DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

figure of the Earth
See cosmic topological defect, Derrick theorem,
spontaneous symmetry breaking.
figure of the Earth The shape of the Earth.
To a first approximation it is an oblate spheroid
with a polar radius of 6357 km and an equator
radius of 6378 km.
filament A structure in the corona consisting
of cool (∼ 7000 K), dense ( 10 12 cm −3 )
plasma supported by magnetic fields and seen
as dark lines threaded over the solar disk. When
seen in emission at the solar limb, a filament
appears as a protuberance: a bright arc of matter
extending high above the photosphere, spanning
latitudes of up to some 10 ◦ . Their density is
about a factor of ten higher than the ambient density
(therefore the arc is bright when seen above
the solar limb), and the filament can extend up to
about 100 times the scale height in the corona.
Filaments are aligned along the separation of
opposing magnetic field patches in the photosphere.
Their existence therefore is related to
solar activity with only a few filaments observed
during solar minimum and a much larger number
during solar maximum. Filaments can have
very long lifetimes, lasting 2 to 3 solar rotations
in some cases. They are found, preferentially,
in two latitude belts on the sun; in a strip at high
latitudes known as the polar crown and in active
mid-latitudes. Typical magnetic fields are ∼ 5
to 10 G in quiescent filaments and may be as
high as 200 G in active region filaments fields.
The cold end dense matter of the filament/protuberance
is held against gravity by
magnetic tension in the anchoring magnetic field
lines. Two configurations can be distinguished
by comparing the photospheric magnetic field
pattern with that of the filament:
1. Kippenhahn–Schlüter configuration, also
called normal configuration: the magnetic field
inside the filament has the same direction as the
photospheric field below it.
2. Raadu–Kuperus configuration, also called
inverse configuration: the magnetic field inside
the filament is directed opposite to the one in the
photosphere. This is possible only because the
anchoring field lines have a neutral point below
the filament. In particular, in large and highrising
filaments, which tend to give rise to coronal
mass ejections, the Raadu–Kuperus configu-
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ration seems to be the dominant one. This is attributed
to the X-point below the filament where
reconnection is likely to occur, leading to the expulsion
of the filament (coronal mass ejection)
and the generation of electromagnetic emission
due to the accelerated electrons (the flare). See
coronal mass ejection, reconnection.
filament channel A broad pattern of fibrils
in the chromosphere, marking where a filament
may soon form or where a filament recently disappeared.
filter layer In civil or coastal engineering, denotes
a layer of material (typically stone, gravel,
or sand, possibly combined with a geotextile
fabric) intended to prevent migration of fine material
into or out of a structure. May also be
employed to reduce settlement. As an example,
breakwaters are often built with a filter layer in
the base to help prevent scouring and settlement.
finestructure In atmospheric dynamics,
structures with scales from tens of meters to
decimeters, covering the processes of internal
waves and intrusions. Finestructure processes
are intermediary between the large-scale (overall
stratification) structures and the small-scale
(turbulence).
fine structure constant Dimensionless constant
relating to strength of electromagnetic interactions:
α = e 2 / (4πɛ0 ¯hc) = 7.297352533(27)×10 −3 .
fingering See double diffusion.
finger regime The double-diffusive regime,
where temperature stabilizes and salinity destabilizes
(i.e., warmer and saltier water is on
top of cooler and fresher water). In this case,
the non-dimensional stability ratio is defined as
Rρ = (α∂/∂z)/(β∂S/∂z). The term “finger”
refers to the saltier finger-like plumes driving
the double-diffusive convection. Classical locations
of the finger regime are under the Mediterranean
Outflow, in the East North-Atlantic and
the C-SALT field east of Barbados.