DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Theoretical power spectra for three models with critical
density. Thick solid line: cold dark matter
(cdm=0.95,Bh2 =0.02,h=0.65); thin solid line: hot
dark matter (hdm=0.95,Bh2 =0.02,h=0.65) and dash
dotted line: cold dark matter with cosmological constant
(to=14×109yr, B=0.7, Bh2 =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
© 2001 by CRC Press LLC
decelleration parameter
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 writ-
ten as
λD =
ɛokBT
2e 2 n e
=
k BT
m e
· 1
.
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.
decelleration parameter q0 = − ¨R/
(R( ˙R) 2 ), where R(t) is the length scale of the