DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

and downcoast areas. Sediment is transported
within the littoral cell, and may enter it via a
source such as a river, or leave via a submarine
canyon or other sink.
littoral current A mean flow of water within
the littoral zone, in the longshore direction. See
longshore current.
littoral drift A term used to denote the sediment
transport along a coast by wave action. See
longshore sediment transport. Often measured
in units of yd 3 /y or m 3 /y.
littoral drift rose A polar graph that illustrates
the potential longshore sediment transport
rate at a site as a function of incident wave energy.
littoral transport Transport of sediment
within the littoral zone. Generally used to refer
to sediment transport in the longshore direction.
See longshore sediment transport.
lobe In general, a roundish division or projection
of an object. See lobe dominated quasars,
lobes, high latitude.
lobe dominated quasars High luminosity,
radio-loud active galactic nuclei whose radio
emission is dominated by extended lobe emission.
Lobe dominated quasars have radio power
and morphologies similar to those of Fanaroff–
Riley class II radio galaxies. In quasars, the
jet appears one-sided, and both core and jet
have higher luminosity than in Fanaroff–Riley
II galaxies. See Fanaroff–Riley radio galaxies.
lobes, high latitude (also known as tail
lobes) Two regions extending into the tail of the
magnetosphere (the magnetotail), located north
and south of the plasma sheet. Each tail lobe
contains a bundle of magnetic field lines connected
to one of the Earth’s polar caps: the
lines of the northern lobe lead into the region
around the north magnetic pole, while those of
the southern lobe come out of the region near
the southern pole.
The magnetic field of the tail lobes is relatively
strong and nearly uniform — about 20 to
30 nano Tesla (nT) near Earth, diminishing to 9
© 2001 by CRC Press LLC
local topological defect
to 10 nT in the distant tail. Their ion density is
very low, around 0.01 ion/cm 3 , and they seem
to stretch well past 100 to 200 RE, at which distance
they become infiltrated by plasma sharing
the flow of the solar wind (possibly originating
in the plasma mantle). The magnetic energy
stored in the lobes is considerable and is widely
believed to be the main energy source of substorms.
local acceleration of gravity The force per
unit mass of gravitational attraction toward the
Earth or other large local concentration of matter,
Fgrav/m. For distance r from a spherical
“Earth” of mass M⊕ the local acceleration of
gravity is
g =−ˆzGM⊕/r 2 ,
where ˆz is a vertical unit vector.
Local Group A group of approximately 30
galaxies, which includes the galaxy and its closest
neighbor galaxies within a distance ≈ 1 Mpc.
Most members are thought to form a gravitationally
bound system; the Local Group is therefore
the closest example of a cluster of galaxies.
The brightest members are our galaxy, the spiral
galaxy Messier 31 (the Andromeda galaxy),
and the Sc spiral M33, although the majority
of galaxies belonging to the Local Group are
dwarf galaxies, either dwarf spheroidal or irregular
galaxies. With present-day instruments,
several galaxies of the Local Group can be resolved
into stars.
local thermodynamic equilibrium (LTE)
The assumption that a localized volume of emitting
gas is in thermodynamic equilibrium such
that the source function for the emitted radiation
is given by the Planck function at the local
temperature. LTE can be assumed to apply to
plasma which is sufficiently dense that most of
the photons are absorbed and thermalized before
they travel a distance over which the temperature
changes considerably.
local topological defect In cosmic topological
defects formed as the result of the breakdown
of a gauge (or local) symmetry, an important
role is played by the gauge fields. In
fact, these help to compensate the growing energy
gradients of the Higgs field far away from
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