DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

transform boundary
shear zone, between two lithospheric plates that
both move parallel with the boundary but in opposite
directions.
transform boundary Represents the boundary
where two plates slide past each other. This
boundary is characterized by strike-slip faulting
and can be a left-lateral or a right-lateral fault,
depending on the movement of the plates involved.
A transform boundary is characterized
by shallow earthquakes but no volcanic activity.
An example of a transform boundary is the San
Andreas Fault which runs through California.
transform fault When two surface plates
slide past each other, the fault on which this
occurs is a transform fault. A transform fault
is also a strike-slip fault. An accretional plate
margin is made up of an orthogonal pattern of
spreading centers (ocean ridges) and transform
faults. The San Andreas fault is an example of
a major transform fault. Transform faults are
usually very active seismically.
transform push Normal force that one lithospheric
plate exerts on another plate across a
transform plate boundary.
transient shock Shock propagating through
a medium, either as a blast wave shock or as
a driven shock in front of an obstacle moving
faster than the local signal speed of the
medium. Transient shocks are also called traveling
shocks; examples are the super-sonic bang
in front of an aircraft, or the shock piling up in
front of a fast coronal mass ejection or a fast
magnetic cloud.
transitional depth A water depth between
shallow water and deep water. The term depth,
as used here, is a relative term and denotes actual
depth divided by wavelength.
transition region In solar physics, the thin
(some hundred kilometers) layer in the solar atmosphere
at a height of a few thousand kilometers
above the photosphere, where the temperature
increases quickly from about 6000 K
to more than 10 5 while the density drops by
about one order of magnitude, and the collision
times increase by more than four orders of mag-
© 2001 by CRC Press LLC
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nitude. Thus, belowthetransitionregion, thesolar
plasma basically behaves like a gas with the
bulk motion of the plasma carrying around the
frozen-in magnetic field, while above the transition
region collisions are rare, and the magnetic
field determines the particle motion. It is observed
mainly in EUV emission lines between
10 4 and 10 6 K.
transition region and coronal explorer
(TRACE) A NASA funded spacecraft
launched in April 1998. It explores the magnetic
field in the solar atmosphere by studying the 3dimensional
field structure, its temporal evolution
in response to photospheric flows, the timedependentcoronalfinestructure,
andthecoronal
and transition region thermal topology. TRACE
has a spatial resolution of 1 arcsec (700 km).
translatory wave A wave that exhibits a net
motion, as opposed to a standing wave.
transmission coefficient In essentially 1dimensional
wave motion, a (complex) dimensionless
parameter giving the ratio of the transmitted
wave (amplitude and phase) at some barrier,
to the incident wave. In some contexts, the
term is used to mean the absolute value (modulus)
of this quantity. For instance, in water
wave descriptions, the meaning is the ratio of
a transmitted wave height to the corresponding
incident wave height. Used to describe the effectiveness
of a coastal structure, such as a breakwater
for reduction of wave height.
transmissivity (T ) A measure of the ability
of a saturated aquifer of thickness b and hydraulic
conductivity K to transmit water: T =
Kb, with units of square meters per day [L 2 /T].
transonic string model See elastic string
model.
transparency The degree of lack of absorption
of a wavelength of interest as light traverses
the atmosphere from the source to an Earthbased
detector, or as light traverses any semitransparent
medium.
transpiration The process whereby plants
give off water vapor through their leaves back