DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Brazil current
A generalization of Brans–Dicke theory is
given by the dilaton gravity theories, in which
the scalar field φ is called the dilaton and couples
directly to all matter fields, thus violating
even the weak equivalence principle. See dilaton
gravity.
Brazil current A warm ocean current that
travels southwestward along the central coast of
South America.
breaker zone The nearshore zone containing
all breaking waves at a coast. Width of this zone
will depend on the range of wave periods and
heights in the wave train and on bathymetry.
breakwater A man-made structure, often of
rubble mound construction, intended to shelter
the area behind (landward of) it. May be used
for erosion control or to shelter a harbor entrance
channel or other facility.
breccias Composite rocks found on the
moon, consisting of heterogeneous particles
compacted and sintered together, typically of a
light gray color.
Bremsstrahlung (German for braking radiation.)
Radiation emitted by a charged particle
under acceleration; in particular, radiation
caused by decelerations when passing through
the field of atomic nuclei, as in X-ray tubes,
whereelectronswithenergiesoftensofkilovolts
are stopped in a metal anode. Bremsstrahlung
is the most common source of solar flare radiation,
where it is generated by deceleration of
electrons by the Coulomb fields of ions.
bremstrahlung [thermal] The emission of
bremstrahlung radiation from an ionized gas at
local thermodynamic equilibrium. The electrons
are the primary radiators since the relative
accelerations are inversely proportional to
masses, and the charges are roughly equal. Applying
Maxwell’s velocity distribution to the
electrons, the amount of emitted radiation per
time per volume per frequency is obtained for a
given temperature and ionized gas density.
Brewster point One of three points on the
sky in a vertical line through the sun at which
© 2001 by CRC Press LLC
the polarization of skylight vanishes. Usually
located at about 20 ◦ below the sun. See Arago
point, Babinet point.
brightness The luminosity of a source [J/sec]
in the bandpass of interest. (For instance, if
referred to visual observations, there are corrections
for the spectral response of the eye.)
Apparent brightness of a stellar object depends
on its absolute luminosity and on its distance.
The flux of energy through a unit area of detector
(e.g., through a telescope objective) is
F = L/4πr 2 [J/sec/m 2 ]. Stellar apparent magnitude
gives a logarithmic measure of brightness:
m =−2.5 log(F ) + K. The magnitude
scale is calibrated by fixing the constant K using
a number of fiducial stars. Absolute magnitude
is defined as the apparent magnitude a source
would have if seen at a distance of 10pc and is a
measure of the absolute brightness of the source.
brightness temperature The temperature of
an equivalent black body with a given intensity
and long wavelengths where the Rayleigh–
Jeans approximation is valid. This temperature
is used in radio and submillimeter astronomy
and is given by
TB =
Iνc 2
2kν 2
where h and k are the Planck and Boltzmann
constants, respectively, ν is the frequency in
Hertz, and I ν is the monochromatic specific intensity
(i.e., the flux per frequency interval per
solid angle) in units of erg s −1 cm −2 Hz −1 sr −1 .
To relate an object’s brightness temperature to
its flux density in Janskys, the size of the source
in stearadians (or the full width of the main beam
of the radiotelescope) is used:
which reduces to
Sν = 2πkTBθ 2 4ln2/λ 2
Sν = 7.35 × 10 −4 θ 2 TB/λ 2
where θ is the width of the telescope’s full beam
at half maximum in arcseconds, TB is the brightness
temperature in Kelvins, λ is the center
wavelength in centimeters, and Sν is the flux
density in Janskys.