DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

celeration process occurs mainly within about
8000 km of Earth.
discrimination (seismic) The use of seismology
to determine whether a seismic event was
caused by the test of an explosive device (such
as the relative slip of rock on either side of a
fault, while explosions generate waves through
a more isotropic compression of the surrounding
rock). The difference in the source mechanisms
and their relative durations (explosions tend to
be very rapid) means that the seismic signals
generated differ in the geometrical pattern of the
radiation, in the proportion of different types of
seismic wave emitted, and in frequency content.
Large explosions that can be detected at long
distances may be reliably discriminated through
study of the seismic signals, although smaller
explosions may be more difficult both to detect
and to distinguish from earthquakes. Potential
complications include the geological setting of
the event (which may not be well known) and
whether an effort has been made to decouple the
explosion from the surrounding rock by setting
it off in a cavity.
disk warp A deviation from planarity of the
disk of a galaxy, in which the outer parts of a
galactic disk gradually tilt. More precisely, one
can think of the disk of a galaxy as a sequence
of concentric, adjacent rings: If the disk is flat,
the rings are coplanar; if the disk is warped, the
inclination and the position angle of the intersection
line between rings (line of nodes) varies
continuously from ring to ring. A disk warp is
more frequently observed in the gaseous than in
the stellar component of a galaxy. The distribution
of the 21-cm hydrogen emission line often
reveals a warp outside the boundaries of the optical
disk. Small warps are probably present in the
outer regions of most disk galaxies (including
the galaxy); strong warps, where the disk plane
tilts by 40 to 50 ◦ , as in the case of NGC 660, are
of rare occurrence.
dislocation climb Dislocation motion when
an edge dislocation moves along the direction
perpendicular to its slip plane under the action of
a shear stress. The climb of dislocation involves
transport of matter. For the dislocation line to
climb one interatomic distance up or down, a
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disparition brusque
line of atoms along the edge of the extra-half
plane has to be removed or added according
to the position of the extra-half plane, which is
completed by diffusion.
dislocation creep A continuous, usually
slow deformation of solid crystalline materials
resulting from the motion (glide, climb, and
cross slip) of dislocations under the action of
shear stress. The resistance to dislocation motion
includes: (1) intrinsic resistance (the Peierls
stress); (2) impurities, and (3) mutual interaction
of dislocations. In general, the largest resistance
force controls the rate of deformation. Dislocation
creep is characterized by (1) a non-linear dependence
of strain rate on stress (n = 3 − 5, n is
stress sensitivity of creep rate at the steady-state
stage); (2) crystals deform on specific crystallographic
planes along certain orientations so that
it can produce strong lattice preferred orientation;
(3) significant transient creep is expected.
dislocation energy (self-energy of dislocation)
The energy stored in a solid by the existence of
a unit length of dislocation line. The magnitude
of dislocation energy depends on the nature of
particular dislocations such as the type and configuration.
For example, the Peierls energy is
the energy associated with the Peierls dislocation,
which can be divided into two portions: the
elastic energy stored in the two half-crystals, and
the misfit energy (core energy) associated with
the distorted bond.
dislocation glide The manner of dislocation
motion when a dislocation moves on its slip
plane under the action of a shear stress. Glide
of dislocation involves no transport of matter by
diffusion: atomic bonds are shifted, but there
is no need for atoms to be brought in or evacuated.
The resistance to the dislocation glide is
intrinsic, mainly from the Peierls stress.
disparition brusque The sudden disappearance,
within the space of a few hours, of a solar
filament. This disappearance begins with a
slow rising motion at a few kms −1 and is typically
associated with a brightening in X-rays
and occasionally by the appearance of Hα flare
ribbons.