DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

stress energy tensor A 4-dimensional symmetric
tensorTµν combining the 3-dimensional
stress tensor of a substance with its energy density
and energy flux:
Too = energy density
Toi = momentum density = energy flux
Tij = 3-dimensional stress tensor.
This tensor is the source of the Einstein equation
for the graviational field:
Gµv = 8πTµv
whereGµv is the Einstein tensor. (Hereµ,ν=
0, 1, 2, 3;i,j= 1, 2, 3.)
striations, auroral A banded structure of auroral
arc, believed to be the overlap of moving
sections that overtake each other.
strike-slip fault A fault where the movement
is horizontal and parallel to the strike (i.e.,
the angle between true north and the direction
of any horizontal planar features) of the fault
plane. Strike-slip faults can be right-lateral or
left-lateral, depending on the direction of displacement.
As one faces a strike-slip fault, if the
rocks across the fault move toward the right, the
fault is then a right-lateral strike-slip fault. Similarly
if the rocks across the fault move toward
the left. Transform boundaries at plate margins
are usually composed of strike-slip faults.
string model See cosmic string, elastic string
model.
string network See scaling solution (cosmic
string).
stromatolite The oldest known fossils, dating
to 3.5 billion years; the oldest are found in
Australia. Colonial structures formed by photosynthesizing
cyanobacteria (blue-green algae)
and other microbes. These prokaryotes (lacking
a cellular nucleus) thrived in warm shallow environments
and built layered reefs of calcium
carbonate and sediment precipitating from the
water, which were trapped within the colony.
The colony then grew upward through the sediment
to form a new layer. In some cases, bacteria
was also fossilized within the mats. This
© 2001 by CRC Press LLC
structure toe
process still occurs today particularly in Shark
Bay in western Australia.
strong anthropic principle The concept that
the universe must be such as to admit intelligent
observers or, even stronger, that the universe
must produce such observers. See anthropic
principle.
strong scattering limit In magnetospheric
plasma physics, a type of MHD for a guiding
center plasma which assumes plasma particles
are always isotropic, i.e., they flow with equal
intensity in all directions. That is equivalent to
assuming that some process keeps scattering the
particles, thoroughly mixing their directions of
motion. The specific volume V is an important
variable in this theory, and Euler potentials are
its natural coordinates.
Strouhal number (Sr) A dimensionless
quantity used in fluid mechanics, defined by
Sr = lf/v, where l is length, f is frequency,
and v is velocity.
structure coefficients The set of functions
Ca bc appearing in the equation
dσ a = 1
2 Ca bc σ b ∧ σ c .
A set of basis 1-forms {σ a } defines the covariant
vector basis. 1-forms are acted on by an
exterior derivative to give a 2-form (an antisymmetric
rank-2 covariant tensor). Here ∧ is the
antisymmetric tensor product, i.e.,
σ b ∧ σ c = 1
σ
2
b ⊗ σ c − σ c ⊗ σ b
.
If {ea} is a vector basis dual to the {σ b }, then the
structure coefficients also give the commutation
rules for the vectors:
[ea , eb] = C d ab ed .
The set of antisymmetric tensor products of the
basis 1-forms is itself a basis for the 2-forms.
See 1-form, 2-form, tensor.
structure toe The place where the bottom of
a structure meets the seafloor. Often a location
of severe scour.
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