DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

plasmas, such as the lower chromosphere and
photosphere.
coesite A dense, heavy form of silica,
formed at extremely high pressures and temperatures
(40 kb at 700 ◦ C to 1700 ◦ C). Coesite is
found naturally only in meteorite impact craters.
Stishovite, an even higher temperature, higher
pressure polymorph of quartz, is also found associated
with impact craters.
cohesionless Without cohesion; used to describe
sediments, such as sand, that have little
tendency to cling together when completely dry
or wet (surface tension will hold moist particles
together if partially wet).
cohesive sediment Sediment in which particles
cling together due to electrical charges on
particle edges or sides. These forces are significant
compared to particle weight. Clays represent
the best example of cohesive sediments.
cold dark matter See dark matter, cold.
cold front The front line along which a
wedge of cold sector air underruns and displaces
the warm sector mass. The gradient of the upper
surface of the cold air is steep, about 1/25 to
1/100. Along the cold front there is a strong instability
that causes cumulonimbus clouds with
rain and thunder. When the cold front passes,
weather changes with significant temperature
decrease, increasing pressure, and wind veering
to northerly or north-westerly in the northern
hemisphere.
cold plasma Formal description of a plasma
neglecting the thermal motion of the particles.
Such cold plasma is the basic assumption in
magnetohydrodynamics: it allows the description
of a plasma as a fluid with all particles having
the same speed, the bulk speed, and is sufficient
to derive concepts, such as frozen-in fields,
magnetic pressure and tension, and reconnection.
Coleman–Weinberg potential Spontaneous
symmetry breaking field-theory models predict
that topological defects (magnetic monopoles,
cosmic strings, domain walls) could survive
© 2001 by CRC Press LLC
colored dissolved organic matter (CDOM)
from the early universe. However, they require
that a scalar field φ evolves in a potential
such that its minimum (which determines
the actual vacuum) is reached for non-vanishing
values of φ. Ad hoc fixing of such an arbitrary
potential appears rather unnatural. However,
Coleman and Weinberg realized in 1973 that in
some cases, quantum radiative corrections to an
otherwise more natural potential could generate
the required shape for symmetry breaking.
See Higgs mechanism, spontaneous symmetry
breaking.
collision boundary See convergent boundary.
collisionless shock The momentum transport
by collisions plays a crucial role in the
formation of gas-dynamic shocks. In the rarefied
plasmas of space, too few collisions occur
between the constituents of the plasma to provide
efficient momentum transport and allow for
shock formation. Nonetheless, shocks do exist
in space plasmas. These shocks are called collisionless
shocks; here the collective behavior of
the plasma is not guaranteed by collisions but by
the collective effects of the electrical and magnetic
properties of the plasma, which allow for
frequent interactions between the particles and,
consequently, also for the formation of a shock
wave.
color-color diagram A plot of one color index
vs. another color index of a collection of
stars. In the Johnson photometry system, the
commonly used color-color diagram plots the
U-B color vs. the B-V color. The placement
of a star on this diagram is a function of its intrinsic
colors (which are a measure of the star’s
temperature) and the strength of the interstellar
reddening. The effect of reddening is to move
stars along a line of increasing U-B and B-V
values.
colored dissolved organic matter (CDOM)
High-molecular-weight organic compounds
(humic and fulvic acids) formed from the decomposition
of plant tissue; they strongly absorb
light at the blue end of the spectrum and can give
water a yellowish color at high concentrations.