DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

T
tachyon A hypothetical subatomic particle
that travels faster than the speed of light. There
is no experimental or observational evidence of
the existence of tachyons.
TAI See International Atomic Time.
tail current Current system in the tail of the
magnetosphere, consisting of the cross-tail current
separating the northern and southern lobe
and the Chapman–Ferraro currents in the magnetopause.
Thus, one closed current encircles
each lobe, with both currents running together
in the cross-tail portion.
Tail current.
Talwani’s method An algorithm concerning
analyses of magnetic anomaly and gravity
anomaly proposed by M. Talwani (1965). For
instance, the magnetic anomaly produced by
a three-dimensional magnetic substance, such
as a seamount, can be calculated, approximating
the configuration of the substance by a
stack of polygonal thin plates with homogeneous
magnetic susceptibility. Similarly, the
free air anomaly produced by a two-dimensional
substance with a density contrast can be calculated,
making an approximation for a crosssection
of the substance by a polygon.
tangential discontinuity See hydromagnetic
wave.
Taylor number
tangential geostrophy A force balance believed
to be appropriate for fluid near the surface
of the Earth’s core and which is useful as an assumption
for constructing models of the flow at
the surface of the core from models of the magnetic
field at the core-mantle boundary, because
it reduces the ambiguity inherent to such flow
modeling. Geostrophy is a balance between
pressure and coriolis forces:
2ρ× u = −∇p
where ρ is the density, is the rotation vector,
u is the velocity, and p the pressure. Tangential
geostrophy assumes that this balance holds
in the horizontal direction (but not necessarily
in the radial direction), i.e., that the Lorentz and
viscous forces, buoyancy, and inertia are only
important in the radial direction, if at all. The
justification for this is that the Lorentz force
may be reduced near the core-mantle boundary,
since if the mantle is insulating, then the toroidal
part of the magnetic field should drop to zero
at the boundary, that gravity is predominantly
radial, and that viscosity and inertia are small.
These assumptions are arguable. However, the
assumptions lead to a constraint on the flow:
∇H · (u cos θ) = 0
where θ is the co-latitude, and ∇H is the horizontal
portion of the divergence operator. As the
inverse problem for the flow is undetermined,
this constraint can be useful for reducing flow
ambiguity. Also, any part of the flow that represents
torsional oscillations in the core will obey
this constraint. See core flow.
Taygeta Magnitude 4.4 type B7 star at RA
03 h 45 m , dec +24.27’; one of the “seven sisters”
of the Pleiades.
Taylor instability Formation of rolls generated
in a column of fluid bounded by differentially
rotating cylindrical walls; governed by the
dimensionless Taylor number: Ta = riω 2 h 3 /
ν 2 , where ri is the radius of the inner cylinder,
ω is the rate of rotation, h is the space between
cylinders, and ν is the kinematic viscosity.
Taylor number A dimensionless number
measuring the influence of rotation on a convecting
system. It is also called rotational Reynolds
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