DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

F
F1-layer Relatively unstable ionospheric
layer at a height of between about 140 and
200 km which fills the gap between the E-layer
and the F2-layer. Ionization is due to EUV radiation.
Formally, the F1-layer can be described
by a Chapman profile.
F2-layer Uppermost ionospheric layer at
heights of between 200 and about 1000 km.
In addition to local ionization and recombination,
large-scale transport processes influence
the chemistry and charge density. In addition,
vertical transport leads to a separation of different
particle species and charge states. Despite
already reduced particle densities, electron densities
are higher than in the other ionospheric
layers. The dominant particle species is ionized
atomic oxygen O + .N + and N +
2 are dominant in
the lower parts, while in the upper part of the F2layer
ionized helium He + and atomic hydrogen
H + become dominant.
Faber–Jackson law An empirical relationship
between the total luminosity and the central
velocity dispersion of elliptical galaxies: L∝
σ 4 , i.e., the galaxy luminosity is proportional to
the fourth power of the velocity dispersion σ.
This law was first discussed by S.M. Faber and
R.E. Jackson in 1976. Since the velocity dispersion
can be measured from the broadening
of absorption lines in the galaxy spectrum, the
Faber–Jackson law can, in principle, be used
to determine the luminosity and, once the apparent
magnitude of the galaxy is measured, to
derive the distance of the galaxy. The Faber–
JacksonlawisanalogoustotheTully–Fisherlaw
for spiral galaxies: The stellar velocity dispersion
substitutes the Hi rotational width, since
there is little atomic gas in early-type galaxies.
See Tully–Fisher law, velocity dispersion.
facula A bright region of the photosphere
seen in white light at the solar limb. Faculae are
fallout
the manifestations of the supergranule boundaries,
seen at the limb.
fading Fading is a common characteristic
of radio propagation and is typified by aperiodic
changes in the received signal amplitude
and other signal characteristics. The depth of
fading can be described by an amplitude probability
distribution and the rapidity of the fading
may be described by autocorrelation functions.
These statistics are required to make allowances
for fading in system design. Understanding the
properties of the fading may also be important in
constructing systems to capitalize or minimize
the effects. Fading on ionospheric propagation
paths can arise from sources that alter the properties
of a single path (e.g., absorption, ionization
changes resulting in pathlength changes or
skip distance changes, ionization irregularities
near the reflection point) and more commonly
from multiple propagation paths (e.g., interference
fading, polarization fading). Understanding
the source of the fading may allow some
control over its effects (e.g., skip fading may
mean using a lower operating frequency; absorption
fading may mean a higher frequency
is required). See flutter fading, ionospheric radio
propagation path, multipath fading.
failed arm See aulacogen.
faint young sun paradox As the sun ages, the
fusion processes at its core gradually intensify,
and models predict that the sun has increased
brightness by about 35% over the last 4 billion
years. The paradox is that one expects the Earth
to have been completely icebound, but geological
evidence shows liquid water at least that far
into the past. A resolution has been proposed
in terms of intense greenhouse effect, driven by
ammonia and hydrocarbon haze, which could
have maintained the surface above freezing.
Falkland current An ocean current flowing
northward along the coast of Argentina, between
Argentina and the Falkland Islands.
fallout Term describing airborne dangerous
particles that deposit from the atmosphere after
days, weeks, or longer in suspension in the air.
Usually referring to radioactive particles which
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