DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

fall speed (velocity)
can be lofted into the air by nuclear explosions,
or via leaks from nuclear power plants, or by fire
associated with catastrophic failure in a nuclear
installation.
fall speed (velocity) The equilibrium speed
reached by a sediment particle falling in a column
of still water. Governed by the size, shape,
and density of the particle, as well as any cohesive
forces between particles, and the density
and viscosity (and thus temperature) of the fluid.
Used for classification of particle size and assessment
of the mobility of the sediment.
Fanaroff–Riley (FR) class I and II radio
galaxies Lobe-dominated radio galaxies
whose luminosity at 178 MHz is below (class
I) or above (class II) 5 × 10 32 ergs s −1 Hz −1 .
In 1974, B.L. Fanaroff and J.M. Riley noted
a dichotomy in the radio morphology of radio
galaxies with a sharp threshold luminosity: FR I
type galaxies, of lower luminosity, show smooth
two-sided and poorly collimated jets and edgedarkened
lobe structures. FR II type galaxies, of
higher luminosity, show edge-brightened lobes,
often with prominent hot spots at the inner end
of the lobes, connected to the nucleus by pencillike
jets that are usually very faint, and in several
sources not visible at all.
Faraday Cup A detector for low-energy
plasma, absorbing incident ions or electrons and
measuring the rate at which their electric charge
is deposited.
Faraday effect The rotation of the plane of
plane-polarized light by a medium placed in a
magnetic field parallel to the direction of the
light beam. The effect can be observed in solids,
liquids, and gasses.
Faraday rotation When propagating
through the ionosphere, a linearly polarized radio
wave will suffer a gradual rotation of its
plane of polarization due to its interaction with
the ionization in the ionosphere in the presence
of the Earth’s magnetic field. The magnitude of
rotation depends on the electron density and the
inverse square of the frequency. Typically, for a
total electron column of 10 16 electrons m −2 and
a frequency of 1 GHz the Faraday rotation is
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168
0.01 radian or 0.57 ◦ and for a total electron column
of 10 18 electrons m −2 the Faraday rotation
is 1 radian or 57 ◦ .
fast magnetohydrodynamic shock A fast
magnetohydrodynamic shock forms when a fast
magnetohydrodynamic wave steepens. The
magnetic field increases from the upstream to
the downstream medium and is bent away from
the shock because the normal component of the
field is constant. In contrast, in a slow magnetohydrodynamic
shock, the magnetic field is bent
toward the shock normal. The normal component
of the upstream (downstream) flow speed
is larger (smaller) than the propagation speed of
fast magnetohydrodynamic waves and both upstream
and downstream flow speeds exceed the
Alfvén speed. Traveling interplanetary shocks
in general and planetary bow shocks always are
fast magnetohydrodynamic shocks.
Change in magnetic field direction across a fast and a
slow shock.
fast shock wave See hydromagnetic shock
wave.
fast solar wind Solar wind escaping from
the open magnetic field regions of the coronal
holes. Plasma speeds range from 400 km/s
to 800 km/s, densities are low with about 3
ions/cm 3 at Earth’s orbit. Particles are mostly
hydrogen; helium amounts to about 4%. Electron
and proton temperatures are 1 × 10 5 K
and 2 × 10 5 K, respectively. Fast solar wind
streams are remarkably stable and variations
from stream to stream are small. Despite the
differences in individual parameters, the average
momentum and total energy flux are similar
in fast and slow solar wind streams.