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decelleration parameter

Theoretical power spectra for three models with critical density. Thick solid line: cold dark matter ( cdm=0.95, B h2=0.02,h=0.65); thin solid line: hot dark matter ( hdm=0.95, B h2=0.02,h=0.65) and dash dotted line: cold dark matter with cosmological constant (to=14×109yr, B =0.7, B h2=0.02). All models are normalized to produce the same quadrupole anisotropy on the Cosmic Microwave Background.

sun (apparent or mean solar time), the ﬁxed stars (sidereal time), or an atomic clock (atomic time).

daylight savings time An adjustment frequently adopted by nations to their civil time, speciﬁcally a subtraction of 1 hour from standard time during a regular period approximating summer in their hemisphere. In the U.S., the changeover currently occurs at 2:00 AM, beginning the ﬁrst Sunday in April and ending the last Sunday in October.

DDO classiﬁcation scheme A variant of the Hubble classiﬁcation scheme for galaxies, named after the David Dunlop Observatory (DDO) where it was developed. The emphasis is on the prominence and length of the spiral arms: The DDO scheme identiﬁes a new class of spirals, the anemic spirals (indicated by the letter A), which are intermediate in terms of arm prominence between the S0 galaxies and the grand-design, or gas-rich, spirals. Other labels are as in Hubble’s scheme. The original DDO scheme has undergone a major revision. The revised DDO type includes a luminosity class in addition to the morphological description. The luminosity class is indicated with a Roman numeral and ranges from I to V, in order of decreasing luminosity. For example, Messier 31 is of type Sb I-II according to the revised DDO scheme. The luminosity class subdivision reﬁnes the separation into the three classes S0, A, and S, since a good correlation is found between

the degree of spiral arm development and luminosity class, and it is therefore possible to assign a luminosity class on the basis of the appearance of spiral arms.

dead zone An area within a ﬂow ﬁeld that has very low velocities and thus will trap any contaminant that enters it.

Debye length In a plasma, the maximum length scale for which substantial deviation from charge neutrality can occur. This length scale is of order vth/ωp, where vth is the electron thermal speed, and ωp is the plasma frequency. The Debye length can be interpreted as the maximal radius of a sphere which in a twocomponent plasma might be depleted of electrons due to their thermal motion. On spatial scales small compared to the Debye length, the quasi-neutrality of a plasma is likely to be violated, while on larger scales the plasma is quasineutral: the kinetic energy contained in the thermal motion is not large enough to disturb the particle distribution over a range wider than the Debye length. For instance, if an unbalanced (but insulated) electric charge is placed in a plasma or electrolyte, ions and electrons near it will shift their average positions in response to its electric ﬁeld. That creates a secondary ﬁeld which cancels the charge’s ﬁeld further away than the Debye length.

With kB as Boltzmann constant, T as temperature, ne as electron density, and ω pe as electron plasma frequency, the Debye length can be written as

λD =	#okBT	=	k BT		·	1	.
	2e2n e			m e		ω pe

In a plasma of absolute temperature T and density n cm−3, D = 743 cm (T /n)1/2. The Debye length is also important in measuring plasma parameters: within the Debye length the electrons are inﬂuenced by the presence of a test charge, such as a satellite in a space plasma, while at larger distances the test charge goes unnoticed. Thus, in order not to inﬂuence the measurement, plasma instruments have to be mounted on sufﬁciently long booms.

	2		q	0	= − ¨
decelleration parameter			q		R/
˙		), where R(t) is the length scale of the
(R(R)		), where R(t) is the length scale of the

decibel

universe and . indicates the time derivative. This deﬁnition assumes an approximately isotropic universal expansion (as is observed). The quan-

tity	˙ is related to the Hubble parameter:	H	0 =
	R	H

˙ .

R/R

decibel A dimensionless measure of the ratio of two powers, P1 and P2, that is equal to 10 times the logarithm to the base 10 of the ratio of two powers (P1/P2). The units expressed this way are one-tenth of a bel and are referred to as decibels. The power P2 may be some reference power. For instance, in electricity, the reference power is sometimes taken as 1 milliwatt (abbreviated to dBm).

declination In terrestrial magnetism, at any given location, the angle between the geographical meridian and the magnetic meridian; that is, the angle between true north and magnetic north is the declination. Declination is measured either east or west as the compass needle points to the east or west of the geographical meridian. East is taken as the positive direction. Lines of constant declination are called isogonic lines, and the one of zero declination is called the agonic line. See dip, magnetic. In astronomy, an angle coordinate on the celestial sphere corresponding to latitude, measured in degrees north or south of the celestial equator.

decollement A near horizontal detachment zone between distinct bodies of rocks.

deep(-focus) earthquake Earthquakes at depths ranging from about 300 to 700 km that occur along the Wadati–Benioff zone, which is inclined from a trench toward a continental side beneath the subduction zone of an oceanic plate. Fault plane solutions with down-dip compression are dominant. Since a deep(-focus) earthquake takes place under high-pressure conditions, where friction is large, it is difﬁcult to explain its generation mechanism by frictional sliding processes such as those for a shallow earthquake. Recent laboratory experiments indicate that shear melting, a self-feedback system of phase transformations of olivine and fault growth, and brittle fracturing due to pore pressure are possible generation mechanisms of deep(-focus) earthquakes.

Deep Space 1 (DS1) A New Millennium spacecraft launched October 24, 1998. It is the ﬁrst mission under NASA’s New Millennium Program to test new technologies for use on future science missions. Its objective is to test 12 advanced technologies in deep space to lower the cost and risk to future science-driven missions that use them for the ﬁrst time. Among these technologies are a xenon ion propulsion system (performing beyond expectations), autonomous navigation, a high-efﬁciency solar array, and a miniature camera/spectrometer. By December 1, 1998, DS1 had accomplished enough testing to satisfy the technology validation aspects of the minimum mission success criteria and is well on its way toward meeting maximum criteria.

It carried out a ﬂyby of the near-Earth asteroid 1992 KD on July 28, 1998 at an altitude of 10 km. The primary mission ended on September 18, 1999. It is now on a new trajectory to encounter Comets Wilson–Harrington and Borrelly.

Deep Space 2 Two microprobes that were onboard the Mars Polar Lander spacecraft launched on January 3, 1999 and lost in the landing on Mars on December 3, 1999. The primary purpose of the Mars Microprobe Mission was to demonstrate key technologies for future planetary exploration while collecting meaningful science data (thus, it was named Deep Space 2). In this case, the scientiﬁc objectives were to determine if ice is present below the Martian surface; to characterize the thermal properties of the Martian subsurface soil; to characterize the atmospheric density proﬁle; to characterize the hardness of the soil and the presence of any layering at a depth of 10 cm to 1 m. See Mars Microprobe, Deep Space 1.

Deep Space Network (DSN) The NASA

Deep Space Network is a world-wide network of large antennas with the principal function of maintaining communications with spacecraft beyond the moon’s orbit. The three main tracking complexes are in Goldstone, California (U.S.), near Canberra (Australia), and Madrid (Spain).

degree (temperature)

deep water wave A wave in water that has a

depth at least half of one wavelength. Then c =

√

gL/(2π), where c is the wave speed, g is the acceleration of gravity, and L is the wavelength. See shallow water wave.

defect See cosmic topological defect.

deferent In the Ptolemaic theory of the Earthcentered universe, the large orbital circle around a point between the Earth and the equant, followed by the center of a planet’s circular epicycle.

deﬁcit angle (cosmic string) Cosmic strings are inhomogeneities in the energy density ﬁeld that form during cosmic phase transitions. In the limit of energy below the Planck energy (1019 GeV), strings can be well described in the weak gravity and thin string limit. The ﬁrst condition allows simpliﬁcation of the relevant equations, while the second assumes that dimensions transversal to the string are effectively negligible compared to the length of the string. In this limit, the Einstein equations predict a metric with the usual Minkowski aspect in cylindrical coordinates

ds2 = dt2 − dz2 − dr2 − r2dθ2

but where the azimuthal angle θ varies between 0 and 2π(1 − 4GU), with G and U being Newton’s constant and the energy per unit length of the string, respectively. While space-time looks locally ﬂat around the string, globally, however, it is non-Euclidean due to the existence of this missing angle δθ ≡ 8πGU (called the deﬁcit angle) which, in usual simple models, is small, of order 10−5 radian. This peculiar feature implies that constant-time surfaces perpendicular to different segments of a string will have the shape of a cone. See Abelian string, cosmic string, cosmic topological defect.

deﬂation A term used to denote the reduction in elevation of a beach or other area subject to sediment transport, due to transport of sediments by wind.

deformation radius	See Rossby radius of
deformation.

degeneracy The condition in which some fermion (a particle with angular momentum = 1/2h¯ and subject to the Pauli exclusion principle) is packed as tightly as quantum mechanical considerations permit. Electrons are degenerate in white dwarfs and neutrons in neutron stars and pulsars. Neutrinos, if they have nonzero rest mass and contribute to hot dark matter, may be degenerate in dwarf galaxies. The ignition of a nuclear reaction in degenerate matter leads to an explosion because the reaction heats the gas. The gas does not expand (because the pressure in degenerate matter depends only on the density, not on the temperature), and so it cannot cool. The reaction goes faster at the higher temperature and releases more energy. The gas gets hotter, and so forth, until ﬁnally it is no longer degenerate, and it expands explosively.

degree (temperature) On the Celsius thermometer scale, under standard atmosphere pressure, the freezing point of water is 0 degrees, and the boiling point of water is 100 degrees. The space between these two temperature points is separated into 100 parts. Each part represents 1 degree, i.e., 1◦ C. On the Fahrenheit thermometer scale, under standard atmosphere pressure, the freezing point of water is 32 degrees, and the boiling point is 212 degrees. Thus,

degree◦F =		9	degree◦C + 32
degree◦F =		5	degree◦C + 32
or
degree◦C =	5	degree◦F − 32 .

	9

The thermodynamic temperature scale, also called Kelvin temperature scale or absolute temperature scale, is an ideal temperature scale based on Carnot cycle theory. It was chosen as the basic temperature scale in 1927. Additionally, the international practical temperature scale has been established, based on the thermodynamic temperature scale. Currently the standard international practical temperature scale in use is the International Practical Temperature Scale 1968, IPTS-68. Symbol is T68 and unit is K. The relation between international practical Celsius temperature scale (t68, unit is ◦C) and international practical temperature scale is

t68 = T68 − 273.15

de Hoffman–Teller frame

de Hoffman–Teller frame Frame of reference in which a magnetohydrodynamic shock is at rest. In contrast to the normal incidence frame, the shock rest frame most commonly used. In the de Hoffman–Teller frame the plasma ﬂow is parallel to the magnetic ﬁeld on both sides of the shock and the v×B induction ﬁeld in the shock front vanishes. Compared to the normal incidence frame, this frame moves parallel to the shock front with the de Hoffman–Teller speed vHT × B = −E.

de Hoffman–Teller frame (right) and normal incidence

frame (left).

Deimos Moon of Mars, also designated MII. It was discovered by A. Hall in 1877. Its orbit has an eccentricity of 0.0005, an inclination of 0.9 − 2.7◦, a precession of 6.614◦ yr−1, and

a semimajor axis of 2.35 × 104 km. Its size is 7.5×6.1×5.5 km, its mass is 1.8×1015 kg, and

its density is 1.7 g cm−3. Its geometric albedo is 0.07, and its surface is similar in reﬂectivity to C-type asteroids. It may be a member of that group that was captured in the past. Deimos orbits Mars once every 1.262 Earth days.

delta An alluvial deposit where a river meets a larger body of water or near the mouth of a tidal inlet. A ﬂood delta is deposited inshore of an inlet by ﬂood tidal currents; an ebb delta is located seaward of the inlet throat and deposited by ebb tidal currents.

Delta Scuti stars Main sequence stars, generally of spectral type A, located within the instability strip on the HR diagram. They are subject to pulsational instabilities driven by hydrogen ionization, but these are generally quite subtle, amounting to brightness changes of 10% or less and with a number of modes (with periods of hours) excited simultaneously. Both the ampli-

tudes and frequencies of the modes can change over a period of years.

Delta surface approximation (Yang, 1987) The effects of the Earth’s sphericity are retained by a quadratic function of y, the meridinal coordinate measured positive northward from the reference latitude.

density Mass per unit volume.

density current A ﬂow that is driven by density variations within a ﬂuid. Typically a result of temperature or salinity gradients within a body of water.

density inversion Situation in which ﬂuid density decreases with depth. The ocean is normally stably stratiﬁed, and the water density increases monotonically with depth. Turbulence created by surface wind stress, internal waves, or tidal ﬂow can disrupt this density proﬁle by mechanical displacement of water parcels. This can lead to situations in which density locally decreases with depth. Density inversions can also be created by the local loss of buoyancy of water at the surface caused by loss, e.g., nighttime cooling of the ocean surface, or by intrusive ﬂows, such as the outﬂow of saline Mediterranean water into the Atlantic at the Strait of Gibraltar.

Since the density of sea water is a (non-linear) function of temperature and salinity, density inversion is usually accompanied by inversions of the temperature and salinity proﬁles. Normally, temperature decreases with depth, and salinity increases with depth. However, the existence of either a temperature or salinity inversion alone does not necessarily create a density inversion.

depleted mantle Mantle that has been depleted of its lightest basaltic components through processes such as partial melting. The residue after extraction of crust.

depletion layer A region adjacent to the magnetopause but outside it, where plasma density has become abnormally low. Usually found during times of northward IMF, it is caused by the compression of magnetic ﬂux tubes as they are pushed against the magnetopause, squeezing out