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solar radiation[′sō·lər ‚rād·ē′ā·shən]
the electromagnetic and corpuscular radiation emitted by the sun. Solar radiation is the principal source of energy for most processes that occur on the earth.
The corpuscular emissions of the sun consist primarily of protons, which have speeds of 300–1,500 km/sec near the earth. The proton concentration in the vicinity of the earth is 5–80 ions/cm3, but it increases with increasing solar activity and after large flares can reach 103 ions/cm3. During solar flares, particles with energies of 5 × 107 to 2 × 10’° electron volts are produced. Mainly protons, these energetic particles form the solar component of cosmic rays and partially account for the variations in the cosmic rays that reach the earth.
The bulk of the electromagnetic radiation emitted by the sun is in the visible part of the spectrum (Figure 1). What is known as the solar constant is the amount of radiant energy received from the sun in 1 min on a surface 1 cm2 in area perpendicular to the sun’s rays and located outside the earth’s atmosphere at the earth’s mean distance from the sun. The solar constant is equal to 1.95 calories/(cm2-min) and corresponds to a flux of 1.36 x 106ergs/(cm2-sec). The emission of solar radiation is believed to increase at times of maximum solar activity. If, however, this increase does occur, it does not exceed a fraction of 1 percent.
The radio-frequency radiation emitted by the sun, or solar radio emission, is transmitted incompletely through the earth’s atmosphere, since in the radio-frequency range the atmosphere is transparent only for wavelengths of a few millimeters to several meters. Solar radio emission is rather weak. It is measured in units of Ф = 10”22 watts/(m2-sec-hertz) and varies from a few <t> to tens and hundreds of thousands of Ф as we move from the meter band, where frequencies are of the order of 108 hertz, to the millimeter band, where frequencies are of the order of 10” hertz. For a terrestrial observer, however, the sun, because of its relatively small distance from the earth, is the most powerful source of cosmic radio-frequency radiation. Solar radio emission consists of the thermal radio emission of the outer layers of the atmosphere of the quiet sun, a slowly varying component associated with sunspots and faculae, and sporadic radio emissions associated with solar activity. The sporadic radio-frequency radiation is often polarized, is more intense than the thermal emission, and varies rather rapidly. It includes noise storms and bursts. Bursts can be classified into five types, which differ both in frequency composition and in the character of the time dependence of the variations in intensity. Most bursts accompany solar flares.
The short-wavelength radiation of the sun is completely absorbed by the earth’s atmosphere. Information on such radiation has been obtained by means of equipment carried by geophysical rockets, artificial earth satellites, and space probes. The intensity of the continuouso solar spectrum decreases abruptly at about 2,085 angstroms (A). The Fraunhofer lines vanish in the vicinity of 1,550 A. Although the= continuous spectrum can be traced to 1,000 A, beyond 1,500 A the spectrum consists principally of emission lines, such as the lines of hydrogen, ionized helium, and multiply ionized carbon, oxygen, and magnesium atoms. There are more than 200 emission lines in the ultraviolet region; the strongest is the resonance line of hydrogen (L0) with a wavelength of 1,216 A. At the earth’s orbit, the flux of short-wavelength radiation from the entire solar disk is 3–6 ergs/(cm2-sec).
Solar X-radiation (wavelengths from 100 to 1 A) consists of continuous radiation and line emissions. The intensity of the X-radiation is strongly dependent on solar activity and varies from 0.13 erg/(cm2-sec) to 1 erg/(cm2-sec) at the earth’s orbit. The X-ray spectrum becomes harder in years of maximum solar activity. During solar flares, the intensity of solar X-radiation is increased by a factor of several tens, and its hardness also increases. Although the ultraviolet radiation and X-radiation from the sun carry comparatively little energy—less than 15 ergs/(cm2-sec) near the earth’s orbit—such radiation has a very strong influence on the state of the upper layers of the earth’s atmosphere. Solar gamma radiation has also been detected, but it remains insufficiently studied.
REFERENCESKosmicheskaia astrofizika. Moscow, 1962. (Translated from English.)
Ul’trafioletovoe izluchenie Solntsa i mezhplanetnaia sreda: Sb. st. Moscow, 1962. (Translated from English.)
Shklovskii, I. S. Fizika solnechnoi korony, 2nd ed. Moscow, 1962.
Solnechnye korpuskuliarnye poloki i ikh vzaimodeistvie s magnitnym polem Zemli: Sb. st. Moscow, 1962. (Translated from English.)
Makarova, E. A., and A. V. Kharitonov. Raspredelenie energii v spektre Solntsa i solnechnaiapostoiannaia. Moscow, 1972.
E. E. DUBOV