DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

adiation cooling
planets (Jupiter, Saturn, Uranus, and Neptune)
also carry such radiation belts, as does Mercury,
but Venus and Mars have very weak magnetic
fields and do not.
radiation cooling The transfer of heat energy
away from an object by the net outward transport
of radiation. In geophysics, at night, there
is no incoming solar radiation, and the outgoing
radiation from the Earth’s surface exceeds
the incoming radiation. The loss of radiant energy
lowers the temperature of the surface, and
the radiational cooling in surface layers of the
atmosphere often causes a strong inversion of
lapse rate. In atmosphere radiational processes,
the vertical variations are much larger than their
horizontal variations. Thus, the radiation cooling
rate CR can be expressed approximately by
its vertical component. That is
CR = 1 ∂FN
ρCp ∂h
where ρ is air density, Cp is specific heat at
constant pressure, FN is vertical component of
net radiation flux, and h is height.
radiation gauge In linearized (weak field)
descriptions of gravity, one may use the restricted
coordinate transformations to achieve
h α α = 0, h0µ = 0, and h αµ ,µ = 0 (where
µ = 0, 1, 2 or 3) for the linearized gravitational
potentialhab in a source-free region. (The
comma denotes partial derivative.) Gravitational
potentials meeting these requirements are
said to be in the radiation gauge. See linearized
gravitation, summation convention.
radiation pressure The transfer of momentum
to an object by radiation through the scattering,
absorption, and emission of the radiation.
The momentum transported by a single photon
is given by p=hν/c where h is Planck’s constant,
ν is the frequency of the photon, and c
is the speed of light. The change in the momentum
of a flux of photons as it is reflected
(or absorbed) results in a net pressure being applied
to the object. If the radiation is reflected,
then the momentum transferred by the photon is
twice the normal incident momentum.
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radiation tide Periodic variations in sea level
primarily related to meteorological changes,
such as the twice daily cycle in barometric pressure,
daily land and sea breezes, and seasonal
changes in temperature. Other changes in sea
level due to meteorological changes that are random
in phase are not considered to be part of the
radiation tides.
radiation zone An interior layer of the sun,
lying between the core and the convection zone,
where energy transport is governed by radiation.
In the radiation zone of the sun, the temperature
is a little cooler than the core, and as a result
some atoms are able to remain intact. Their
opacity influences the flow of radiation through
this zone.
radiative-convective equilibrium An equilibriumstatewheretheoutgoingradiationwould
be equal to the absorbed radiation at all latitudes.
radiative transfer equation The linear integrodifferential
equation that describes the rate
of change with distance of the radiance in a collimated
beam at a specified location, direction,
and wavelength; the equation accounts for all
losses (e.g., due to absorption and scattering out
of the beam) and gains (e.g., by emission or scattering
into the beam).
radio absorption Absorption occurring to
radio waves as a result of interaction between
an electromagnetic wave and free electrons in
the ionosphere. See ionospheric absorption.
radioactive Containing isotopes that undergo
radioactive decay to another isotope, or
another element, by emitting a subatomic particle:
typically electron (beta particle) or helium
nucleus (alpha particle). In some cases, the
radioactive nucleus undergoes an internal rearrangement
and emits an energetic gamma ray (a
photon), without changing its atomic number or
atomic weight.
radioactive decay The process by which
unstable atomic nuclei (radioisotopes) spontaneously
break down into one or more nuclei of
other elements. During this process, energy and
subatomic particles are released. The radioac-