DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

sudden stratospheric warming
the ionosphere. When the D-region ionization
increases, the reflection height is lowered correspondingly
and the altered path-length is responsible
for the SPA. See short wave fadeout.
sudden stratospheric warming A sudden
increase temperature in the stratosphere that occurs
in winter and can be driven by upwardpropagating
planetary waves. In a major event
of this type, the temperature at the 10 mb (about
31 km) level at the North Pole may increase by
40 to 60 K in less than 1 week.
sulfur dioxide SO2, a colorless, toxic gas.
Freezing point −72.7 ◦ C; boiling point −10 ◦ C.
Volcanic eruptions provide a natural source of
sulfur dioxide in the atmosphere. However, sulfur
dioxide is a major anthropogenic primary atmospheric
pollutant, originating from fossil fuel
combustion. In the atmosphere, sulfur dioxide
is oxidized to form sulfur trioxide, that is extremely
soluble in water. In the presence of
water, droplets of sulfuric acid (acid rain) are
formed. Sulfuric acid droplets also appear to act
as nucleation centers, increasing the amount of
rainfall. High airborne concentrations of sulfur
dioxide may aggravate existing respiratory and
cardiovascular disease, forming sulfuric acid in
the bronchia.
summation convention In equations involving
vectors or tensors, and in matrix equations,
one often encounters sums of the form
AαB α ,
where {Aα} and {B α } are collections of perhaps
different kinds of objects, but of equal number
labeled by and summed over the full set of integers
α ∈[αmin ,αmax].
Einstein noted that in a large number of situations,
the expressions unambiguously require
the
sum. In such cases we can omit writing the
. This is the Einstein summation convention.
α
In the unusual circumstance where one wants to
examine the symbol AαB α for one fixed value
of α only, one appends a “no sum” to the expression.
Formulae in this dictionary use the
summation convention. Similar notations have
been extended to continuous sums (i.e., integration).
© 2001 by CRC Press LLC
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α
summer solstice The point that lies on the
ecliptic midway between the vernal and autumnal
equinoxes and at which the sun, in its apparent
annual motion, is at its greatest angular
distance north of the celestial equator. On the
day of the summer solstice, that occurs on about
June 21, the length of daytime is at its maximum
in the northern hemisphere. After summer
solstice, the northern length of daytime will decrease
until the winter solstice. Because of complicated
interactions with atmosphere ocean and
land heat reservoirs, northern surface temperatures
continue to increase for a period of time
after the summer solstice. In the southern hemisphere,
the longest day, and the beginning of
southern summer, occur on the winter solstice,
about December 21.
sun Our star; an incandescent, approximately
spherical star around which there exists a system
of nine planets (the “solar system”), rotating in
elliptical orbits. The sun is a main sequence star
of type G2 V with an absolute magnitude of 4.8.
The sun has a mass of 1.99 × 10 30 kg (roughly
99.9% of all the matter in the solar system) a
radius of 696,000 km, a mean distance from the
Earth of 150 million kilometers (denoted an Astronomical
Unit), a surface gravity of 274 m/s 2 ,
a radiation emission of 3.86 × 10 26 W, an equatorial
rotation rate of 26 days and an effective
temperature of 5785 K. The sun has a radiative
interior surrounded by a convective zone both of
which participate in the transportation of energy
from the nuclear burning core to the surface.
The solar atmosphere consists of a number of
distinct temperature regimes: the photosphere,
chromosphere, transition region, and corona.
sunlit aurora Aurora occurring in the upper
atmosphere around the polar cusps of the Earth,
observed in the ultraviolet by satellite imagers.
sunspot A generally irregular dark spot, with
considerable internal detail on the surface of the
sun. A well-developed sunspot consists of a
central circular or elliptical umbra, of lower luminosity,
surrounded by a brighter penumbra.
Sunspots are violent eruptions of gases cooler
than the surrounding surface areas. The temperature
of sunspots is about 1000 K lower than
the typical temperature of the photosphere of