238 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1955 



Since the end of the war the radiation from the sun on radio wave- 

 lengths has been extensively investigated. It proves to be complex in 

 nature. There is radiation from the normal quiet sun, and much more 

 intense radiation associated with various aspects of the active sun. 



The radiation from the normal quiet sun is much more intense than 

 can be attributed to a black body with a temperature of 6,000° K. The 

 intensity of the radiation on wavelengths from one centimeter to 

 several meters has been measured. It is usual to express the intensity 

 at any wavelength in terms of the effective temperature of a black 

 body that would emit radiation of the observed intensity at that wave- 

 length. Expressed in this way it is found that the effective tempera- 

 ture required to generate the centimeter waves is about 10,000° K., 

 and that with increase in wavelength the effective temperature rapidly 

 increases, so that a temperature exceeding 1,000,000° K. is required 

 to account for the observed intensity at wavelengths of several meters. 



Wlien a large smispot appears on the disk of the sun the radiation 

 in radio wavelengths is much enhanced; its greatest intensity occurs 

 at about the time of central meridian transit of the spot. The radia- 

 tion is found to be circularly polarized, which suggests that it is con- 

 nected in some way with the magnetic field associated with the spot. 

 Whenever a flare occurs an outburst of radiation on radio wavelengths 

 is produced, which is of a temporary character, lasting for several 

 minutes only. Such outbursts do not occur simultaneously with the 

 peak intensity of the flare but follow it with a lag of several minutes. 

 This lag is not the same at all wavelengths or frequencies but is greater 

 on the lower frequencies. The outburst occurs first on the short waves 

 of high frequency and then on the longer waves, with lags increasing 

 progressively with the wavelength. This suggests that there is an 

 effect that travels outward from the sun. 



The key to the solution of many of the perplexing problems which 

 the sun presents will ultimately be provided by the corona, the faint 

 tenuous appendage to the sun which extends outward from the sun for 

 more than two million miles. Until recent years the corona, because 

 of its f aintness, could be observed only on the rare occasions of a total 

 eclipse of the sun. But Lyot has shown that by making observations 

 under favorable conditions at a high altitude, when the scattering of 

 sunlight by the atmosphere of the earth is much reduced, and by 

 careful design and construction of apparatus, so as to eliminate instru- 

 mental scattering, it is possible to see the corona by producing an 

 artificial eclipse. The shape and structure of the corona change 

 through the sunspot cycle. At sunspot minimum, there are long equa- 

 torial streams and, in the polar regions, short plumes or tufts; at 

 sunspot maximum the corona shows a more uniform distribution. 



