DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

diurnal motion
the non-periodic variation caused by synoptic
situations (such as advection processes). The
strength of diurnal variation is related to location,
stronger in continental areas and weaker in
maritime areas, e.g., the strongest diurnal variation
region is the Tibetan Plateau. The diurnal
range in equatorial areas exceeds the annual
variation in average temperature. More strictly,
diurnal pertains to occurrences during the day,
as opposed to nocturnal occurrences; diel occurrences
are those that happen once per day.
diurnal motion Apparent motion of objects
on the celestial sphere due to the rotation of
Earth from west to east, which causes objects
to appear to rise in the east and set in the west
daily.
divergence Branching off or moving in different
directions. Also denotes a mathematical
operation; the divergence of a vector u is denoted
by ∇ ·u.
divergence law for irradiance See Gershun’s
law.
divergence theorem Also called Gauss’ theorem:
B ·n dA = ∇·B dV
A
with V being a volume enclosed by surface A
and n is a unit normal toA. Used, for instance, to
convert between integral and differential forms
of Maxwell’s equations.
divergent boundary In tectonics, two plates
moveapartfromeachotheratadivergentboundary.
Magma moves up from the Earth’s asthenosphere
at the divergent boundary. The magma
rises to the surface, where it cools and solidifies
as the volcanic rock basalt. Continued
magma ascent forces the newly formed basaltic
crust to move to the sides as the process repeats.
Thus, divergent boundaries are areas where new
crust is formed and where the plates move apart.
Divergent boundaries are characterized by volcanism
(quiet eruptions with very fluid lavas)
and shallow earthquakes. Divergent boundaries
are believed to occur over the uprising portions
of convection cells within the Earth’s astheno-
© 2001 by CRC Press LLC
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sphere. The Mid-Atlantic Rift and the East
African Rift Valley are examples of divergent
boundaries.
divergent plate boundary See divergent
boundary, seafloor spreading.
diversity reception Often, a radio circuit will
have many different possible paths between the
transmitter and receiver. There may be differences
in the quality of the service on these paths
and the quality may vary in time, space, and frequency.
Diversity methods seek to exploit these
differences. In the simplest form, signals on two
different paths may be received and the maximum
signal selected. More complex diversity
systems may make use of redundancy between
signals collected from several paths. See ionospheric
radio propagation path.
dividedbar Thenameofanapparatususedto
measure thermal conductivities of disk-shaped
rock samples. Two metal, usually brass, heads
with circular cross-sections are maintained at
different constant temperatures. Rock samples
with known and unknown thermal conductivities
are sandwiched in between. The heat flux
q through the axis (z) of the system is determined
from the measured temperature gradient
dT/dz across the rock sample of known thermal
conductivityλk from Fourier’s law of heat conduction.
The thermal conductivity of the other
sample is then given by this heat flux and the
measured temperature gradient across it. See
conductive heat transfer.
D-layer The D-layer is the lowest ionospheric
layer at heights between 60 and 85 km.
In contrast to the other ionospheric layers, it is
still inside the mesosphere. Ionization is primarily
due to energetic particles of solar and galactic
origin and some UV lines which penetrate deep
into the atmosphere, such as the Lyman-α-line.
The main constituent is NO, which results from
a combination of atomar oxygen O and molecular
nitrogen N2. The layer can be described
very well by a Chapman profile. Due to the
high densities, free electrons are rare and ion
clusters and negative ions form instead. Polar
cap absoptions (PCAs) and sudden ionospheric
disturbances (SID) strongly modify this layer.