DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

spring
tion and produces fluctuations in signals coming
from the outer ionosphere known as Scintillations.
Spread F tend to occur in the night. See
F region, ionosphere.
spring Season of the year in the northern
hemisphere between the vernal equinox, about
March 21 and the summer solstice, about June
21; in the southern hemisphere between the autumnal
solstice, about September 21, and the
winter solstice, about December 21. See also
cosmic spring.
spring tide Tide which occurs when the
Earth, sun, and moon are nearly co-linear. Under
these conditions the gravitational field gradients
of the sun and moon reinforce each other.
The high tide is higher and low tide is lower than
the average. Spring tides occur twice a month
near the times of both new moon and full moon.
sprites Short-lived luminosities observed
at high altitudes above thunderstorms, apparently
associated with upward discharges of thunderstorm
electricity. They appear as columnar
diffuse reddish glows between 30 km and
80 km above ground, branching into the upper
atmosphere lasting tens of milliseconds, following
large positive cloud-to-ground lightning
strokes. It is currently believed that such lightning
strokes leave in the cloud a residual charge
of 200 Coulombs or more, creating a significant
voltage difference between the cloud top and
the ionosphere. This induces a heating of the
middle atmosphere, which produces the sprites
when electrons collisionally excite atmospheric
nitrogen, that emits red light in flashes with several
milliseconds duration.
Studied from the ground, aircraft and from
the space shuttle, sprites appear to be most common
above large mesoscale convection features,
such as the storm systems of the American Midwest.
Ground observations indicate that an active
system can generate as many as 100 sprites
in the course of several hours. Discovered in
1990. See elves, blue jet.
squall A sudden strong wind or brief storm
that persists for only a few minutes, often associated
with thunderstorm activity and sometimes
accompanied by rain or snow.
© 2001 by CRC Press LLC
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s (slow) process The capture of free neutrons
by nuclei of iron and heavier elements on a time
scale slower than the average for beta-decays of
unstable nuclei (cf. r process). The s process is
the primary source of those isotopes of elements
from Z = 30 to 82 with the largest binding energies
(the “valley of beta stability”). It occurs
in asymptotic giant branch stars and in the corresponding
evolutionary stage of more massive
stars. The main sources of free neutrons (which
require some mixing between zones dominated
by different nuclear reactions) are
13 C(α, n) 16 O and
14 N(α, γ ) 18 O(α, γ ) 22 Ne(α, n) 25 Mg
(the latter being more important in massive
stars). Among the important products of the s
process are barium (because it is dominated by
s process nuclides and can be seen in the atmospheres
of cool, evolved stars) and technitium
(because of its short half life). s-process production
of elements does not reach beyond lead
and bismuth because of the set of unstable elements
before reaching uranium and thorium. A
typical s-process chain is
174
Y b(n, γ )
175
Yb(e
−
¯νe) 175 Lu(n, γ ) 176 Lu(n, γ )
177
Lu(e
−
¯ν)
177
Hf (n, γ )
178
Hf (n, γ )
179
Hf
179
Hf (n, γ )
180
Hf (n, γ )
181
Hf (e
−
¯νe)
181
T a(n, γ )
182
Ta e
−
¯νe
182 W(n, γ ) 184 W(n, γ )
and so forth, onward to Re,Os, etc. The time
scales of the beta decays range from minutes to
months.
The cross-section to capture the next neutron
is smallest for nuclides with filled shells,
so that products of the s process with N =
26, 50, 82, and 126 are particularly abundant.
See asymptotic giant branch star, beta decay, r
process.
S stars Cool (3000 to 3600 K) giant stars
which have greatly enhanced s-process elements
(such as zirconium, barium, yttrium, and technetium)
and have an atmospheric C/O number
ratio very close to unity. Their optical spectra
are characterized by strong absorption bands of
ZrO, as well as TiO, and carbon-rich compounds
such as CN and C2. Most are believed to be on
the asymptotic giant branch (AGB) of the H-R