DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

folds In geophysics, when the continental
crust is deformed under compression the result
is often a near periodic, sinusoidal structure.
These are folds, and they can occur on scales
from a meter or less to hundreds of kilometers.
Folding is usually associated with layered sedimentary
rocks with some layers being rigid and
others more ductile.
following spot A sunspot or sunspot group
lying on the eastern side of an active region complex,
i.e., following in the direction of solar rotation.
Following spots tend to be smaller and
greater in number than preceding (or “leader”)
spots. During the evolution of an active region,
the following spots tend to remain at the same
longitude or move backwards (relative to the direction
of solar rotation).
Fomalhaut 1.16 magnitude star of spectral
type A at RA22 h 57 m 38.9 s , dec 19 ◦ 37 ′ 20 ′′ .
forbidden lines Spectral emission lines violating
quantum mechanics selection rules for
electric dipole emission. In both permitted and
forbidden transitions, the photon of a spectral
line is emitted when an electron moves from an
upper to a lower energy level. The photon energy
is equal to the difference between the energy
of the two levels. In the case of forbidden
lines, the probability of a spontaneous transition
between the upper and lower energy level is very
small, and the electron remains a much longer
time in the excited state than in the case of a permitted
transition. In this case, the upper level is
said to be metastable. Forbidden lines in several
astronomical sources are collisionally excited,
i.e., the electron bound to an ion is brought to
a higher, metastable level via the collision with
a free electron or with another ion. At densities
typical of the terrestrial environment, subsequent
collisions would quickly de-excite the
atom without emission of radiation. At electron
densities ne < ∼ 10 3 − 10 7 electrons cm −3 , the
probability of a collision is much lower, and the
electron can decay to a lower level with the emission
of a photon of the forbidden line. Forbidden
lines are, therefore, very sensitive indicators of
density in several gaseous nebulæ, for example
Hi regions, or planetary nebulæ.
© 2001 by CRC Press LLC
forbidden orbits See Störmer orbits.
force
forbidden region A term used in the study of
cosmic ray ions or particles convecting into the
Earth’s magnetic field. It is a region in space, or
among the directions of arrival, which (for some
stated energy) contains no orbits that connect to
infinity, but only trapped orbits. Since the cosmic
ray ions, or the convecting ions, come from
great distances and are not trapped, the forbidden
region contains none of them. Forbidden
regions for convection particles are responsible
for Alfvén layers.
Forbush decrease The sudden dramatic reduction
in the flux of cosmic rays detected at the
Earth (e.g., with a neutron monitor) due to the interaction
of solar flare induced shock waves with
the interplanetary medium. The shock waves
sweep across the geomagnetic field lines and
deflect the galactic cosmic ray particles. One to
three days after a strong flare counting rates decrease
sharply by a few percent. This decrease
coincides with the passage of the shock. Sometimes
the decrease occurs in two steps, the first
step coinciding with shock passage, the second
with the arrival of the magnetic cloud. Although
the decrease is sharp, the recovery is slow, lasting
for some days.
force In Newtonian physics, a vector quantity
describing the external influence on an object
which tends to accelerate the object. Force
may be measured by its effect in compressing a
spring:
F =−kx ,
where k is a constant, and x is the amount of displacement
of the spring from its resting position.
Newton’s second law relates force to acceleration:
F = ma ,
where m is the mass, and a is the acceleration.
Thus, force is the rate of change of momentum
with time.
Units: dynes = gm cm/sec 2 ; Newtons =
kg m/sec 2 .
In relativity, a number of formulations of
force have been given, all of which reduce to
the Newtonian form in the limit of small velocities.
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