SUNLIGHT AS A SOUKCi: OF KADIATION 99 



prominences, or the continuum — are strong. The quantity a\h in Eq. 

 (3-1) is small throughout the chromosphere, and light from the underlying 

 layers is readily transmitted. Since the chromosphere is a weak absorber, 

 it is a weak emitter. 



The corona is the outermost observable region of the sun, being observa- 

 ble only during a solar eclipse or by use of the coronagraph. The corona 

 begins in the region where the total continuous radiation from the solar 

 atmosphere is about equal in intensity to the total line emission, and this 

 region lies about 12,000 km above the photosphere. The corona extends 

 outward for very great distances. S. P. Langley is said to have observed 

 a coronal streamer extending to 12 solar diameters during the eclipse of 

 1878. Photographs usually show the corona extending to about 1 solar 

 diameter because of the rapid decrease in its brightness with increasing 

 height. 



The visible light from the corona consists principally of light from the 

 photosphere scattered by electrons, but emission lines of highly ionized 

 calcium, iron, nickel, and argon are also present, the most intense being 

 the green line at 5303 A due to Fe(XIV), that is, iron with half its 26 elec- 

 trons removed. Temperatures of about one million degrees are required 

 to produce the states of ionization and other effects observed in the 

 corona. Nevertheless, the entire visible light from the corona is about 

 half that of the full moon, or about one one-millionth that of the sun, and 

 its contribution to the visible light and ultraviolet radiations which reach 

 the surface of the earth is inconsequential. The corona and upper 

 chromosphere are, however, of interest as the source of radio waves and of 

 X rays emitted by the sun. Although these emissions are probably too 

 weak to be of biological importance, they deserve a brief description. 



Radio emissions originating outside the earth, presumably from inter- 

 stellar space, were discovered in 1932 by Jansky (1933) in experiments 

 with 30-meter waves, but emissions from the sun were not known until 

 1945 when the improved sensitivity of receiving techniques brought 

 about their detection (Hagen, 1951). It was found that the solar radio 

 waves originate in the upper chromosphere and corona in thick regions 

 of the solar atmosphere which center at heights above the photosphere of 

 approximately 8000, 10,000, 13,000, and 18,000 km for wave lengths 0.8, 

 3, 10, and 50 cm, respectivel3^ For example, during optical totality of 

 the solar eclipse of May 20, 1946, the solar radiations in the respecti^'e 

 wave lengths were reduced to 1, 6, 19, and 33 per cent of their values for 

 the uneclipsed sun. To account for the observed radio-wave energy the 

 temperatures of the regions which were emitting the wave lengths 0.8, 3, 

 10, and 50 cm were calculated to be 7000, 10,000, 2(),000, and212,000°K. 



In contrast with the limb darkening of the sun for visible light, when 

 observed with radio waves, the sun brightens at the limb, the l)rightening 

 increasing with the wave length. The solar radio emission is not constant 



