SPECTRUM DENSITY 



South Equatorial Current — Any of several 



ocean currents driven by the southeast trade 

 winds flowing over the tropical oceans of the 

 Southern Hemisphere. 



In the Atlantic Ocean it is known as the At- 

 lantic South Equatorial Current and flows west- 

 ward with its axis through 2°N., 25°W. Part, 

 flows northwest along the northeast coast of 

 South America (the Guianas) as the Guiana 

 Current. The other part turns below Natal 

 and flows south along the coast of Brazil as the 

 Brazil Current. 



In the Pacific Ocean, the Pacific South Equa- 

 torial Current flows westward between approxi- 

 mately 3°N and 10°S. Much of it turns south 

 in midocean, forming a large anticyclonic whirl. 

 The portion that continues across the ocean 

 divides as it approaches Australia, part moving 

 north toward New Guinea and part turning 

 south along the east coast of Australia as the 

 East Australia Current. 



In the Indian Ocean, the Indian South Equa- 

 torial Current axis is located at about 12 °S and 

 as it approaches the east coast of Africa it turns 

 south, joining the Agulhas Current. See also 

 monsoon current. 



South Indian Current — An eastward flowing 

 current of the southern Indian Ocean that is 

 continuous with the northern edge of the West 

 Wind Drift. 



South Pacific Current — An eastward flowing 

 current of the South Pacific Ocean that is con- 

 tinuous with the northern edge of the West 

 Wind Drift. (5) 



spark-type luminescence — A display of biologi- 

 cal light appearing as innumerable flickering 

 pinpoints of light, particularly conspicuous in 

 the wake of a ship, along the hull line, or in 

 agitated waters. Crustaceans, such as cope- 

 pods and euphausiids, cause this type of dis- 

 play. See bioluminescence. 



spat — The spawn or young of bivalve mollusks. 



species — See classification of organisms. 



specific acoustic impedance — The ratio of 

 acoustic pressure in a medium to the particle 

 velocity of the medium. Tlie meter-kilogram- 

 second unit of specific acoustic impedance is a 

 kilogram per square meter second or rayl. (33) 



specific activity — 1. The activity of a radioiso- 

 tope of an element per unit weight of element 

 present in the sample. 



2. The activity per unit mass of a pure 

 radionuclide. 



3. The activity per unit weight of any sample 

 of radioactive material. 



(41) 

 specific gravity — The ratio of the density of a 

 given substance to that of distilled water usually 

 at 4°C and at a pressure of one atmosphere. 

 Since the density of pure water depends on its 

 isotopic composition, unless the isotopic com- 

 position of the distilled water can be specified, 



the term specific gravity should not be used 

 when the intention is to state a precise value. 

 Values derived from Knudsen's Tables are of 

 specific gravity, not density. 



specific heat — The heat capacity of a system per 

 unit mass, that is, the ratio of the heat absorbed 

 (or released) by unit mass of the system to the 

 corresponding temperature rise (or fall). 



The amomit of heat required to raise the tem- 

 perature of 1 gram of water by 1°C. The spe- 

 cific heat of water is usually measured at 

 constant pressure but may also be measured at 

 constant volume. 



The specific heat of water, which for pure 

 water at 17.5°C (63.5°F) is 1 calorie per gram, 

 decreases with increasmg temperature and 

 salinity. (5, 54) 



specific volume — The volume per imit mass of a 

 substance, or the reciprocal of density. (5) 



In oceanographic practice, specific volume is 

 taken as the reciprocal of specific gravity. 



specific volume anomaly — (or anomaly of spe- 

 cific volutne; also called steric anonrval'i/; symbol 

 8). In oceanography, the excess of the actual 

 specific volume of the sea water a*t any point in 

 the ocean over the specific volume of sea water 

 of salinity 35 per mille and temperature 0°C at 

 the same pressure. The integral of specific vol- 

 ume anomaly with depth is the dynamic height 

 anomaly. See thermosteric anomaly. (5) 



Since oceanographic tables actually list re- 

 ciprocal density rather than specific volume, the 

 quantity used in oceanography is actually "re- 

 ciprocal density anomaly." However, a "reci- 

 procal density anomaly" is numerically equal 

 to a specific volume anomaly to as many signifi- 

 cant figures as are desirable in oceanographic 

 work. 



spectral distribution curve — A curve represent- 

 ing the spectral concentration of a quantity as 

 a function of wavelength (or frequency). (8) 



spectrophotometer — A device for the measure- 

 ment and analysis of both emitted and absorbed 

 radiant energy. Appropriate identifications 

 of the composition or the constituents of organic 

 and inorganic matter may be made. Quantita- 

 tive information derived from a determination 

 of the intensity or density of specific spectral 

 lines may also be measured. 



spectrum — A visual display, a photographic rec- 

 ord, or a plot of the distribution of the intensity 

 of energy dispersion of a given kind as a func- 

 tion of its wavelength, energy, frequency, 

 momentum, mass, or any related quantity. 



spectrum density — The mean-square amplitude 

 of tlie output of an ideal filter with unity gain 

 resjionding to the oscillation, per unit band- 

 width; that is, the limit for vanishingly small 

 bandwidth of the quotient of the mean-square 

 amplitude divided by the bandwidth. 



153 



