DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

ut a rotational discontinuity. The propagation
speed parallel to the magnetic field equals the
Alfvén speed; thus, the intermediate shock is
sometimes also called Alfvén shock.
intermittency Even in non-stratified, homogeneous
turbulence, variations of the dissipation
of turbulent kinetic energy (ε) and scalar variance
(χ) are large and follow lognormal probability
distributions. In natural waters, intermittency
is larger because, in addition to the inherent
intermittency, the production and transport
of turbulent kinetic energy is usually variable in
time and patchy in space. Due to the variations
of the physical processes and intensities leading
to turbulence, turbulence parameters, such as ε
and χ, are usually not lognormal distributed.
The standard deviation of ln(εi) is sometimes
called intermittency factor.
internal energy The total energy in a thermodynamic
system not associated with bulk motion;
an extensive thermodynamic potential U
given by
U=ST +PV,
where S is the entropy, T is the temperature,
P is the pressure, and V is the volume of the
system. The change of the internal energy is
the maximum work that can be extracted from
a reversible closed system at constant V . For
a reversible process at constant S and V , work
stored as internal energy can be recovered completely.
internal friction angle A term used in the
study of soil mechanics; denotes the maximum
sustainable angle for a pile of the soil. Also
referred to as angle of repose.
internal wave In oceanography, a wave that
occurs within seawater whose density changes
with depth either gradually or abruptly at an interface
discontinuity.
International Atomic Time (TAI) A
weighted average of atomic clocks in many
countries, each having its rate adjusted by a very
small amount to bring its rate, as transmitted to
sealevel(thegeoid)bytelemetry, tothatwhichit
would exhibit if situated on the geoid. Because
the combination of the results from many clocks,
© 2001 by CRC Press LLC
International Temperature Scale (ITS-90)
weighted according to their estimated accuracy,
requires hours or days to calculate, TAI is not
available in real time, but signals from several
sources, such as NIST and the USNO, are very
close to TAI and corrections are available after
date. First, “Free-running Atomic Time” (EAL)
is produced and then TAI is computed from it
after frequency adjustments are applied to correct
for instrumental errors in the length of the
second, as determined from measurements of
primary frequency standards (atomic clocks) at
timing laboratories. Like Coordinated Universal
Time (UTC), TAI is referenced to the Greenwich
meridian time zone. Because it increases
continuously, when UTC is held back by the introduction
of leap seconds, TAI may be thought
of as being equal to UTC + (cumulated leap
seconds). Thus, TAI noon precedes Greenwich
(UTC) solar noon by that number of seconds.
See geoid.
international dateline A broken line running
from the north to south pole on the Earth
roughly along the meridian at 180 ◦ East longitude
= 180 ◦ West longitude, 180 ◦ away from
the prime meridian through Greenwich. The
line along which a discontinuity in the current
date occurs. Legal dates are arranged across this
line so that the Eastern side of the line has a date
24 hours earlier than the Western side. Thus,
on traveling eastward across this line, the legal
date becomes one day earlier, e.g., Sunday August
10, 1997 became Saturday August 9, 1997.
For political reasons the legal date line does not
follow exactly the 180 ◦ longitude.
International Geomagnetic Reference Field
(IGRF) See harmonic model.
International System of Units (SI) The unit
system adopted by the General Conference on
Weights and Measures in 1960. It consists of
several base units (meter, kilogram, second, ampere,
kelvin, mole, candela), plus derived units
and prefixes.
International Temperature Scale (ITS-90)
The official international temperature scale
adopted in 1990. It consists of a set of fixed
points and equations that enable the thermody-
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