DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

shock spike
The shock speed then can be calculated from
the mass conservation across the shock (see
Rankine–Hugoniot relations) as
vs = ϱdud − ϱuuu
ϱd − ϱu
· n
with ϱ being the plasma density, u being the
plasma speed, and the indices u and d indicating
the upstream and downstream medium, respectively.
shock spike In magnetospheric physics, a
short intensity increase, lasting for some 10 min,
around the time of shock passage. Shock spikes
are observed at quasi-perpendicular shocks
only; they are a typical feature of shock drift
acceleration.
shock wave A shock wave is associated with
a sudden change in properties of a continuous
medium, in particular a sudden increase in gasdynamic
pressure. A shock forms when a propagation
speed exceeds the typical signal speed
in the medium: a body might move through
a medium faster than the signal speed (e.g.,
a supersonic jet, traveling shock) or a supersonic
flow might be slowed down at an obstacle
(e.g., the solar wind at a planetary magnetosphere,
standing shock). Another shock wave
is the blast wave following an explosion. General
characteristics of shock waves are: (1) the
disturbance propagates faster than the signal
speed, (2) at the shock front, the properties of
the medium change abruptly, and (3) behind
the shock, a transition back to the state of the
undisturbed medium must occur. Here a reverse
shock might form. The jump of properties at
the shock front is described by the Rankine–
Hugoniot relations. Shocks transport information;
however, since their speed exceeds the signal
speed no information can propagate ahead
of the shock. The passage of a shock changes
the properties of the medium irreversibly.
Shoemaker–Levy A comet that impacted on
the planet Jupiter after breaking into at least 21
major pieces. The impacts lasted from July 16
through July 22, 1994. The impacts created
fireballs and raised plumes of up to 3000 km
above the Jovian cloudtops. Remnants left on
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Jupiter for days included circular and arcate dark
smudges.
shoreline A loosely used term indicating the
line separating water from land. This line actually
moves back and forth as wave and tide
conditions change. More precise definitions involve
description of the mean water line, still
water line, mean high tide line, etc.
short wave fadeout (SWF) A solar flare is
accompanied by an increase in X-ray emissions.
The increased X-ray flux ionizes the dayside D
region of the ionosphere leading to an increase in
absorption of radio waves passing through it. A
fadeout occurs when a radio frequency becomes
too weak to use as a result of this absorption.
The event is called a short-wave fade because
all short-wave frequencies passing through the
daytime ionosphere will be affected. In the past,
when communications depended more heavily
on HF radio, this meant that all short-wave HF
radio transmissions could simultaneously cease
to be heard. Most M-class and all X-class flares
will cause short-wave fades. The increased ionization
leads to a number of other effects. See
magnetic crochet, solar flare, sudden frequency
deviation (SFD).
side-cast dredge A dredge that discharges
dredged material off to one side of the dredge
centerline. Often used to dredge navigation
channels.
sidereal day The interval of time required for
a given right ascension to return to a particular
observer’s meridian; 23 h 56 m 4 s .
sidereal month See month.
sidereal time Time measured in terms of the
rotation of the Earth with respect to the stars.
Because the Earth’s sense of rotation is in the
same direction as its revolution around the sun,
it must rotate through more than 360 ◦ from noon
to noon. Hence, there is one more revolution
with respect to the stars, than with respect to
the sun, in a given year. Sidereal time equals
the right ascension of objects on the observer’s
meridian. Equivalently it is the hour angle of the
autumnal equinox. Sidereal time includes the