DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

orbital elements
higher than the OWF that will be better to use.
Conventionally, the OWF has probably been the
most important ionospheric parameter deduced
for HF propagation. The OWF is also sometimes
called the Frequency of Optimum Traffic
(FOT). See ionospheric radio propagation path.
orbital elements Seven quantities that must
be specified to specify the position of a point
mass in a Newtonian orbit: the longitude of the
ascending node (), the inclination i of the object’s
orbital plane to the ecliptic (or to the plane
of the sky, for extrasolar objects), the argument
of the perihelion (ω), the semimajor radius of
the orbit, a, the eccentricity of the orbit, ɛ, and
the mean anomaly,M, which increases linearly
with time. See separate entries for these quantities.
orogeny The creation of a mountain range.
orographic cloud A cloud that forms because
of forcing of the air motion by the Earth’s
topography, as cap clouds, lee wave clouds, etc.
Orowan’s equation The main relationship
for the physics of plastic deformation, expressing
the strain rate (ε) caused by the motion (glide
or climb) of dislocations of Burgers vector b,
with an average velocity ( ¯v) and the density of
mobile dislocations being ρm,
ε=ρmb ¯v.
It is the master equation, from which many constitutive
equations expressing microscopic deformation
models can be derived.
orthogonal Perpendicular; lines drawn perpendicular
to the crests of water waves are referred
to as wave orthogonals.
Osborn–Cox method See Cox number, dissipation
of temperature variance.
Osborn model Method to infer turbulent
density fluxes from the measurement of the dissipation
rate of turbulent kinetic energy ɛ. The
vertical turbulent density flux is normally expressed
via the eddy diffusivity Kρ
w ′ ρ ′ ∂ρ
=−Kρ
∂z
© 2001 by CRC Press LLC
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wherew is the vertical velocity in thez direction,
the primes denote turbulent fluctuations from
the mean (see Reynolds decomposition), and the
over-bar denotes a suitable spatial average that
is longer than the typical length scales of the
turbulence, but shorter than any energy containing
scales. Under the assumption that the turbulence
is steady in time and homogeneous in
space, the equation describing the turbulent kinetic
energy (tke) reduces to the so-called tkebalance
equation
P = B + ɛ
where P =−u ′ w ′ (∂U/∂z) represents the mechanical
production of turbulent kinetic energy,
B = g/ρ −1 w ′ ρ ′ the destruction of tke by
buoyancy, and ɛ is the rate of dissipation of
tke. The buoyancy term can be parameterized
by the non-dimensional flux Richardson number
Rif ≡ B/P. Substitution of Rif into the tkebalance
equation expresses the vertical density
flux in terms of the dissipation rate ɛ
gρ −1 w ′ ρ ′ = Rif
1 − Rif
ɛ = Ɣɛ .
The quantity Ɣ = Rif /(1 − Rif ) is often referred
to as the mixing efficiency.
oscillating universe If the universe is closed,
i.e., it has enough mass or equivalently expands
slowly enough for the gravitational pull of matter
to cause it to contract back to zero size in the
far future, this may be repeated as a cycle. The
temperature and density would increase without
limit as the contraction intensifies, would
go formally to infinity, and then decrease again
as the universe expands. This model of cosmic
evolution parallels the ancient idea of the cyclic
universe — one that undergoes a never-ending
sequence of rebirths, each time rising phoenixlike
from the ashes of its previous demise. In
the modern viewpoint, the bounce may occur in
a very high temperature quantum gravity epoch.
O star Star of spectral type O.
Ostriker–Thompson–Witten scenario (1986)
Closed loops of superconducting cosmic strings
formed during string network interactions will
oscillate and radiate gravitationally. This energy