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Dictionary of Geophysics, Astrophysic, and Astronomy.pdf
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Tethys Ocean

Tethys Ocean In geophysics, the continents of Laurasia and Gondwanaland were separated by the Tethys Ocean approximately 200 million years ago. This ocean has closed, and the Mediterranean Sea is a remnant. See continental drift.

texture See cosmic texture.

Thalassa Moon of Neptune also designated NVII. Discovered by Voyager 2 in 1989, it is a small, irregularly shaped body approximately 40 km in radius. Its orbit has an eccentricity of 0.00016, an inclination of 0.21, a precession of 551yr1, and a semimajor axis of 5.01 × 104 km. Its mass has not been measured. It orbits Neptune once every 0.3115 Earth days.

Tharsis Province A broad Martian topographic rise centered on the equator at longitude 115W. It stands as much as 10 km above the reference datum, measures 8000 km across, and it occupies 25% of the surface area of Mars. It is the most pronounced region of central vent volcanism on Mars. It has four large shield volcanoes (Olympus Mons, Ascraeus Mons, Pavonis Mons, and Arsia Mons, the summits of which are concordant, standing 27 km above the reference datum). Olympus Mons stands 25 km above the plains, and the other three stand 17 km above the plains. All are considered to have formed by successive eruptions of low-viscosity lavas. The volcanoes account for half the planet’s volcanic production from the late Hesperian until the Amazonian.

The province is asymmetrical being twice as steep at the northwestern limit as it is on the southeastern limit. At the NW, it forms a continuous slope (only broken by remnants of old cratered terrain, e.g., Tempe Terra) with the sparsely cratered northern lowland terrain, and on the SE, it forms a continuous slope grading into the high-standing, highly cratered southern hemisphere terrain. It demonstrates a complex and extended tectonic record, being at the center of a vast radial fracture system that affects half the planet, and a substantial free-air gravity anomaly. Additionally, the huge equatorial canyons of Noctis Labyrinthus start at the center of the bulge and extend down the eastern flank to form Valles Marineris.

Thebe Moon of Jupiter, also designated JXV. Discovered by S. Synnott in 1979, its orbit has an eccentricity of 0.015, an inclination of 0.8,

and a semimajor axis of 2.22 × 105 km. Its size is 55 × 45 km, its mass 7.60 × 1017 kg, and its

density 1.6 g cm3. It has a geometric albedo of 0.05 and orbits Jupiter once every 0.674 Earth days.

thermal anisotropy See kinetic temperature, plasma stress tensor.

thermal bar Due to cabbeling, mixing of two water masses with identical density but different temperature and salinity generates denser water. The thermal bar relates to the special case of mixing two water bodies with temperatures below and above 4C. The mixed water has close to maximum density and subsequently sinks as thermal plumes. The term “bar” refers to the phenomenon that the original waters cannot cross the sinking plumes.

thermal boundary layer A portion on the edge of a body with a high thermal gradient. In convective systems (such as the Earth’s mantle), this is commonly associated with the edges of each convecting layer. The oceanic lithosphere is one example of a thermal boundary layer and illustrates clearly the importance of this boundary layer for the convective process: the layer thickness is very small at mid-ocean ridges where new boundary layer material is erupted from beneath, but the thickness increases moving away from the ridge as heat is removed from the plate by diffusion. This production of a cool boundary layer causes negative buoyancy, which allows the oceanic slab to plunge down into the interior of the convective region, transporting a heat deficit inwards (and by implication, causing a net transport of heat towards the surface). It is quite likely that there is another thermal boundary layer at the base of the mantle, where the seismically distinct D” layer is to be found and where perhaps a reverse process is taking place except that boundary layer material appear to detach from the core-mantle boundary in the form of blobs rather than slabs.

thermal bremsstrahlung See bremstrahlung [thermal].

© 2001 by CRC Press LLC


thermal conductivity The property of a material characterizing the ease with which heat is conducted through the material at steady state. Thermal conductivity λ appears in Fourier’s law of heat conduction: q = −λ · T , where q is the heat flux vector, and T is the temperature gradient. For an anisotropic material, λ is a tensor.

thermal diffusive sublayer The layer along a contiguous boundary, within which momentum (shear stress) is transferred by molecular processes; i.e., τ = ν∂u/∂z.

thermal diffusivity A property that determines the rate of the diffusion of heat in a transient state, defined as α = λ/ρc, where λ is the thermal conductivity, ρ is density, and c is the specific heat. For an anisotropic material, α is a tensor.

thermal Doppler line broadening Broadening of a spectral line due to the random motion of gas particles in a Maxwellian distribution, which thus depends on temperature. The average speed of a molecule or atom of gas is given by

v0 =



where k is Boltzmann’s constant, T is the kinetic temperature, and µ is the atomic weight in molecules. Then the number of gas particles with a velocity in the range of v from v0 + 5v to v0 5v is given by









e(v/v0) dv .


Nv0 π

This expression can be combined with the Doppler shift and an expression for the Doppler half width as the distance from the line center where the intensity of the line falls to 1/e of the


D = λ0 v0 . c

Then the normalized Doppler profile is








φλ =




/5λD d5λ .




In general, the thermal Doppler broadening for heavy elements is less than for light elements

at the same temperature, since heavier elements have lower mean velocities at any given temperature. For example, at a temperature of T = 6000 K, the n = 3 to n = 2 electronic transition of neutral hydrogen (Balmer α at 6563 Å) will have a thermal Doppler line width of 5λ/λ v/c 1/4 Å.

thermal expansivity Coefficient α expressing the relative change of density due to the change in potential temperature at constant salinity and pressure; i.e., α = −ρ1 (∂ρ/∂8)S,p [K1]. α is strongly temperaturedependent and is always positive: α > 0 for ocean water; in low-saline water α < 0 below temperature of maximum density. See actual values in UNESCO [1983], Algorithms for computation of fundamental properties of seawater,

Unesco Tech. Pap. Mar. Sci., 44, 53 pp.

thermal wind relationship The relation between density gradient (on an isobaric surface) and wind shear in the hydrostatic and geostrophic flow. This relationship takes the form

f ∂v/∂z = −1(∂ρ/∂x)p, f ∂u/∂z = 1(∂ρ/∂y)p

where u, v are eastward and northward wind components, respectively, g is the gravity, ρ is density, f is Coriolis parameter, and x, y, z are eastward, northward, and upward coordinates, respectively.

In the atmosphere when temperature decreases toward the poles, winds become more westerly with height. For an ideal atmosphere, the relation is given by

f ∂v/∂z = gT 1(∂T /∂x)p, f ∂u/∂z = −gT 1(∂T /∂y)p

where T is temperature.

thermobaric effect

See thermobaricity.

thermobaricity The compressibility of water depends on potential temperature as well as salinity. Laterally displacing water without performing work against gravity (following neutral tracks), and will lead to displacements relative

© 2001 by CRC Press LLC


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 nightto dayside. Viewed from a high flying satellite, this arc combined with the auroral oval closely resembles the Greek letter 8 (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 = 5.6h and dec = −69.1, and the main cluster subtends approximately 7 arcmin. This is a re-

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 = e2/mc2 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 =







0.665 × 10




for electrons.







t’Hooft–Polyakov monopole



particular exactly describable magnetic-like monopole involving a vector Higgs field φa a = 1, 2, 3 (connected to a phase transition from a higher temperature configuration), in which each of the field components φa is equal in value to the corresponding spatial coordinate xa. 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

© 2001 by CRC Press LLC

tidal forces

turbulence is not two-dimensional, the displacement d1 is not necessarily the distance the water parcel has traveled vertically (see Thorpe scale).

Thorpe displacements are computed from measured vertical space series of density. A sorting algorithm is applied to the sampled density profile, with ordering beginning at the shallowest depth, which re-orders the samples in ascending order. If a sample at position n is moved to position m, the associated entry in the Thorpe displacement series is then dn.

Thorpe scale Measure of the mean eddy size in turbulent oceanic flows. The Thorpe scale LT is defined as the root-mean-square of the Thorpe displacement, d,


LT = d2

where the over-bar signifies an appropriate spatial average that depends on the vertical extent of the turbulent process. If the mean vertical density gradient is much larger than the mean horizontal gradient, the Thorpe scale is proportional to the mean eddy size of the turbulence. See also Ellison scale. Since experimental evidence indicates that the Thorpe scale is nearly identical to the Ozmidov scale LO , the Thorpe scale is often considered as the maximum size of the vertically overturning eddies. See Thorpe displacement.

thrust fault A dip-slip fault upon which the deformation is compressional. See reverse fault.

Tibetan Plateau Located to the southwest of China. Its area is about 200 × 104 km2, and average sea level elevation is about 4 km. Since the Tibetan Plateau is at the middle level of the troposphere (about 600 hPa level), it strongly affects the atmospheric circulations by its orographic thermal and dynamic effects. In the summer, almost every meteorological observation element over the Tibetan Plateau has the most strong diurnal variations in the world; and cumulus convective activities are very frequent and active due to the large orographic heating effects. In the summer, the low levels of the Tibetan Plateau are a thermal low pressure region, and the high level at 200 hPa is the south Asian high, which is maintained by the high

temperature and high vertical moisture transport from the lower levels. Such vertical transport is mainly carried out by the strong and frequent cumulus convective activities over the plateau. In contrast, in winter, at low levels they are cold high pressure systems, and at high levels they are cold low pressure systems. The opposite plateau pressure systems in winter and summer cause the special plateau monsoon. To the atmosphere, the plateau is a heat source during both winter and summer; and the atmosphere over the plateau to the atmosphere around it is a heat source in summer and a cold source in winter. The thermal effects of the plateau in the winter enhance the Hadley circulation over the plateau; in the summer, they create the monsoon meridional and zonal circulation systems, which all come from the ascending air over the plateau and flow to the east Pacific, northern Africa, and southern hemisphere. The seasonal change of the cold and heat source is also an important factor to cause the seasonal change (jump) of the east Asian circulations.

tidal bore A translating wave found in coastal areas that resembles a hydraulic jump. Found at locations with very large (O(10 m)) tide range.

tidal currents A current that is driven by pressure gradients within the wave that results in tidal fluctuations in coastal areas. Often bidirectional (ebb and flood) in estuaries or rotary in open water.

tidal delta A deposit of sediments transported by a combination of waves and tidal currents at a tidal inlet. Many inlets have both ebb and flood deltas.

tidal energy dissipation Conversion of work done to a celestial body by tidal forces into heat due to the anelastic tidal response of the body. For example, the total rotational energy of the Earth–moon system decreases as a result of tidal energy dissipation, while the angular momentum is conserved. The rate of lunar tidal energy dissipation is about 3 × 1012 W.

tidal forces Differences in the gravitational force on opposite ends of an extended body caused by the different distances of those ends

© 2001 by CRC Press LLC

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