DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

thermocline
to isopycnal surfaces. As a result, meso-scale
eddies lead to a diapycnal flux. In contrast to
cabbeling, where mixing is involved, the thermobaric
effect arises from displacement only
and does not require mixing.
thermocline The region of large temperature
gradient in oceans or lakes. Often there is
a region of large temperature gradient near the
surface of the ocean that appears only in summer
and autumn; this is called seasonal thermocline.
In low and middle latitudes, there is an
ocean thermocline present all the time at depths
between 200 and 1000 m, called the main or permanent
thermocline. The e-folding thermocline
depth scale in the ocean is about 1 km; in lakes
it is much less and depends on the clarity of the
water.
thermohaline circulation A circulation that
is driven by the buoyancy force. See also winddriven
circulation.
thermosphere See ionosphere.
Theta aurora Rare form of the aurora extending
across the polar cap from night- to dayside.
Viewed from a high flying satellite, this arc
combined with the auroral oval closely resembles
the Greek letter (Theta). Observations of
Theta aurora are limited to time periods when
the interplanetary magnetic field has a northward
component. See polar cap arc.
thick-target A plasma in which a nonthermal
population of energetic electrons is thermalized
while generating radiation. The thermalization
may be due to Coulomb collisions of
the electrons with ambient particles or collective
interactions with each other.
thin-target A plasma that has no appreciable
effect on an injected spectrum of non-thermal
particles passing through it. A thin-target scenario
would be applicable to electrons injected
outwards through the corona.
30 Doradus A star formation region in the
Large Magellanic Cloud. It is located at RA =
5.6 h and dec = −69.1 ◦ , and the main cluster
subtends approximately 7 arcmin. This is a re-
© 2001 by CRC Press LLC
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gion of very active and current star formation,
often referred to as a “starburst” and is the closest
and most visible example of such a region. It
contains a large collection of very early O-type
and Wolf–Rayet stars. The core of the cluster, R
136 (HD 38268), was once thought to be a single
supermassive star but has now been resolved
into a very dense cluster of young stars.
Thompson circulation theorem See Kelvin
circulation theorem.
Thomson scattering Scattering of electromagnetic
radiation by a charged particle that
moves nonrelativistically in the process. For unpolarized
incident radiation:
dσ/d = e 2 /mc 2 2
(1/2)(1 + cos θ) ,
where θ is the scattering angle, and e and m are
the charge and mass of the scatterer.
The total Thomson cross-section is
σT = 8π/3 e 2 /mc 2 2
for electrons.
= 0.665 × 10 −24 cm 2
t’Hooft–Polyakov monopole (1974) A
particular exactly describable magnetic-like
monopole involving a vector Higgs field φ a a =
1, 2, 3 (connected to a phase transition from
a higher temperature configuration), in which
each of the field components φ a is equal in value
to the corresponding spatial coordinate x a . See
cosmic topological defect, inflation, monopole,
monopole excess problem, winding number.
Thorpe displacement The distance a water
parcel must be moved vertically so that it is
in stable equilibrium with the surrounding water.
Turbulence can generate local overturns of
water parcels that lead to inversions of the density
profile. If a water parcel at depth z1 must
be moved to depth z2 to generate a monotonically
increasing density profile, the displacement
d1 = z2 −z1 is called the Thorpe displacement.
Thorpe displacements are useful as an aid
for defining the vertical extent of oceanic mixing
events and overturns. However, since ocean