DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

adio axis and the line of sight form a larger
angle in lobe-dominated objects. The jet onesidedness
suggests that radiation is boosted by
relativistic beaming: If the emitting particles are
moving at a velocity close to the speed of light,
the detection of the jet on the approaching side
is strongly favored. In this case, a very large
dynamical range is needed to detect the radiolobes,
which, seen pole-on, may appear as a faint
fuzz surrounding the core.
core flow Magnetic field may be generated
in a conducting fluid by fluid flow, as described
by the induction equation of magnetohydrodynamics.
Such flow in the Earth’s core is thought
to be the generating mechanism for the bulk of
the Earth’s magnetic field. The motions and
field within the core cannot be directly calculated
from observations of the magnetic field at
the Earth’s surface (although it may be possible
to indirectly infer certain parts of the internal
flow and field). However, observations of the
surface field can be used to calculate the field
at the core-mantle boundary by assuming that
the electrical currents in the mantle are negligible,
in which case the flow at the surface of the
core is constrained using the radial component
of the induction equation of magnetohydrodynamics
in the frozen-flux limit (i.e., assuming
that magnetic diffusion within the core may be
neglected):
∂Br
∂t =∇H·(Bru)
where Br is the radial field and u the velocity
vector. Since the flow does not penetrate
the core-mantle boundary, there are two components
of the velocity but only one constraining
equation, which means that although the above
equation can be used to invert time varying models
of Br for possible flows, they will not be
uniquely determined. Extra constraints on the
flow have been used to alleviate the nonuniqueness.
See nonuniqueness.
core-mantle boundary At around 3480 km
from the center of the Earth, the material composition
is thought to change from molten iron plus
dissolved impurities (the outer core) to crystalline
silicate rock (the mantle). This coremantle
boundary is in terms of absolute den-
© 2001 by CRC Press LLC
core-mantle coupling
sity contrast the Earth’s major transition, with
a jump from 5.6 g/cm 3 at the base of the mantle
to 9.9 g/cm 3 at the top of the core. There
is also a significant contrast in viscosity (although
the viscosities of both sides are poorly
constrained) and also, quite possibly, conductivity
(although it has been proposed that the
conductivity of the base of the mantle is highly
elevated). The degree to which chemical exchange
occurs across the core-mantle boundary
is a frequent topic of study. Analysis of seismic
data has indicated significant lateral variation in
seismic wave speeds at the base of the mantle
and also anisotropy, and there have also been
claims of seismic observations of topography of
the boundary itself. See core-mantle coupling.
core-mantlecoupling Thistermmaybeused
to refer either to the exchange of material across
the core-mantle boundary (e.g., via chemical
reaction), or the exchange of momentum between
parts of the core and the mantle. The
mechanism for the latter type of coupling is disputed,
as is the degree to which the former type
of coupling occurs at all. Changes in the rotation
rate and direction of the Earth’s mantle
on timescales of decades are usually attributed
to momentum exchange between the core and
the mantle, and there have been claims that the
core is important on even shorter timescales,
even though on subannual timescales the atmosphere
is the predominant driving force for
changes in Earth’s rotation. There are several
possibilities for how momentum exchange may
occur: viscous coupling, electromagnetic coupling,
topographic coupling, and gravitational
coupling. Viscous coupling is usually ruled out
as the cause of the observed changes in Earth rotation
because the viscosity of the core is usually
regarded as very small. Electromagnetic coupling
would occur via magnetic linkage between
the liquid metal of the core and conductivity in
the mantle, in which electrical currents would be
induced by changes in the magnetic field. Topographic
coupling would occur through the resistance
of topography on the core-mantle boundary
to flow at the top of the core, while gravitational
coupling would happen if lateral variations
in the density of the mantle and core are
arranged so as to yield a torque between the two.
See core-mantle boundary.