DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

oson
boson An elementary particle of spin an integer
multiple of the reduced Planck constant ¯h.
A quantum of light (a photon) is a boson.
boson star A theoretical construct in which
the Klein–Gordon equation for a massive scalar
fieldφ:
✷φ +m 2 φ= 0
is coupled to a description of gravity, either
Newtonian gravity withφg the gravitational potential
∇ 2 φg= 4πG 1
2 (∇φ)2
or General Relativistic gravitation:
Gµv = 8πGTµv(φ)
whereTµv(φ) is the scalar stress-energy tensor.
In that case the wave operator applied toφ is the
covariant one. ✷φ =φ α ;α. Stable localized solutions,
held together by gravity are called boson
strings. Nonstationary and nonspherical boson
stars may be found by numerical integration of
the equations.
Bouguer correction A correction made to
gravity survey data that removes the effect of
the mass between the elevation of the observation
point and a reference elevation, such as the
mean sea level (or geoid). It is one of the several
steps to reduce the data to a common reference
level. In Bouguer correction, the mass
in consideration is approximated by a slab of
infinite horizontal dimension with thickness h,
which is the elevation of the observation point
above the reference level, and average densityρ.
The slab’s gravitational force to be subtracted
from the measured gravity value is then △g =
2πGρh, whereG is the gravitational constant.
Bouguer gravity anomaly Much of the
point-to-point variation in the Earth’s gravity
field can be attributed to the attractions of the
mass in mountains and to the lack of attraction
of the missing mass in valleys. When the attraction
of near surface masses (topography) is used
to correct the “free-air” gravity measurements,
the result is a Bouguer-gravity map. However,
major mountain belts (with widths greater than
about 400 km) are “compensated”. Bouguer
gravity anomalies are caused by inhomogeneous
© 2001 by CRC Press LLC
lateral density distribution below the reference
level, such as the sealevel, and are particularly
useful in studying the internal mass distributions.
Thus, the primary signal in Bouguer gravity
maps is the negative density and the negative
gravity anomalies of the crustal roots of mountain
belts.
Bouguer–Lambert law (Bouguer’s law)
See Beer’s law.
bounce motion The back-and-forth motion
of an ion or electron trapped in the Earth’s magnetic
field, between its mirror points. This motion
is associated with the second periodicity of
trapped particle motion (“bounce period”) and
with the longitudinal adiabatic invariant.
boundary conditions Values or relations
among the values of physical quantities that
have to be specified at the boundary of a domain,
in order to solve differential or difference
equations throughout the domain. For instance,
solving Laplace’s equation ∇φ = 0ina3dimensional
volume requires specifying a priori
known boundary values for φ or relations among
boundary values of φ, such as specification of
the normal derivative.
boundary layer pumping Ekman pumping.
Boussinesq approximation In the equation
of motion, the density variation is neglected except
in the gravity force.
Boussinesq assumption The Boussinesq assumption
is employed in the study of open channel
flow and shallow water waves. It involves
assumption of a linearly varying vertical velocity,
zero at the channel bottom and maximum at
the free surface.
Boussinesq equation The general flow equation
for two-dimensional unconfined flow in an
aquifer is:
∂
∂x
h ∂h
∂x
+ ∂
h
∂y
∂h
=
∂y
Sy ∂h
K ∂t
where Sy is specific yield, K is the hydraulic
conductivity, h is the saturated thickness of the
aquifer, and t is the time interval. This equation