100 UADIATION Hlol-OGY 



in intcihsity but varies in an iin'{;iiiai- and unpredictable manner. At the 

 shortest wave length the intensity is most constant. The correlatioii 

 with sunspot area is poor at the shortest wave length and improves at a 

 wave length of about 10 cm. At longer wave lengths the fluctuations in 

 the intensity become more erratic and abrupt. \'ig(>i-()us investigation of 

 all these phenomena is in progress. 



X rays and short ultraviolet radiations from the sun which had never 

 been detected at sea level were measured by narrow-band photon counters 

 carried aloft in a rocket (Friedman et al., 1951) on Sept. 29, 1949. Ultra- 

 violet radiation in the wave-length band 1150-1350 A was observed 

 above 05 km altitude, and in the band 1425-1700 A above 100 km. 

 Solar X-ray emission was first recorded at 85 km with a counter sensi- 

 tive from to 10 A, which indicated, because of the known absorption 

 of the atmosphere, that the solar emission became undetectable below 

 1 A. The measured intensities recjuired effective temperatures of the 

 emitting regions, again probably the upper chromosphere and corona, of 

 4500°, 5000°, and 10«°K for the bands 1425-1700, 1150-1350, and 7-10 A, 

 respectively. It seems nearly certain that X rays longer and softer than 

 10 A are emitted by the sun, and further rocket experiments have been 

 planned to investigate the subject. 



SUNLIGHT ON TOP OF THE ATMOSPHERE 



Two methods have been used to determine the solar spectral energy on 

 top of the atmosphere: (1) by measuring the spectral intensity of the 

 sunlight reaching the surface of the earth and correcting for the trans- 

 mission of the terrestrial atmosphere, and (2) by sending apparatus on 

 rockets to altitudes above most of the atmosphere. Both methods are 

 difficult; the first has been in use for many years but can obviousl}^ give 

 information about only those wave lengths which are detected at the 

 earth's surface; the second is relatively recent and has yielded important 

 new results in the ultraviolet portion of the spectrum. In the first 

 method, unfocused radiation from the entire sun is allowed to fall on the 

 slit of a double monochromator, that is, two monochromators in series in 

 order to reduce contamination, by radiation scattered by the optical 

 parts of the instrument, of the spectrum falling on the exit slit. Back of 

 the exit slit, in a position to intercept the portion of the spectrum emerg- 

 ing from the monochromator is mounted a bolometer, a thermocouple, or 

 a calibrated photoelectric cell with which the intensity may be measured 

 point by point throughout the spectrum or in selected portions of it. 

 Glass or quartz prisms and lenses in the monochromator are used to 

 measure the major portion of sunlight reaching the surface of the earth, 

 which lies between 2900 A and 2.5 ju- Rock-salt pi-isms and diffraction 

 gratings are used to observe the infrared solar spectrum from 2.5 to 25 n. 

 This portion of the spectrum has been of little direct interest in bio- 



