DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

ν 1/4 q 1/2 ɛ −1/4 the nondimensional vertical
wavenumber, ν the kinematic viscosity of water,
χT the dissipation rate of temperature variance,
and ɛ the dissipation rate of turbulent kinetic
energy; LB is the Batchelor scale (Batchelor,
1959). The constancy of the turbulence
parameter q, usually taken as q = 3.4 (Dillon
and Caldwell, 1980), is subject to debate.
The Batchelor spectrum of salt follows the same
function with the molecular diffusivity κS of
salt (instead of κT ) and the dissipation rate of
salt variance χS (instead of χT ). For small
k, the spectrum φT (kz) drops as k −1 (viscousconvective
subrange), and for k larger than the
Batchelor wavenumber kB ,φT (kz) rolls off at a
much steeper rate (viscous-diffusive subrange).
These two subranges are commonly referred to
as Batchelor spectrum.
Batchelor wavenumber The inverse of the
Batchelor scale, expressing the wavenumber kB,
above which scalar fluctuations (such as those
of temperature or salt) become eradicated by
the smoothing effect of molecular diffusion (i.e.,
Batchelor spectrum drops off sharply above kB).
Two definitions are common: kB = (ε/νκ 2 ) 1/4
[rad m −1 ], or kB = (2π) −1 (ε/νκ 2 ) 1/4 [cycles
m −1 ]. κ is the molecular diffusivity of the scalar
(κ = κT for temperature or κ = κS for salt).
batholith Large igneous body where molten
rocks have solidified at depth in the ocean. A
typical batholith has a thickness of a few kilometers
and horizontal dimension of tens to hundreds
of kilometers.
bathymetry Measure of the depth of the sea
floor below sea level.
b-boundary (or bundle boundary) In relativity,
assorted boundary points of the spacetime
manifold defined by equivalence classes
of curves in the orthogonal frame bundle of the
space-time (B. Schmidt). The goal of this work
is to evade the difficulties of setting up a topology
on a manifold with an indefinite metric by
using the positive-definite measure on the frame
bundle. Examples of cosmological space-times
have been found which show that the notion of
the b-boundary is not suitable for defining the
© 2001 by CRC Press LLC
beams, ion
boundary of a space-time, due to topological
problems.
beach cusps Periodic (in space), transient,
cuspate features which commonly appear in
beach planforms. Wavelengths typically range
from 1 to 50 meters, with amplitudes less than
a wavelength.
beachface The relatively planar portion of
a beach in the vicinity of the still water line.
Subject to swash by wave action; therefore, alternately
wet and dry.
beach mining The process of removing sand
for some human activity, often industrial.
beach morphology The study of the shape
and form of a beach or coastal area.
beach nourishment The process of placing
sand on an eroding beach to advance the shoreline
seaward. Sand is typically transported by
hydraulic dredge via a pipeline, delivered by a
split-hull barge, or transferred overland by truck.
Typical project sand volumes are 10,000 to 10
million m 3 .
beach profile A cross-shore slice through a
beach, illustrating how bathymetry varies in the
shore-normal direction.
beach ridge A linear ridge of sand that appears
behind the modern beach and indicates a
prograding beach. Multiple beach ridges are often
visible. Referred to as “Cheniers” in Texas
and Louisiana.
beam attenuation coefficient The limit of
the ratio of the spectral absorptance plus spectral
scatterance to the distance of photon travel as
that distance becomes vanishingly small [m −1 ];
equal to the sum of the absorption and scattering
coefficients.
beams, ion Beam-like upwards flow of
ions, usually O + (singly ionized oxygen), observed
above the atmosphere during polar auroras.
First seen in 1976 by the S3-3 satellite,
ion beams are believed to be accelerated
by the same parallel electric field that accel-