DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

flat universe
flat universe A model of the universe in
which the average density is exactly equal to the
critical density at which it neither expands freely
nor collapses again. It is called flat because at
any instant the global geometry of space is Euclidean;
space is flat rather than curved.
flexure Bending of a plate. In tectonics, the
Earth’slithosphereissubjectedtoflexureinavariety
of geological settings. Examples include
the bending of the lithosphere prior to subduction
at an ocean trench, bending under the load
of a volcanic island, for example the Hawaiian
Islands, and the bending associated with sedimentary
basins.
floating breakwater A breakwater which
does not rest on the seafloor, but is instead anchored
in place and has a buoyant force equal to
its weight.
flocculation The process in which suspended
soil forms lumps during the mixing of freshwater
with seawater, especially in estuaries. Flocculation
occurs as the result of molecular attractive
forces known as the van der Waals forces.
These forces are weak, and are only significant
as clay particles are brought close together,
such as during mixing turbulence. Normally, in
fresh water clay minerals carry a negative charge
which repels particles from each other. In saline
waters, these charges are neutralized, and the attractive
van der Waals forces dominate.
flood current The tidal current that results
when the water in bays and estuaries is lower
than that in the adjoining sea. The opposite is
referred to as an ebb current.
flood delta A deposit of sediments immediately
inshore of a tidal inlet, deposited by flood
tidal currents.
flood shoal See flood delta.
flood tide See flood current.
Flora Eighth asteroid to be discovered, in
1847. Orbit: semimajor axis 2.2015AU, eccentricity
0.1562, inclination to the ecliptic
5 ◦ .8858, period 3.27 years.
© 2001 by CRC Press LLC
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Florida current Ocean current flowing
northward along the south-east coast of the U.S.
from the Florida Straits to Cape Hatteras, feeding
into the Gulf Stream. This current transports
approximately 26 million cubic meters per second
through the Florida Straits.
flow regime A categorization of open channel
flow based on flow speed and bed form. The
lower flow regime corresponds to lower flow
speeds and ripples or dunes on the bed. A transitional
flow is somewhat faster and will yield
dunes, a planar bed, or antidunes. The upper
flow regime is faster and yields a planar bed,
antidunes, or more complicated flow and bed
patterns.
fluence The time integrated flux.
fluorescence The inelastic scattering process
in which a photon is absorbed by a molecule and
shortly thereafter (10 −11 to 10 −8 s) another photon
of greater wavelengh is emitted; the emitted
radiance is unpolarized.
flutter fading Very rapid fading of radio signals
in an ionospheric circuit (5 to 10 fades per
second, or 5 to 10 Hz fades, and faster). Called
flutter fading because of the audible effect it has
on signals. Fades can be deep, dropping below
the local noise levels and resulting in the
signal being drowned in the noise. It is a common
problem at low latitudes where it is associated
with equatorial spread F. Flutter fading
is also observed at high latitudes on non-great
circle paths, suggesting large ionization gradients,
and near the auroral oval where it has been
linked with slant-type sporadic E. See fading.
fluvial Refers to processes or features produced
by the flow of some liquid, usually water.
River channels are among the most common
features produced by fluvial processes. These
features can be produced by surface flow or by
groundwater flow. On Mars, large outflow channels
have been produced by catastrophic surface
floods while the smaller valley networks have
been created by the sapping process (removal
of groundwater, causing the overlying terrain to
collapse).