DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Fraunhofer lines
where M is the mass of the central body, and
v is the velocity of the body in its orbit. For
a 650-km altitude orbit around the Earth, this
contribution is of order 6.6 arcsec/year.
Proposed space experiments would measure
the orbital dragging (LAGEOS-III experiment)
and the gyroscope precession (Gravity Probe B,
GPB).
In strong field situations, as in close orbit
around (but still outside) a rotating black hole
dragging is so strong that no observer can remain
at rest with respect to distant stars.
Fraunhofer lines Absorption lines in the solar
spectrum (first observed by Fraunhofer in
1814).
free-air correction A correction made to
gravity survey data that removes the effect of
the elevation difference between the observation
point and a reference level such as mean
sealevel. It is one of several steps taken to reduce
the data to a common reference level. The
free-air correction accounts only for the different
distances of the two elevations from the center
of the earth, ignoring the mass between the
two elevations. The commonly used formula
for free-air correction for elevation difference h
is g = 2h gh/R, where gh is the measured
gravity value, and R is the average radius of the
Earth.
free-air gravity anomaly The difference between
the measured gravity field and the reference
field for a spheroidal Earth; the correction
to the value of the gravitational acceleration (g)
which includes the height of the measuring instrument
above the geoid. It is called “free air”
because no masses between the instrument and
the geoid are included in the correction. Inclusion
of the excess gravitational pull due to
masses between the measuring instrument and
the geoid gives rise to the Bouguer anomaly.
free atmosphere The atmosphere above the
level frictionally coupled to the surface, usually
taken as about 500 m.
free bodies A physical body in which there
are no forces applied. Such a body will maintain
a uniform motion (until a force acts upon
© 2001 by CRC Press LLC
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it) according to the first law of Newtonian Mechanics.
free-bound continuum emission Radiation
produced when the interaction of a free electron
with an ion results in the capture of the electron
onto the ion (e.g., recombination emission). The
most energetic spectral lines of importance in
solar physics belong to the Lyman series of Fe
XXVI. The continuum edge for this series lies
at 9.2 keV, so that all radiation above this energy
must be in the form of continua, both free-bound
and free-free. As the energy increases, the contribution
of the free-bound emission to the total
emission falls off relative to the free-free emission.
free convection Flow of a fluid driven purely
by buoyancy. In the case of thermal free convection,
the necessary condition for its onset is
that the vertical thermal gradient must be greater
than the adiabatic gradient. Whether free convection
actually occurs depends on geometrical
constraints. Parts of the Earth’s mantle and core
are believed to be freely convecting, but little is
known about the patterns of convection.
free-free continuum emission Radiation
produced when the interaction of a free electron
with an ion leaves the electron free (e.g.,
bremsstrahlung).
free oscillations When a great earthquake occurs
the entire Earth vibrates. These vibrations
are known as free oscillations.
freeze A meteorological condition in which
the temperature at ground level falls below 0 ◦ C.
F region The F region is the part of the ionosphere
existing between approximately 160 and
500 km above the surface of the Earth. During
daytime, at middle and low latitudes the
F region may form into two layers, called the
F1 and F2 layers. The F1 layer exists from
about 160 to 250 km above the surface of the
Earth. Though fairly regular in its characteristics,
it is not observable everywhere or on all
days. The F1 layer has approximately 5 × 10 5
e/cm 3 (free electrons per cubic centimeter) at
noontime and minimum sunspot activity, and in-