DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Doppler broadening
Doppler broadening The broadening of a
spectral line in a gas because of red and blue
shifts associated with thermal motion in the gas.
Doppler dimming A means by which to determine
outflow plasma velocities from coronagraph
measurements of the solar corona, where
the outflows are perpendicular to the line-ofsight.
In the solar wind of the outer corona,
Doppler dimming observed in Lyman α at
1216 Å provides a diagnostic tool for determining
the outflow speed. At heights of interest,
∼ 4R⊙, densities are too small for significant
coronal Lyman α emission, so the observed
radiation is produced by resonance scattering
of chromospheric Lyman α off the HI
locally present in the corona. The intensity of
the detected radiation is determined both by the
chromospheric intensity and by the systematic
velocity of the scattering HI . This scattering is
largest when the velocity is zero and Doppler
dimming occurs when the velocity, relative to
the chromosphere, increases.
Doppler effect The observed frequency of
the wave signal from a standard source is a function
of the motion of the emitter and the detector.
In Newtonian physics with a universal timet, for
waves moving at a given speed vp with respect
to a medium (e.g., for sound waves in air):
received frequencyf ′ =
1 ±vo/vp
f
1 ∓vs/vp
where f is the source frequency, measured at
rest near the source, vs is the source speed, vo
is the observer speed relative to the medium,
both measured along the line joining source and
observer; the upper sign is for motion reducing
the relative separation. In Newtonian physics
only the component along the line between the
source and observer contributes to the Doppler
effect.
When considering the Doppler effect of light
or other electromagnetic signals, one uses the
special relativistic formula. Here there is no
medium, so the concept of motion relative to
the medium is meaningless. The relativistic formula
is:
f ′ =
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1 −v/ccosθ
√ (1 −v 2 /c 2 ) f,
where v is the velocity of relative motion of the
source and observer, θ is the angle between the
direction of propagation of the photon and the
velocity of the observer, measured in the rest
frame of the emitter. (θ = 0 corresponds to
motion increasing the separation.) Notice that in
relativistic systems there is a transverse Doppler
shift arising from the denominator, even when
there is no motion along the line between source
and observer.
Doppler shift Doppler effect.
double couple A seismological model that
a focal mechanism is produced by release of
double-couple torques with mutually opposite
direction at a hypocenter. In contrast, a model
that a focal mechanism consists of a single
torque is referred to as single couple. Although
radiation patterns for initial motion of P-waves
are the same for both models, those for S-waves
become of four quadrant type for double couple,
whereas they are of two quadrant type for
single couple. Radiation patterns for Love wave
and Rayleigh wave and their amplitude ratios are
also different between the two models. At the
beginning of the 1960s, a point source model
equivalent to lateral faulting was theoretically
proved to be double couple. The force and
moment are zero for a double couple, and the
amount of one of the two torques is called seismic
moment.
double diffusion Mixing process resulting
from differential (“double”) diffusion of salt and
heat between two water masses. In the absence
of mechanical stirring caused by shear currents,
this type of mixing occurs in two layer situations,
in which two water types of different heat
and salt composition are stacked vertically. A
necessary condition for double diffusion is that
the gradients of temperature and salinity have
the same sign. See also stability ratio.
Two scenarios are possible: in the first,
warmer and saltier water is above colder, fresher
water. Since the rate of molecular diffusion for
heat is about 100 times larger than that of salt, the
upper water loses heat to the lower water faster
than it loses salt. This results in a loss of buoyancy
of the upper water in the vicinity of the interface.
If the initial density difference between