DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

creases to roughly 2 × 10 6 e/cm 3 during maximum
sunspot activity. The density falls off to
below 10 4 e/cm 3 at night. The F1 layer merges
into the F2 layer at night. The F2 layer exists
from about 250 to 400 km above the surface of
the Earth. The F2 layer is the principal reflecting
layer for HF communications during both
day and night. The longest distance for one-hop
F2 propagation is usually around 4000 km. The
F2 layer has about 10 6 e/cm 3 and is thus usually
the most densely ionized ionospheric layer.
However, variations are usually large, irregular,
and particularly pronounced during ionospheric
storms. During some ionospheric storms, the
F region ionization can reduce sufficiently so
that the F2 region peak electron density is less
than the F1 region peak density. See ionosphere,
ionogram, ionosphericstorm, spreadF,traveling
ionospheric disturbance, winter anomaly.
frequency of optimum traffic (FOT) See
Optimum Working Frequency (OWF).
Fresnel reflectance The fraction of radiant
energy in a narrow beam that is reflected from a
surface at which there is an index of refraction
mismatch.
Fresnel zone Any one of the array of concentric
surfaces in space between transmitter
and receiver over which the increase in distance
over the straight line path is equal to some integer
multiple of one-half wavelength. A simplification
allowing approximate calculation of
diffraction.
fretted channels One channel type considered
to indicate fluvial activity on Mars. Fretted
channels are a channel type affecting much of
the fretted terrain, thus straddling the highland–
lowland boundary of Mars. Their formation
is restricted to two latitude belts centered on
40 ◦ N and 45 ◦ S and spanning ≈ 25 ◦ wide. They
are most extensive between longitude 280 ◦ W
to 350 ◦ W. They extend from far in the uplands
down to the lowland plains and represent broad,
flat-floored channels, in which flow lines are a
common feature. See fretted terrain.
fretted terrain Part of the highland–lowland
boundary region of Mars, lying along a great
© 2001 by CRC Press LLC
friction slope
circle having a pole at ≈ 145 ◦ W and 55 ◦ N. The
terrain also exists around the high-standing terrain
retained in the northern lowland plains. It is
characterized by flat-topped outliers of cratered
uplands, termed plateaux, mesas, buttes, and
knobs depending on their size. The differences
in size are thought to reflect different
extents of fracturing and subsequent modification,
whereby greater fracturing and modification
created the smaller landforms.
Plateaux, mesas, and buttes are generally
considered to have formed during creation of
the relief difference. Alternatively, it has been
proposed that they represent flat-topped table
mountains that formed due to the interaction
of basaltic lava and ground water at low eruption
rates. The final form of the fretted terrain
has also been accounted for as representing the
shorelines of a sea and by scarp retreat owing
to mass wasting and sublimation of volatiles, or
ground-water sapping.
friction The process whereby motion of one
object past another is impeded, or the force producing
this impediment. Friction is caused by
microscopic interference between moving surfaces,
and/or by microscopic fusion between the
surfaces, which must be broken to continue motion.
Friction is dissipative, producing heat from
ordered kinetic energy. The frictional shear
stress on a surface S ′ f is a fraction of the normal
stress on the surface Sn so that
S ′ f
= fSn
where f is the coefficient of friction. This is
known as Amontons’s law and f has a typical
value of 0.6 for common surfaces in contact, and
in geophysical processes. Friction controls the
behavior of geophysical faults.
friction factor A coefficient that indicates
the resistance to a flow or movement. Generally
empirical and dimensionless (e.g., Darcy–
Weisbach friction factor).
friction slope Energy loss, expressed as head
(energy loss per unit weight of fluid) per unit
length of flow. Appears in the Manning Equation.
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