about the smallness of the fluctuations enters: The only limitation is a clear separation between the fluctuating part and the average behavior of the plasma. See perturba- tion theory.

quasi-neutral equilibrium A plasma condition apparently occurring on magnetic field lines in the auroral oval, in which a parallel electric field is balanced by the mirror force on precipitating electrons. The parallel voltage may originate in the circuit that powers Birkeland currents linking the auroral ionosphere with the distant magnetosphere. It accelerates the auroral electrons and at the same time expands the loss cone, allowing an increase in the current intensity.

quasi-separatrix layer Generalizations of magnetic separatrices to magnetic configurations with a non-zero magnetic field strength everywhere in a given region. They are the locations where drastic changes in field-line linkages occur in the solar atmosphere.

quasi-single-scattering approximation A radiative transfer model that accounts for only single scattering of photons with the assumption that forward-scattered light is treated as unscattered.

quasi stellar object See QSO.

quasi-viscous force A force transferring momentum through the magnetopause, from the solar wind flowing past Earth to the plasma inside the geomagnetic tail. A quasi-viscous force was proposed in 1961 by Ian Axford and Colin Hines as a possible mechanism for causing global convection in the Earth’s magnetosphere. Magnetic reconnection, proposed the same year by James Dungey, is an alternative mechanism, and many researchers believe both mechanisms are active, with the quasi-viscous force being the weaker but steadier factor.

© 2001 by CRC Press LLC

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