DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

stiff fluid
stiff fluid A fluid in which the energy-density
ρ equals p/c 2 at every point (where p is the pressure
and c is the speed of light), so that the velocity
of sound is equal to the velocity of light. This
is a theoretical construct (no known physical
system has such a large pressure). The equality
between the velocity of sound and the velocity
of light greatly simplifies the dynamical properties
of such a medium, and several problems
that would be difficult to solve in realistic matter
become solvable in a stiff fluid. One example is
the propagation of soliton waves through such a
medium. Also, solutions of Einstein’s equations
in which a stiff fluid is a source are easier to find
than solutions with more realistic sources.
still water level The level at which the water
in an ocean or lake would be if all waves were absent.
Differs from the mean water level because
of the influence of wave setup and setdown.
stishovite A high pressure polymorph of
quartz having a density of about 1.6 times that
of ordinary quartz thought to be naturally stable
only in the Earth’s mantle. Stishovite was first
synthesized in the laboratory by Sergei Stishov
in 1960, but later small crystals were discovered
in nature at the site of the Barringer Meteor
Crater in Arizona. Stishovite has the rutile structure
and is one of the very few known materials
in which Si occurs in octahedral coordination
with oxygen.
stochastic acceleration The energization of
a population of particles in which the spread in
momenta of the accelerated particles increases at
the same time as the average energy per particle.
Physically, stochastic acceleration of a particle
occurs via a series of random kicks from the
interaction with ambient particles.
Stokes’ law The statement, valid when viscous
forces dominate inertial effects, that the
viscous force F experienced by a sphere of radius
a moving at velocity v in a medium of viscosity
η is given by F =−6πηav.
Stokes parameters Four parameters used to
describe polarized radiation; one component describes
the radiance, two components describe
© 2001 by CRC Press LLC
448
the states of linear polarization, and one component
describes the state of circular polarization.
Stokes parameters Four quantities obeying
s0 2 = s1 2 + s2 2 + s3 2 , describing the polarization
state of a beam of light. s0 is the total
intensity, s1 is the difference in intensity of
the two polarizations. The other two parameters
describe the relative phases of the two polarization
states. Stokes parameters can be defined
in terms of linear, circular polarizations. (The
resulting parameters differ in the two bases, for
the same beam of light.)
Stokes polarimetry The means by which
to measure the amount of polarization in light
from the sun in order to yield information about
the direction of the magnetic field in threedimensions.
An accurate determination of the
vector magnetic field, using the Zeeman effect,
needs a precise measurement of the four Stokes
parameters.
Stokes’ Theorem In three dimensions, the
surface integral over a topological disk, of the
dot product of the curl of a vector with the normal
to the disk (n · ∇ × A ) equals the line
integral around the boundary of the surface of
A · dl, where dl is the directed line element in
the boundary. In general, the integral of an exact
differential form dω in a region equals the
integral of its potential ω in the boundary of the
region.
Stokes wave theory A theory for description
of surface water waves. Generally refers to
first-order (linear) wave theory, but higher-order
terms may be retained during the derivation to
yield an nth-order Stokes wave theory.
stony-iron meteorite A meteorite that shows
a combination of stone and nickel-iron. This
type of meteorite may have been formed from
the core-mantle boundary of a parent body that
broke up to produce the meteorite, or may have
been formed in an energetic collision that partly
melted the colliding objects.
storativity (S) The volume of water produced
from a confined aquifer per unit area per
unit decline in the potentiometric surface is a di-