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secular variation of the geomagnetic ﬁeld

the “International System of Units” (SI). Specifically, since 1968, the second has been deﬁned as the duration of 9192631770 cycles of the radiation produced by the transition between the two hyperﬁne levels of the ground state of the cesium-133 atom at a temperature of 0 K. A leap second is occasionally added to Universal Time.

secondary craters Impact craters distinguishable from primary craters by their morphology. Primary craters are often circular with a distinct raised rim, of approximately uniform height. Secondary craters, however, are generally elongate, have low irregular rims, and are more shallow than primary craters of corresponding size. Additionally, they tend to occur in clusters or crater chains at a distance of one crater diameter or more from the primary crater. They form when crater ejecta from the primary crater has sufﬁcient velocity to form a secondary crater upon landing (thus, secondary craters always exist circumferentially to a primary crater).

secondary rainbow The fainter rainbow that occasionally forms outside a primary rainbow, with its colors in reverse order from those of the primary (i.e., red on the inside, at about 52◦ from the center of the arc, to blue on the outside of the arc at about 54◦.5 from the center). The secondary rainbow arises from multiple reﬂection paths of light through the water droplets creating the rainbow.

second fundamental form Let S be a p- dimensional submanifold of the n-dimensional Riemannian manifold X. Let u be the tangent vector to S at the point x and v a differentiable vector. The second fundamental form kx is the mapping kx : TxS × TxS → (TxS) deﬁned by

(ux, vx) → kx ≡ ( xv)x

where ( xv)x is the component of the covariant derivative of v that is normal to S.

In string theory, one views the history of the string as a 2-dimensional world sheet, so the extrinsic curvature extends into the remaining dimensions of the spacetime. In general relativity, one often treats 3-spaces of constant time embedded in the full 4-dimensional spacetime. Then the second fundamental form is a vector

valued tensor in the 3-space, which takes values proportional to the unit normal of the 3-space.

sector boundary In the solar wind, the area of demarcation between sectors, which are large-scale features distinguished by the predominant direction of the interplanetary magnetic ﬁeld, toward or away from the sun.

sector structure Polarity pattern of the solar magnetic ﬁeld in the plane of ecliptic. The sector boundaries between the different magnetic ﬁeld sectors are the neutral line on the source surface (see source surface) as it is carried outward by the solar wind. Thus, during solar minimum, when the neutral line does not deviate much from the heliographic equator, a pattern of two or four magnetic ﬁeld sectors can be observed. During solar maximum conditions, on the other hand, the extremely wavy neutral line shows excursions to high latitudes and often crosses the heliographic equator. In turn, the sector structure carried outwards into space is more complex with a larger number of magnetic ﬁeld sectors.

secular (long-period) resonances In a gravitationally interacting system of three or more bodies such as our solar system, neither the longitude of perihelion nor that of ascending node is constant as a consequence of their mutual perturbations. These two longitudes slowly change with secular precession rates gi and si (i = 1 for Mercury through i = 8 for Neptune). Secular resonances occur when a linear combination of the average precession frequency of longitude of

perihelion of a small body < 0˙	> and that of
its longitude of ascending node <	˙	matches
	1 >

a linear combination of the natural frequencies of the gravitationally interacting system. In the asteroid belt, the mean motion resonances are unstable and manifest themselves as the Kirkwood gaps.

secular variation of the geomagnetic ﬁeld

Slow and small changes (with a time scale of decades) in the main ﬁeld of the Earth. For instance, the dipole component of the ﬁeld diminishing between 1840 and 1970 by about 5% per century, a rate which may have increased since then to 7% per century.

sedimentary

sedimentary Referring to rock that has been formed by deposition and consolidation of preexisting material. Divided into types: clastic — consilidation (typically under water) of ﬁne “sand” particles; carbonate — including oolitic limestones, which are consolidated diatom shells; organic — coal, which formed from compressed organic material; and evaporite — rock salt, gypsum. Only sedimentary stones contain fossils, since they form in ways that will preserve the structure of organic material.

sedimentary basin In geophysics, a region in which the Earth’s surface has subsided and has been covered by sediments. One mechanism for subsidence is thermal subsidence: the lithosphere cools and thickens, and due to the increased density of the cooler rocks the lithosphere sinks. A typical thickness of a sedimentary basin is a few kilometers but some basins have a thickness of 10 km or more. Subsidence is often impeded by the rigidity of the elastic lithosphere and the resulting ﬂexure of the lithosphere results in near circular or linear basins with a radius or width of about 200 km.

sediment budget See sand budget.

sediment load The amount of sediment being transported by moving water. Consists of bed load, suspended load, and wash load.

sediment sink A place or process that results in sediment removal from a beach or littoral system. A submarine canyon might serve as a sediment sink, and sand mining provides another example.

sediment source The opposite of a sand sink; a source of sand for the littoral system or beach. Cliffs that back a beach, or a river might serve as sediment sources.

sediment transport The movement of sediment due to the action of wind or water.

seeing The phenomenon of time dependent refraction in the Earth’s atmosphere which moves stellar images around, and often breaks them into multiple subimages. For groundbased telescope, seeing is the dominant factor

limiting resolution. Seeing arises principally in the air within 50 m of the telescope. Seeing is reported in seconds of arc, as the size of the apparent disk of a point source (star). The best Earth-based sites have seeing of the order of 2 arcseconds. Active optics in telescopes can reduce the effects of seeing by factors of up to 10, thereby allowing ground-based observations closer to the diffraction limit.

seiche A stationary water wave usually caused by strong winds, changes in barometric pressure, or seismic activity. It is found in lakes, semi-enclosed bodies of water, and in areas of the open ocean. The period, T , of a seiche in an enclosed rectangular body of water is usually

represented by the formula 2L , where L is the

√

length, d is the average depth of the body of water, and g is the acceleration of gravity. Typical periods range from a few minutes in small lakes to a few hours in gulfs.

seismic coupling factor A parameter used almost exclusively for subduction zone thrust faults. It is deﬁned as the ratio of the long-term average rate of fault slip taking place during subduction earthquakes to the rate of plate convergence for a given fault area.

seismic gap First, a region that has a potential to cause large earthquakes, but where large earthquakes have not occurred for a long time (seismic gap of a ﬁrst kind). Second, a region where seismicity of usual small earthquakes decreases drastically for some time (seismic gap of a second kind). The latter sometimes precedes a forthcoming large earthquake. Seismic gaps of the ﬁrst and second kinds could be important clues for long-term prediction of large earthquakes.

seismic intensity Numerical values which represent the intensity of earthquake motion at a point, divided into several classes. Seismic intensity is determined mainly based on degree of shaking that the human body feels, also referring to shaking of surrounding materials, degree of damage, and phenomena associated with the earthquake. Seismic intensity scales such as MM, MSK, and JMA seismic intensity scales, which are appropriate to each country, are used.

seismology

Since seismic intensity is determined promptly without using instruments, it has been widely used for prompt reports of earthquakes, giving rough information on shaking at different regions and magnitude of earthquakes. Recently, the Japan Meteorological Agency (JMA) developed a seismic intensity meter, taking account of period, displacement, velocity, and duration time of earthquake motion, and deployed seismic intensity meters at different places to provide automatic reporting of seismic intensity.

seismicity Earthquake activity in a particular region.

seismic moment A quantity related to magnitude of an earthquake, based on one of the two torques composing double couple in earthquake faulting. According to dislocation theory, the seismic moment is represented by the product µDS of rigidity of rocks composing source region (µ), the amount of average slip on a fault plane (D), and a fault area (S). Recently, the seismic moment for relatively large earthquakes (Ms ≥ 5.5) has been made available within a short time wherever they occur throughout the world by extracting this information from broadband seismographic records.

seismic parameter The ratio of the bulk modulus to the density within a planetary body. For the Earth (and any other planet where the speeds of seismic waves can be determined as a function of depth), the seismic parameter can be used together with the assumption of hydrostatic equilibrium to compute the density as a function of depth.

seismic wave Longitudinal, transverse, or mixed body or surface elastic wave which propagates through the Earth generated by earthquakes, atmospheric and oceanic disturbances, and lunar activity, for instance intentionally by explosions or “thumpers” for the purpose of geophysical subsurface visualization. Seismic waves propagating the Earth’s interior are called body waves (P-wave, S-wave), while those propagating only along the Earth’s surface are called surface waves (Rayleigh wave, Love wave). Typical velocities of propagation are 1 to 5 km/sec. Seismic waves arrive in the order

of P-wave, S-wave, and surface waves on a seismogram. Surface waves are the waves that have large amplitude near the Earth’s surface, which are generated by interference of many reﬂected and refracted body waves. Velocity structure of the Earth’s interior, attenuation structure, and materials and their state in the Earth can be estimated through investigation of the propagation of seismic waves.

seismogram The output of a seismometer during an earthquake. A record of the ground motions caused by the compressional and shear body waves and the surface waves.

seismograph A recording accelerometer that records earth motions (as especially during earthquakes) and from which the intensity and timing of the ground vibrations can be determined.

seismology The study of the motions of the solid Earth (earthquakes and motions initiated by impacts or explosions), and attempts to explain these motions, and to determine the interior structure of a body. It is the source of the most detailed information about planetary interiors. The science of seismology is based on the study of stress and strain within planetary materials. When brittle material can no longer withstand the stress/strain that is imposed on it, the material will fracture and the released energy, felt as an earthquake, propagates through the surroundings. Seismic energy is dispersed in two types of waves: body waves (which travel through the interior of the body) and surface waves (which travel only along the surface). Body waves can be either primary (P) waves, which are longitudinal waves (i.e., direction of particle motion and the direction of energy propagation are parallel), or secondary (S) waves, which are transverse waves (i.e., direction of particle motion and direction of energy propagation are perpendicular). S-waves cannot travel through liquids, and both P and S waves will vary in velocity depending on the characteristics of the material through which they pass. Thus, by having several seismic stations on the surface pick up the seismic waves from a distant earthquake, the paths of each of those seismic waves can be reconstructed and the structure of

seismometer

the body’s interior can be determined. Surface waves are subdivided into several types, but all are concentrated along the surface rather than propagating through the body. Surface waves can cause much damage to structures on the surface of the planet but give little information about the body’s interior structure.

seismometer An instrument that measures ground motions.

seismotectonics A research ﬁeld to investigate the tectonics by means of earthquakes. Seismotectonics pursues the mutual relation between regional characteristics of earthquake generation and observed various geoscientiﬁc structure and phenomena. To be more concrete, the conﬁguration of plate boundaries, relative plate motion, interplate coupling, intraplate deformations, and stresses are investigated based on data such as seismicity, fault parameters like fault plane solutions and seismic moment, stress drop, and rupture process.

selective heating Solar process which, in impulsive ﬂares, leads to an enrichment of 3He and heavier ions such as Fe compared to coronal or solar wind abundances. See impulsive ﬂare.

self-generated turbulence		See diffusive
shock acceleration.
semiclassical gravity	The approximation in

which gravitational degrees of freedom are assumed to be sufﬁciently well deﬁned by classical general relativity or another classical theory of gravity, but sources (e.g., matter) are taken in terms of a quantum description of matter.

semi-diurnal tides Tidal components that have a cycle of approximately one-half day. The largest amplitude tidal constituent, M2, is semidiurnal, and therefore the tidal motion in most of the world is dominated by two high waters and two low waters each tidal day. The tidal current is said to be semi-diurnal when there are two ﬂood and two ebb periods each day. All tidal constituents with a subscript 2 are semi-diurnal tides.

semimajor axis Half the major axis of an ellipse.

separator A magnetic ﬁeld line that connects any two null points of a magnetic conﬁguration. The separator deﬁnes the intersection of two separatrix surfaces.

separatrix In space plasmas, the magnetic surface that divides topologically distinct magnetic ﬂux regions, for instance separating bundles of magnetic ﬁeld lines with different connections, e.g., open and closed ﬁeld lines.

sequence stratigraphy A subdiscipline of stratigraphy (the study of stratiﬁed rocks, either volcanic or sedimentary). The phrase sequence stratigraphy is not uniquely deﬁned but is generally understood to involve the division of rocks in a sedimentary system into distinct packages, which are then related to each other in time through observation of their spatial and lithological relationships and the application of some stratigraphic rules. In practice, this has been associated with the development of algorithms for interpreting seismic proﬁles of sedimentary basins (“seismic stratigraphy”), as it became clear that packets of rocks are laid down in similar fashions in different basins. These packets are related to different stages of local transgression and regression of the sea, which occurs across a broad range of frequencies. By inferring that local sea level is related to global sea level, and correlating sequences between different basins, one may derive a curve of global sea level with different “orders” of cycles from timescales of around 100 Ma (ﬁrst order cycles, associated with supercontinent assembly and break-up) through tens of thousands of years (fourth order or higher, perhaps due to changes in global glaciation); however, the global correlation of higher order cycles is disputed and local subsidence may often be important.

Sérsic–Pastoriza (1965) galaxies Galaxies that exhibit an anomalous luminosity proﬁle near their center, and often structures due to unresolved, compact emission regions (“hot spots”), which are sites of intense star formation. Several Sérsic–Pastoriza galaxies have a nucleus showing a Seyfert-type or LINER