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Southern Oscillation Index (SOI)

cally receive different daily periods of sunlight. At the solstices the location of the planet in its orbit is such as to provide the maximum tilt of one hemisphere toward the sun, and this hemisphere receives more direct and longer duration light than does the opposite hemisphere. Thus, the northern hemisphere experiences long days at the summer solstice, the official beginning of northern summer, while the southern hemisphere experiences short days on this date, the official beginning of southern winter. Similarly the northern hemisphere experiences short days at the winter solstice, the official beginning of northern winter, while the southern hemisphere experiences long days on this date, the official beginning of southern summer. “Solstice” has the meaning “sun standing still” and marks the extreme of the northward or southward motion of the sun on the celestial sphere.

sound wave A longitudinal wave propagating in a supporting medium, with a frequency audible to humans: about 20 to 20,000 Hz.

source functions The terms in the radiative transfer equation that describe the inelastic scattering and true emission contribution to a beam of radiation; some authors also include elastic scattering in the source term.

source surface Fictious surface in the solar corona at a height of about 2.5 solar radii. Graphically, the source surface separates the lower altitudes where open and closed magnetic field structures co-exist from the higher altitudes where all field-lines are open, extending into the interplanetary medium. The source surface can be determined from the photospheric magnetic field pattern using potential theory. Constraints on the field are then: (a) the magnetic field at the source surface is directed radially and (b) currents either vanish or are horizontal in the corona. The magnetic field pattern on the source surface then also gives the polarity pattern carried outward into with the solar wind. In particular, a neutral line separating the two opposite polarities can be identified. This neutral line is carried outwards as the heliospheric current sheet; the maximum excursion of the neutral line from the heliographic equator defines the tilt angle. See sector structure.

South Atlantic Anomaly A region of strong radiation (principally protons) above the Atlantic Ocean off the Brazilian Coast, arising from the inner Van Allen belt. The Van Allen belts are energetic particles trapped by the Earth’s magnetic field. The offset between the Earth’s geographical and magnetic axes leads to an asymmetry in the belt position, and the

South Atlantic Anomaly is the resulting region of minimum altitude (about 250 km). Some of the quasi-trapped charged particles mainly electrons in the kev range from the radiation belts may dip to very low heights at around 100 km in the Earth’s atmosphere, where collisions with neutral particles can cause ionization and photochemical excitation. The anomaly extends as far as the coast of Africa. The South Atlantic Anomaly is thus a region of energetic particles through which low-orbiting satellites frequently pass, and the radiation density can be harmful to electronics and to human crew. See Van Allen Belts.

South Equatorial current An ocean current flowing westward along and south of the equator.

southern oscillation Quasi-periodic phenomenon in sea level pressure, surface wind, sea surface temperature, cloudiness, and rainfall over a wide area of the tropical Pacific Ocean and adjacent coastal areas, south of the equator with a period from two to four years, characterized by simultaneously opposite sea level pressure anomalies at Tahiti, in the eastern tropical Pacific and Darwin, on the northwest coast of Australia. See El Niño, La Niña.

Southern Oscillation Index (SOI) An index that is calculated to monitor the atmospheric component of the El Niño-Southern Oscillation (ENSO) phenomenon. It is usually defined as the monthly averaged sea level pressure anomaly at Tahiti minus the pressure anomaly at Darwin, Australia. The SOI is often normalized by the standard deviation over the entire record. Anomalously high pressure at Darwin and low pressure at Tahiti usually indicate El Niño (warm) conditions. See also El Niño.

© 2001 by CRC Press LLC

space-like infinity

space-like infinity The endpoint i0 of all space-like geodesics of a space-time.

space-like vector An element t of a linear space with a Lorentzian metric g of signature (, +, +, +) such that the norm is g(t, t) = gabtatb > 0. The norm of a space-like four vector in the theory of relativity represents the spatial distance of two events that appear simultaneous to some observer.

spacetime The collection of all places and all moments of time in the whole universe. For various problems simplified models of our physical spacetime are constructed. For example, cosmology often ignores details of the geometry and matter distribution in the universe that are smaller than a group of galaxies. In investigating planetary motions, typically all stars other than the sun are ignored, and it is assumed that the empty space around the sun extends to infinite distances. A mathematical model of spacetime is a four-dimensional space (one of the four coordinates is time) with a given metric; two spacetimes are in fact identical if their metrics can be transformed one into the other by a coordinate transformation. Points of the spacetime are called events. Two events p and q can be in a timelike, lightlike (null), or spacelike relation. The metric makes it possible to calculate the lapse of time between the events p and q when they are in a timelike relation or the distance between them when they are in a spacelike relation. In fact the metric contains all the relevant information about the geometry and physics in its underlying spacetime, although some of the information may be technically difficult to extract. In general relativity, Einstein’s equations relate the matter distribution to the geometry of the spacetime. Geodesics can be calculated to determine the trajectories of particles, and of light rays, moving under the influence of gravitational forces. If the properties of the spacetime are found to correspond to part of the observed world, then the mathematical spacetime model is considered realistic in the appropriate range of phenomena. Very few such analytically exact realistic models exist. Among them are the flat Minkowski spacetime that is the geometrical arena of special relativity, the spacetime of the Schwarzschild solu-

tion that describes spherical black holes, and the sun’s dominant, spherically symmetric gravitational field in the solar system (but without taking into account the planets’ own gravitational fields or the rotation and rotational deformation of the star), the spacetimes of the Friedmann– Lemaître and Robertson–Walker cosmological models that are used to model the whole universe, the Kerr spacetime describing the gravitation field of a stationary rotating black hole, and a few more spacetimes corresponding to simple patterns of gravitational waves and isolated structures in the universe (the Lemaître–Tolman cosmological model is among the latter). However, computational modeling is beginning to provide a much longer list of accessible realistic spacetimes.

space weather The conditions and processes occurring in space which have the potential to affect the near Earth environment and, in particular, technological systems. Space weather processes include the solar wind and interplanetary magnetic field, solar flares, coronal mass ejections from the sun, and the resulting disturbances in the Earth’s geomagnetic field and atmosphere. The effects can range from the unexpected (e.g., disruption of power grids, damage to satellites) to the common (e.g., failure of HF systems). Although space weather effects have been recognized and studied for many years, it has only recently developed as a recognized field of unified activity which attempts to forecast solar flares, magnetic storms and other space-related phenomena.

spallation A nuclear reaction in which an atomic nucleus is struck by an incident high energy particle. As a result, particles typically heavier than an α-particle are ejected from the

nucleus. Astrophysical amounts of the lightly bound isotopes 6Li, 9Be,10B, and 11B are be-

lieved to have been formed by spallation by energetic cosmic rays.

special relativity A description of mechanical and electromagnetic phenomena involving sources and observers moving at velocities close to that of light, but in the absence of gravitational effects. Maxwell’s theory describes the dynamics of electric and magnetic fields and predicts

© 2001 by CRC Press LLC

specific photosynthetic rate

a propagation velocity, c, which is numerically equal to the observed speed of the light.

Minkowksi, Lorentz, and especially Einstein in his 1905 exposition, modified concepts of space and time in a way that accommodates the fact that every observer, regardless of his motion, will always measure exactly the same speed for light as experimentally observed by Michelson and Morley. To accomplish this required accepting that time is involved in Lorentz transformation, the change of coordinate frame between moving observers, just as spatial position is involved. The Lorentz transformation also predicts length contraction, time dilation, and relativistic mass increase, and led Einstein to his famous result E = mc2.

A consequence of the universality of the speed of light is that velocities are not additive. If the system B (say, an airliner) moves with respect to the system A (say, the surface of the Earth) with a given velocity v1 (say, 900 km/h) and an object C (say, a flight attendant) moves in the same direction with a velocity v2 (suppose, 4 km/h), then the velocity of the object C with respect to the system A is not simply v1 + v2,

but V = v1+v2 , where c is the velocity

1+(v1v2/c2)

of light (in the example given, the flight attendant moves with respect to the surface of the Earth with the velocity 904 · (1 0.31 · 1012) km/h). As is seen from the example, at velocities encountered in everyday life the corrections provided by special relativity to Newton’s mechanics are negligible. However, for velocities large compared to c, the difference is profound. In particular, if v2 = c, then V = c. Also, if v1 c and v2 c, then V c, i.e., the velocity of light cannot be exceeded by adding velocities smaller than c. The geometric arena of special relativity is not the three-dimensional Euclidean space familiar from Newton’s theory, but a four-dimensional Minkowski spacetime. The Euclidean space is contained in the Minkowski spacetime as the subspace of constant time, but it is not universally defined, i.e., every observer will see a different Euclidean space in his/her own reference system. See Lorentz transformation, Michelson–Morley experiment.

specific The adjective used to express a quantity per unit mass.

specific absorption coefficient The absorption coefficient [m1] per unit mass of material, e.g., for unit chlorophyll a concentration (Units: [mg chl a m3]); one obtains a specific absorption coefficient with units [m2 (mg chl a) 1].

specific discharge See Darcy velocity.

specific energy A term used in the study of open channel flow to denote the energy of a fluid relative to the channel bottom. Specific energy is defined as E = y + V 2/2g, where y is flow depth, V is flow speed, and g is acceleration of gravity. Specific energy has units of length and corresponds to energy per unit weight of fluid.

specific gravity Ratio of the density of a substance to that of water.

specific heat Specific heats, also called specific heat capacity or heat capacity, are defined under constant volume (cv, Cv, also called isochoric specific heat) and constant pressure (cp, Cp, also called isobaric specific heat). The constant volume specific heat of a pure substance is the change of molecular internal energy u for a unit mass (or 1 mole) per degree change of temperature when the end states are equilibrium states of the same volume:

 

v

∂T

v

v

∂T

v

c

 

 

∂u

and C

 

 

∂u¯

,

 

 

 

 

 

where cv is for unit mass and Cv is called the molal specific heat, u is the specific internal energy, u¯ is internal energy for 1 mole; Cv = Mcv. The constant pressure specific heat of a pure substance is the change of enthalpy for a unit mass (or 1 mole) between two equilibrium states at the same pressure per degree change of temperature:

cp

∂h

p

and Cp

 

¯

p

∂T

∂T

 

 

 

 

 

∂h

 

where h = (u+pv) is the specific enthalpy, and

¯ is the enthalpy for 1 mole;

C

p =

Mc

p

.

h

 

 

 

specific humidity

The mass of water vapor

per unit mass of air.

 

 

 

 

 

 

specific photosynthetic rate

 

Photosynthetic

rate, net or gross, per unit biomass or per unit

© 2001 by CRC Press LLC

specific storage (Ss )
specific storage (Ss)
volume, e.g., [µmoles CO2 h1].

(or O2) (mg chl)1

The amount of water per unit volume of a saturated formation that is stored or released from a saturated aquifer in response to a unit increase or decrease in hydraulic head, with units of meter1 (L1): Ss = ρg(α + nβ), where ρg is the fluid weight, α is the compressibility of the aquifer, and β is the compressibility of the fluid.

specific volume In a dipole-like magnetic configuration, like that of the not too distant magnetosphere of the Earth, the integral V =

ds/B along a closed magnetic field line. (The integral is usually assumed to be computed from one end of the line to the other, but the value obtained between the two intersections of the line with the ionosphere is not much different.) The variable V is a useful quantity in the simplified formulation of MHD in which the plasma in any convecting flux tube is assumed to stay inside that tube and to remain isotropic. The equations of this formulation (e.g., the Grad– Vasyliunas theorem) are often used in theories and simulations of global convection. In particular, an adiabatic law pv5/3 is expected to hold then, with p the plasma pressure.

spectral The adjective used to denote either wavelength dependence or a radiometric quantity per unit wavelength interval.

spectral energy distribution The relative intensity of light (or other electromagnetic radiation) at different frequencies, measured over a range of frequency; usually expressed in units of power per unit frequency or per unit wavelength. In astronomy often referring to the continuum spectrum of an astronomical object, with the absorption or emission line spectrum not considered. The spectral energy distribution is very different for different astronomical sources and emission mechanisms. For example, stars emit radiation whose spectral energy distribution show minor deviations from the Planck function, as expected for high-temperature black bodies; synchrotron processes produce a spectral energy distribution that can be described,

© 2001 by CRC Press LLC

over a wide spectral range, by a power-law as a function of frequency or of wavelength.

spectral energy distribution of active galactic nuclei A spectral energy distribution which is very different from the those of stars and nonactive galaxies and which is characterized by significant, in a first approximation almost constant, energy emission over a very wide range of frequencies, from the far IR to the hard X- ray domain (power-law function of frequency (fν να), with a spectral index α 1; hence, the emitted energy νfν does not depend on ν and is constant). In radio-loud AGN, roughly constant emission extends to radio frequencies; in radio quiet AGN, emission at radio frequencies is typically 100 times lower. Superimposed to the power-law emission there is a broad feature, the big blue bump, which extends from the visible to the soft X-ray domain, with maximum emission in the far UV. The big blue bump has been ascribed to thermal emission, possibly by the putative accretion disk. This interpretation of the AGN continuum is, however, the subject of current debate. There are also significant differences between AGN subclasses. See big blue bump, spectral energy distribution.

spectral gap Refers to a substantial local minimum in the energy spectrum φ(ω) at frequency ωg, which divides the energy spectrum in hydrodynamics into a mean flow (large-scale, advective) (ω < ωg) and turbulence (ω > ωg). The presence of a spectral gap is an ideal prerequisite for the Reynolds decomposition.

spectral line The specific wavelengths of light which are exceptionally bright or exceptionally faint compared to neighboring wavelengths in a spectrum. When passed through a spectrograph, these are seen as bright or dark transverse lines in the spectrum. Bright lines arise from emission from specific atomic states; dark lines are absorption on specific atomic states.

spectral

line profiles

The shape of an ab-

sorption

or emission line profile depends on

many factors. Among them are thermal Doppler broadening, pressure broadening, collisional damping, radiation damping, rotational broad-

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