DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

which are successively poorer in volatile content.
C1 chondrites do not contain chondrules
but are chemically very similar to other chondrites.
chondrite, ordinary The most common
form of meteorite. Composed mostly of olivine
with significant amounts of other metallic silicates.
Ordinary chondrites are further classified
according to their iron content into H (high iron),
L (low iron), and LL (very low iron) chondrites.
An additional subdivision, E (enstatite), is characterized
by lower Mg/Si ratios, which give rise
to a composition richer in enstatite and containing
little olivine.
chondrule Small spheroidal (∼ 1 mm)
glassy bead of silicates (olivine or enstatite)
found within stony meteorites, called chondrites.
Chondrules generally have a composition
very similar to the matrix material of the
chondrite in which they are embedded, but this
material has undergone an episode of melting
and rapid quenching early in the history of solar
system formation. The actual process causing
the melting is poorly understood.
Christoffel symbol See affine connection,
metricity of covariant derivative.
chromatic aberration A situation arising in
lens-based (refracting) optical instruments, in
which light of different colors cannot be brought
into focus at the same point, arising because
glass or other lens material is dispersive and so
produces different deviation of the path of light
of different colors.
chromosphere The layer in the sun or a cool
star between the photosphere and corona. The
layer is relatively thin and at a temperature of
about 10,000 K in the sun. During a total solar
eclipse, it is seen as a red ring (hence the name)
around the moon’s shadow. Outside eclipse, it
contributes emission lines to the spectrum of the
sun (or star). The intensity of the chromosphere
varies through the solar cycle.
chromospheric evaporation The upward
flow of hot plasma in a solar flare resulting
from the fast deposition of energy in the chro-
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cigar distribution
mosphere. In the chromospheric evaporation
process, the energy released during a solar flare
rapidly dissipates in the chromosphere (typically
assumed to involve non-thermal electron
beams or a thermal conduction front). The chromosphere
is suddenly heated to coronal temperatures
and subsequently expands upward. Evidence
for these flows generally comes from
blueshifts measured in spectral lines, such as
those observed by a Bragg Crystal Spectrometer.
chromospheric heating The process behind
the enhanced temperature of the sun’s chromosphere
above that of the sun’s surface (photosphere).
Radiation is the bulk energy loss mechanism
in the chromosphere, conductive losses
being negligible, and both the photosphere and
corona contribute to its heating. The amount of
heating required to balance radiative losses in
the chromosphere is about 4 × 10 3 Wm −2 for
quiet sun and coronal hole regions. This rises
to ∼ 2 × 10 4 Wm −2 for the chromosphere in
active regions. The exact nature of the heating
mechanism is uncertain.
chronological future/past The chronological
future/past I ± (S) of a set S is defined as
the union of all points that can be reached from
S by a future/past-directed time-like curve. (A
time-like curve must be of non-zero extent, thus
isolated points are excluded.)
CHUMP (Charged Hypothetical Ultra
Massive Particle) In the framework of grand
unification models, hypothetical CHUMPs are
among the candidates to explain the missing
mass problem in the universe (the fact that the
observed luminous matter cannot account for the
dynamical properties observed on large scales).
As they carry an electric charge, they need to
be very massive, otherwise they would already
have been detected. An example of such a particle
is the vorton, which would appear in theories
having current-carrying cosmic strings. See
dark matter, vorton.
cigar distribution In plasma physics, a pitch
angle distribution in which the highest intensities
are field aligned, so that a contour of the flux