DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

tion and plane are associated with the Burgers
vector and glide plane, respectively, of gliding
dislocations. The correlated combination of the
Burgers vector and the glide plane in a given
crystal system is then defined as a slip system.
In general, one would expect glide planes to be
the closest-packed plane, or these planes have
the lowest {hkl} indices; the shorter of the possible
perfect-dislocation Burgers vectors should
correspond to the slip direction.
slough A long, thin tidal estuary.
slow earthquake An earthquake that emits
seismic waves with much longer periods than
usual earthquakes because rupture velocity on a
fault plane is abnormally slow and rupture duration
time is abnormally long. Rupture duration
times for usual earthquakes are almost determined
according to their magnitude. For instance,
rupture duration time for a great earthquake
with a magnitude of 8 class is tens of several
seconds. Sometimes rupture duration time
for slow earthquakes which occur below ocean
bottoms becomes more than several minutes,
leading to a tsunami (tidal wave). Furthermore,
when a slow earthquake ruptures spending more
than tens of several minutes, its seismic waves
cannot be recorded even using long-period seismometers.
In such a case, no tsunami is generated.
Such an earthquake is called a silent
earthquake, for which detection efforts are being
made, using high-sensitivity extensometers.
slow magnetohydrodynamic shock A slow
magnetohydrodynamic shock results from the
steepening of slow MHD waves. The magnetic
field decreases from the upstream to the downstream
medium and the field is bent toward the
shock normal because the field’s normal component
stays constant. The upstream flow speed
exceeds the sound speed but is smaller than the
Alfvén speed. Thus far, in the interplanetary
medium less than a handful of slow magnetohydrodynamic
shocks have been observed. In
the corona, slow MHD shocks might be more
common because both the Alfvén speed and the
sound speed are higher. See fast magnetohydrodynamic
shock.
© 2001 by CRC Press LLC
SNC meteorites
slow shock wave See hydromagnetic shock
wave.
slow solar wind The properties of the slow
solarwind arehighlyvariable. Oftenlarge-scale
structures such as magnetic clouds or shocks are
embedded in the slow stream. Plasma speeds
range from 250 km/s to 400 km/s, proton temperatures
are about 3 × 10 4 K, electron temperatures
are twice as much, and densities at 1 AU
are about 8 ions/cm 3 . The helium content is
highly variable, averaging 2%. Despite the differences,
momentum flux and total energy flux
on average are similar in fast and slow solar wind
streams. Two possible sources of the slow solar
wind have been suggested: the slow wind might
stem from the closed magnetic field regions in
the streamer belt or it might be over-expanded,
and therefore slowed down, solar wind from the
outer skirts of the fast solar wind stemming from
the coronal holes.
small amplitude wave A sinusoidal water
wave. Corresponds to linear or Airy wave theory.
Linear wave theory derivation involves assumption
of a small wave amplitude.
Small Magellanic Cloud (SMC,NGC 292)
An irregular galaxy in the southern constellation
Tucana at right ascension 0 h 50 m , declination
−73 ◦ , at 65 kpc distance. The SMC has angular
dimension of 280 ′ × 160 ′ , about 10 kpc. It has
a positive radial velocity of −30 km/s (toward
us). Both the Large Magellanic Cloud and the
Small Magellanic Cloud orbit the Milky Way.
See Large Magellanic Cloud.
small-scale turbulence Nearly isotropic,
eddy-like state of fluid random motions, where
the inertial forces in the eddies are larger than
the buoyancy and viscous forces (i.e., Reynolds
and Froude number exceed critical values). The
length scale of the small-scale turbulent motion
is smaller than the Ozmidov-scale but at least an
order of magnitude larger than the Kolmogorov
scale.
SNC meteorites A subclass of achondrites
named after the first three examples found:
Shergotty, Nakhla, and Chassigny. The abundances
of isotopes of trapped nitrogen and rare
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