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Dictionary of Geophysics, Astrophysic, and Astronomy.pdf

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SNC meteorites

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.

slow shock wave See hydromagnetic shock wave.

slow solar wind The properties of the slow solar wind are highly variable. Often large-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 × 104 K, electron temperatures are twice as much, and densities at 1 AU are about 8 ions/cm3. 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 0h50m, 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

Snell’s law

gases differ from those of other meteorites but are similar to abundances in the Martian atmosphere. As a result it is believed that these meteorites formed on Mars, and were launched into space by a large impact. See Martian meteorites.

Snell’s law The law that describes the refraction of light at an interface between two media that have different real indices of refraction: n1 sin θ1 = n2 sin θ2 where n1, n2 are the indices of refraction and θ1 θ2 are the angles to the normal of the interface.

snow Frozen precipitation (small ice crystals, or “ﬂakes”) that formed in clouds by accretion onto accretion centers at temperatures below freezing, and fell to Earth without being melted. Hence, it preserves the hexagonal symmetry with which the ice crystal formed.

soft gamma repeaters Fractional-second increases in the count rate of 20-keV photons from space that were ﬁrst found with Russian gamma-ray burst instrumentation about 20 years ago. Unlike typical gamma ray bursts, these events repeat from the same sources in sporadic episodes, from several per year to several per day. The total number of known sources now totals only four, each localized at or near supernova remnants in the Large Magellanic Cloud or in our galaxy. Exhibiting many of the properties of hard X-ray burst events, such as the rate of change of a several-second period, the sources are clearly conﬁrmed as neutron stars that are highly magnetized (“magnetars”). In addition, only twice in 30 years has a 1000-fold more intense soft gamma outburst been detected, originally confusing as to its possible gammaray burst identity (see March 5 event), but now thought to be a neutron star crustquake.

Soft X-ray Telescope (SXT) A broad-band X-ray imager on board the Yohkoh spacecraft. The SXT provides images of the solar corona over the soft X-ray wavelength range, 3 to 60 Å, with a maximum spatial resolution of 4.9 arcsecs (pixel size = 2.45 arcsec) or 3500 km. The dynamic range of the SXT and the ﬂexible exposure control enables it to image a wide range of solar phenomena from ﬂares to quiescent coro-

nal loops. SXT is sensitive to plasma at temperatures 1 to 2 million K.

solar abundance The relative abundances of the elements in the early sun. When used in the context of lower temperature environments, where the atoms combine into molecules, solar abundance refers to the abundances of the atoms themselves, regardless of the molecules in which they are found.

solar activity Solar activity can be described on several time scales. Two are particularly important: short time scales of days or less and long time scales of several years. The two time scales are linked because short-term solar activity varies over a period of 11 years, called the solar cycle. Short-term solar activity can encompass any change in the sun’s appearance or behavior. The most commonly referred to change is the appearance, evolution, and disappearance of active regions and sunspots over periods of one to three solar rotations ( 27 days when viewed from the Earth). Shorter timescale transient perturbations of the solar photosphere, evolving in seconds and lasting for hours, are solar ﬂares; dramatic visual forms of short-term solar activity. Changes in solar activity can directly affect the Earth. See solar cycle.

Solar and Heliospheric Observatory (SOHO)

A joint European Space Agency and NASA mission launched in December 1995 designed to study the internal structure of the sun, its extensive outer atmosphere and the origin of the solar wind, the stream of highly ionized gas that blows continuously outward through the solar system. SOHO consists of 12 separate instruments including helioseismology experiments, atmospheric imagers and spectrometers, white light and UV coronagraphs, and in situ particle detectors.

solar atmosphere The solar atmosphere starts above the photosphere, the visible surface of the sun where most of its light is emitted, and consists of three layers: the chromosphere, the transition region, and the corona. The bottom layer of the solar atmosphere is the chromosphere, a thin (a few thousand kilometers thick)

solar dynamo

layer, which can be seen as a small reddish ring during a total eclipse. Its name, chromosphere, means colored sphere and stems from this reddish color, which results from emission in the Hα line of hydrogen. The chromosphere shows highly variable structures, giving it the nickname “burning prairie”. In the chromosphere, the density decreases by about three orders of magnitude while the temperature stays roughly constant. The next layer is the transition region, where the temperature increases by a factor of about 200, the density decreases by one order of magnitude and the collision time increases by more than 4 orders of magnitude — over a thin layer which is only a few hundred kilometers thick. The outer solar atmosphere, the corona, ﬁlls the entire heliosphere as solar wind. Aside from just above the transition region, temperature and density are roughly constant. The corona can be seen as a broad structured ring around the occulting disk of the moon during a total eclipse. These structures, in particular arcs, rays, or helmet streamers, reﬂect the magnetic ﬁeld pattern in the corona, allowing the distinction between open and closed magnetic ﬁelds. Despite its high temperature, the corona does not radiate like a black body because its density is too low. Instead, the radiation is photospheric light scattered by the electrons. Thus, the light intensity also reﬂects the electron density in the corona. See chromosphere, corona, photosphere, transition region.

solar B-angle The angle, relative to the center of the solar disk, between the north pole of the sun and the zenith, measured from north to west in degrees.

solar constant The rate at which solar radiant energy at all wavelengths is received outside the atmosphere on a surface normal to the incident radiation, at the Earth’s mean distance from the sun. The amount of variability is still a subject of debate, but is certainly very small (less than 1%) apart from the long-term development in the history of the sun. The symbol of solar constant is S. Unit of S is Wm−2 or cal · cm−2· min−1. Since 99.9% of the solar radiation energy is within the wave band 0.2 to 10.0 µm, the measurment of solar constant does not need to involve a wide wave band. In 1981, World

Meteorological Organization suggested the use of S = 1367 ± 7 W/m2 as the solar constant value, equivalent to 1.945 cal · cm−2· min−1.

solar cycle The solar cycle, the 11-year periodicity in the occurrence of sunspots, ﬁrst was recognized in 1843 by H. Schwabe. Its features can best be seen in a butterﬂy diagram: at the beginning of a solar cycle a few sunspots start to appear at latitudes around 30◦. These spots are relatively stable and can often be observed over a couple of solar rotations. The spots move toward the equator while at their original latitudes new spots appear. The number of sunspots increases until solar maximum. Afterwards, only a few new sunspots emerge while the spots at lower latitude dissolve. The total number of sunspots decreases until just after solar minimum when new sunspots begin to emerge at higher latitudes. The average duration of such a cycle is 11 years with variations between 7 and 15 years. Successive cycles have opposite magnetic polarity, so that the full cycle is actually 22 years. The maximum number of sunspots also differs from cycle to cycle by up to a factor of 4. Occasionally, the solar cycle, and with it the sunspots, can disappear nearly completely over time scales of some 10 years. The best documented case of such “missing sunspots” is the Maunder Minimum. Solar Cycle #23 began in October 1996. See butterﬂy diagram, Hale’s polarity law, Maunder Minimum, Spoerer’s law, sunspot, sunspot cycle.

solar day The length of time from noon to noon; It varies from the mean solar day (24 h) as described by the equation of time.

solar disk The visible surface of the sun projected against the sky.

solar dynamo The process by which the interaction between magnetic ﬁeld and plasma deep in the solar interior is thought to occur. The dynamo process results in the intensiﬁcation of magnetic ﬁelds via the induction of plasmas trying to cross ﬁeld lines. The action of the solar dynamo is thought to explain the existence of the solar cycle, the butterﬂy diagram, Spörer’s Law, and the reversal of the sun’s polar ﬁelds near sunspot maximum.

solar eclipse

solar eclipse The blocking of the solar disk due to the passage of the moon between the sun and the Earth. The moon casts a shadow on the Earth — or from the Earth, the sun is screened off by the moon. Solar eclipses can come in one of three forms: total, annular, or partial. A total solar eclipse occurs when the moon and sun are in perfect alignment and the relative distances are such that they have the same angular diameter in the sky. Because of the eccentricity of the Earth’s orbit around the sun and in the orbit of the moon around the Earth, the angular diameter of the sun is sometimes less than, or sometimes greater than the angular diameter of the moon. The annular eclipse also occurs when the sun and moon are in perfect alignment but when the moon is more distant, so has a slightly smaller angular diameter than the sun creating a bright ring or annulus at the time of maximum eclipse. Partial eclipses are more common and refer to the occasions when the moon only partly covers the solar disk as seen from the viewpoint on Earth. Of these three, the total eclipse is by far the most spectacular, occurring on average once per year somewhere on Earth with durations ranging from 1 to 7 min. During a total solar eclipse, the corona and chromosphere become visible.

solar electromagnetic radiation

On aver-

age, the sun emits a total of 3.86 × 1023 kW integrated over its surface or 6.3 × 104 kW/m2. This radiation can be divided into ﬁve frequency bands:

1.X-rays and extreme ultraviolet (EUV) with

λ< 1800 Å contributes to about 10−3 of the total energy output and is emitted from the lower corona and the chromosphere. This hard radiation varies strongly with the solar cycle; during ﬂares the emission can be enhanced by up to orders of magnitude.

2.Ultraviolet (1800 Å < λ ≤ 3500 Å) contributes to about 9% of the total ﬂux and is emitted from the photosphere and the corona. Variations with the solar cycle are similar to the ones in X-rays, although the amplitude is smaller.

3.Visible light (3500 Å < λ < 7500 Å) contributes to 40% of the total ﬂux and is emitted from the photosphere. Variations with the solar cycle are small (less than 0.1%); only in ex-

tremely strong ﬂares can a local brightening in visible light be observed (white-light ﬂare).

4.Infrared (7400 Å < λ < 107 Å) contributes 51% to the energy ﬂux. Like the visible light, it is emitted from the photosphere and shows only small variations with the solar cycle.

5.Radio-emission (λ > 1 mm) contributes only 10−10 to the total energy ﬂux. It is emitted from the corona and can be enhanced signiﬁcantly during solar ﬂares.

solar ﬂare A violent re-organization of intense magnetic ﬁelds in and above the solar photosphere. The event is accompanied by an increase in X-ray emissions from the vicinity of the ﬂare. The extent of the X-ray emission depends on the size of the ﬂare. The magnitude, or class, of the ﬂare can be indicated by the intensity of the X-ray emissions. The main classes of X-ray ﬂare are: C class, between 10−6 and 10−5 watts meter−2; M Class, between 10−5 and 10−4 watts meter−2; and X class, greater than 10−4 watts meter−2. Solar ﬂares usually occur in active regions on the sun and are more common near solar maximum. See short wave fadeout.

solar ﬂux unit Unit of radio emission from the sun. 1 sfu ≡ 10−22 Wm−2.

solar granulation Solar granules consisting of upwelling convective regions in the sun’s photosphere, of order 1000 km in size, separated by dark, intergranular lanes, where cool material is descending. The typical lifetime of a granule is of order 5 to 10 min.

solar limb The apparent edge of the sun as it is seen in the sky.

solar magnetic sectors When solar plasma moves deep into the interplanetary space, the embedded magnetic ﬁeld adopts a spiral structure due to the solar rotation. Magnetic ﬁelds in the low speed solar wind are more tightly wound than those in the high speed solar wind, which are more radially aligned. The angle that the magnetic ﬁeld makes with the sun-Earth line at the Earth’s orbit is about 45◦ in the solar equatorial plane. The strength of the magnetic ﬁeld is of the order of a few nanotesla. The direction of

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