DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

differential heating/cooling
differential heating/cooling Due to different
exposure (to the atmosphere; solar radiation,
wind, etc.), changing water transmissivity (algae,
kelp forests, etc.), or varying depth, neighboring
water bodies may experience different
rates of cooling or heating. Differential heating/cooling
leads subsequently to lateral density
gradient and may drive lateral convection.
differential rotation Change of the rotation
speed with distance from the rotation axis.
1. Change of the solar rotation speed with heliographic
longitude: the sun rotates faster close to
the equator and slower towards the poles. Typical
values for the sidereal rotation period of the
photosphere at different latitudes are given in
the table.
Sidereal Rotation
Periods of the Sun
Latitude Period [days]
equator 26.8
30 ◦ 28.2
60 ◦ 30.8
75 ◦ 31.8
In addition, there is also a differential rotation
in radius in the convection zone. In particular
at latitudes above about 30 ◦ , the rotation speed
of the sun increases towards the bottom of the
convection zone. At lower latitudes there is a
rather small decrease in rotation speed while at
latitudes around 30 ◦ there is no differential rotation
in radius. Observed in the photosphere,
low latitudes rotate at a faster angular rate (approximately
14 ◦ per day) than do high latitudes
(approximately 12 ◦ per day). This pronounced
latitude dependence appears to be the result of
convective flows driven radially by the buoyancy
force and deflected horizontally by the Coriolis
force due to solar rotation. Differential rotation
plays a crucial role in the understanding of solar
activity and the solar dynamo.
2. Differential rotation can also be observed in
systems of stars when the rotation speed varies
with distance from the center of rotation.
© 2001 by CRC Press LLC
differentiation In geophysics, the process
of separation of different materials, in geological
or astrophysical settings, via different physical
properties. For instance, the atmosphere
of the Earth is differentiated in that hydrogen
is very rare, having escaped from the upper atmosphere,
while oxygen and nitrogen have not.
Their larger molecular weight gives them lower
speeds at comparable temperatures, so they do
not achieve escape velocity at the top of the atmosphere.
Similarly, internal heating (caused by accretion,
radioactive decay, tidal heating, etc.)
causes the interior of a solid body to become
partially or completely molten. Materials making
up the body’s interior separate, depending
primarily on their densities — denser material
(such as iron) sinks to the center while less
dense materials (such as silicates) float to the
surface. This differentiation thus creates the layered
structure of the crust, mantle, and core suspected
in most of the larger solar system bodies
(those with diameters > 1000 km).
In mathematics, the linear operation (′) obeying
Leibnitz’ law: (ab) ′ = a ′ b + b ′ a which retrieves
the slope of a function if it exists, in some
suitable generalization, if not.
diffraction A term relating to the spread of
wave energy in a direction lateral to the dominant
direction of wave propagation. Used in the
description of light and water waves. Results in
interference patterns (constructive and destructive)
and accounts for the spreading of energy
into areas that would otherwise be shadowed.
diffraction grating A light analyzer, used to
disperse different chromatic elements in a light
beam, based on the principle of light diffraction
and interference by a series of parallel slits.
A typical diffraction grating consists of a large
number of equally spaced, tilted grooves (∼ 100
to 1000 grooves per millimeter). The spectral
resolving power increases with the number of
grooves per millimeter for a fixed incident bandpass.
A diffraction grating can either reflect or
transmit light. To avoid loss of light because of a
maximum of the diffraction pattern at zero order,
the grooves are tilted to shift the maximum of
the diffraction pattern toward the first or, occa-