DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

ning signal was heard as a sharp, short duration
crackle on a radio receiver. This bursty crackle
of interference was called an atmospheric, to
distinguish it from the internal and local site interference.
The sum of many atmospherics from
remote lightning strokes all over the world produces
a steady background noise limit at these
radio frequencies called atmospheric noise. Atmospherics
were observed at lower frequencies
and used as a measure of thunderstorm activity.
Early receivers for this application were sometimes
caller spheric receivers.
atmospheric tide Oscillations in any atmospheric
field with periods that are simple integer
fractions of either a lunar or a solar day. In
addition to being somewhat excited by the gravitational
potential of the sun and moon, atmospheric
tides are strongly forced by daily variations
in solar heating. The response of these
forcings is by internal gravity waves. Unlike
ocean tides, atmospheric tides are not bound
by coastlines but are oscillations of a spherical
shell.
atomic mass The mass of an isotope of an
element measured in atomic mass units. The
atomic mass unit was defined in 1961, by the
International Union of Pure and Applied Physics
and the International Union of Pure and Applied
Chemistry, as 1/12 of the mass of the carbon
isotope counting 6 neutrons (and 6 protons) in
its nucleus.
atomic number The number of protons in
the nucleus of a given element.
atomic structure calculations — one-electron
models The calculation of possible states of
an electron in the presence of an atomic nucleus.
The calculations consist in obtaining the electron
distribution or wave function about the nucleus
for each state. This is achieved by solving
the Schrödinger equation for the electron
wave function in a fixed Coulomb potential generated
by the nucleus of the atom. The quantified
nature of the possible solutions or states
appear naturally when the conditions of continuity
and integrability are applied to the wave
functions. An important characteristic of the
one-electron models is that they can be solved
© 2001 by CRC Press LLC
aurora
exactly; the wave functions may be expressed
in terms of spherical harmonics and associated
Laguerre polynomials. Relativistic treatment is
done through Dirac’s equation. Dirac’s equation
leads to the fine structure as a relativistic correction
to Schrödinger’s solution. Another important
result of Dirac’s equations is that even for
non-relativistic cases one finds that the electron
has two possible states, generally interpreted as
two possible states of intrinsic angular momentum
or spin.
atomic time Time as measured by one or
more atomic clocks, usually a cesium-beam
atomic clock or a hydrogen maser. Measured
since January 1, 1958, it is the most uniform
measure of time available and has, therefore, replaced
Universal Time as the standard.
attenuation coefficient In propogation of a
signal, beam, or wave through a medium, with
absorption of energy and scattering out of the
path to the detector, the attenuation coefficient
α is
α = d −1 ln (S/S0) ,
where this is the natural logarithm, and S and S0
are the current intensity and the initial intensity.
Since α is an inverse length, it is often expressed
in terms of decibel per meter, or per kilometer.
See beam attenuation coefficient, diffuse attenuation
coefficient.
attenuation efficiency factor The sum of the
absorption plus scattering efficiency factors.
aulacogen Mantle plumes create regions of
elevated topography which typically have three
rift valleys at about 120 ◦ apart; these are aulacogen.
These are also known as triple junctions,
and they participate in the formation of new
ocean basins. An example is the southern end of
the Red Sea. Typically two arms participate in
the opening of an ocean, and the third is known
as a failed arm. The St. Lawrence river valley is
a failed arm associated with the opening of the
Atlantic Ocean.
aurora Polar lights. The aurora borealis
(northern lights) and aurora australis (southern
lights). Energetic electrons are trapped from the
solar wind and spiral around the field lines of the