DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

charge exchange
an ordinary differential equation problem. The
solution may be obtained graphically or by numerical
techniques. Used in the study of open
channel flow and elsewhere.
charge exchange An atomic process that
may occur when a fast ion collides with a neutral
atom, in which an electron is transferred from
the atom to the ion. As a result, the atom becomes
a slow ion, while the ion becomes an energetic
neutral atom (ENA). It strongly depends
on energy (also on the type of ion) and is much
more pronounced around 5 keV than at 50 keV.
Charge exchange is believed to be the major
mechanism by which ions produced in a magnetic
storm are lost, leading to the decay of the
enhanced ring current associated with the storm.
Ionsofthatringcurrentaremagneticallytrapped
by the Earth, but if they change into ENAs, the
magnetic field no longer affects them, and they
move in straight lines, making them a possible
means of remotely sensing the ring current.
Charles’ Law At constant pressure, the volume
of an ideal gas is directly proportional to
the absolute temperature of that gas.
Charon Moon of Pluto, although it is close
enough in size to Pluto to be considered a double
planet. It was discovered by Christy in 1978. Its
orbithasaneccentricityof0.0001, aninclination
of 98.3 ◦ , and a semimajor axis of 1.96 ×10 4 km.
Both Pluto and Charon rotate in such a way as to
always keep the same face towards each other.
Its radius is not well determined, with measurements
giving between 590 and 630 km. Its mass
and density are not well known. Its geometric
albedo is 0.36, and it orbits every 6.387 Earth
days.
chemically peculiar star A star whose surface
composition deviates from the standard set
by the sun (3/4 hydrogen, 1/4 helium, a few percent
of everything else in standard proportions:
See cosmic abundance). This happens for one of
three reasons: (1) products of nuclear reactions
in the star itself have been mixed to the surface
(as in carbon stars or R Coronae Borealis stars),
(2) products of nuclear reactions in a companion
star have been transferred to the observed,
or (3) a combination of radiation pressure and
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a very steady atmosphere has separated out unusual
atoms, often in regions of strong magnetic
fields. Europium and strontium are among the
elements sometimes greatly overabundant in the
third sort of star, called an Ap star (A for its spectral
type, p for peculiar).
chenier Beach ridge.
Chezy coefficient, Chezy equation The
Chezy equation is used to relate flow speed to
flow geometry and other parameters. The equation
is V =C RSf , where V is flow speed,
C is the Chezy coefficient, R =A/P = hydraulic
radius, and Sf is friction slope (head
loss per unit length). The Chezy coefficient is
determined primarily by channel roughness but
decreases with increasing roughness. An alternative
to the Manning equation.
chlorinity The concentration of chloride ions
by mass in seawater (g/kg). Typical ocean chlorinity
is of order 19 g/kg of seawater. There is
an observed constant ratio of chlorinity to salinity:
Salinity in parts per thousand = (1.80655)
(chlorinity in parts per thousand). See salinity.
chlorophyll Chemical compounds occurring
in plants that enable radiant energy to be converted
to chemical energy in the process of photosynthesis;
there are several types (e.g., denoted
as chl a, b, etc.) with chl a typically the
most abundant.
chondrite A relatively abundant stony meteorite
characterized by the presence of small
(∼ 1 mm) glassy spherical inclusions called
chondrules. Chondrites are further subdivided
into ordinary, carbonaceous, and enstatite chondrites,
depending on details of composition and
thermal processing.
chondrite, carbonaceous A form of chondrite
with high abundances of water, organic,
and other volatile materials. This, and the fact
that the heavier elements are present in roughly
solar proportions, indicates that these bodies
have been little processed. These meteorites
are further classified by their volatile content
into CI (or C1), which are most nearly solar in
composition, CII (CM) and CIII (CO or CV),