DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

de Hoffman–Teller frame
de Hoffman–Teller frame Frame of reference
in which a magnetohydrodynamic shock
is at rest. In contrast to the normal incidence
frame, the shock rest frame most commonly
used. In the de Hoffman–Teller frame the plasma
flow is parallel to the magnetic field on both
sides of the shock and the v×B induction field in
the shock front vanishes. Compared to the normal
incidence frame, this frame moves parallel
to the shock front with the de Hoffman–Teller
speed vHT × B =−E.
de Hoffman–Teller frame (right) and normal incidence
frame (left).
Deimos Moon of Mars, also designated MII.
It was discovered by A. Hall in 1877. Its orbit
has an eccentricity of 0.0005, an inclination of
0.9 − 2.7 ◦ , a precession of 6.614 ◦ yr −1 , and
a semimajor axis of 2.35 × 10 4 km. Its size is
7.5×6.1×5.5 km, its mass is 1.8×10 15 kg, and
its density is 1.7 g cm −3 . Its geometric albedo
is 0.07, and its surface is similar in reflectivity
to C-type asteroids. It may be a member of that
group that was captured in the past. Deimos
orbits Mars once every 1.262 Earth days.
delta An alluvial deposit where a river meets
a larger body of water or near the mouth of a
tidal inlet. A flood delta is deposited inshore of
an inlet by flood tidal currents; an ebb delta is
located seaward of the inlet throat and deposited
by ebb tidal currents.
Delta Scuti stars Main sequence stars, generally
of spectral type A, located within the instability
strip on the HR diagram. They are subject
to pulsational instabilities driven by hydrogen
ionization, but these are generally quite subtle,
amounting to brightness changes of 10% or less
and with a number of modes (with periods of
hours) excited simultaneously. Both the ampli-
© 2001 by CRC Press LLC
tudes and frequencies of the modes can change
over a period of years.
Delta surface approximation (Yang, 1987)
The effects of the Earth’s sphericity are retained
by a quadratic function of y, the meridinal coordinate
measured positive northward from the
reference latitude.
density Mass per unit volume.
density current A flow that is driven by density
variations within a fluid. Typically a result
of temperature or salinity gradients within
a body of water.
density inversion Situation in which fluid
density decreases with depth. The ocean is normally
stably stratified, and the water density increases
monotonically with depth. Turbulence
created by surface wind stress, internal waves,
or tidal flow can disrupt this density profile by
mechanical displacement of water parcels. This
can lead to situations in which density locally
decreases with depth. Density inversions can
also be created by the local loss of buoyancy of
water at the surface caused by loss, e.g., nighttime
cooling of the ocean surface, or by intrusive
flows, such as the outflow of saline Mediterranean
water into the Atlantic at the Strait of
Gibraltar.
Since the density of sea water is a (non-linear)
function of temperature and salinity, density inversion
is usually accompanied by inversions of
the temperature and salinity profiles. Normally,
temperature decreases with depth, and salinity
increases with depth. However, the existence of
either a temperature or salinity inversion alone
does not necessarily create a density inversion.
depleted mantle Mantle that has been
depleted of its lightest basaltic components
through processes such as partial melting. The
residue after extraction of crust.
depletion layer A region adjacent to the magnetopause
but outside it, where plasma density
has become abnormally low. Usually found during
times of northward IMF, it is caused by the
compression of magnetic flux tubes as they are
pushed against the magnetopause, squeezing out