98 UADIATION lUOLOCiY 



of liij^h phototiiiapliic contrast. Duiiiiji a tcjtal oclipso of tlic .sun when 

 the moon ohscurots the pliotosphcrc, llie reversing layer and regions 

 al)ove it are .seen to emit l)riliiant spectral lines, the so-called "flash 

 spectrum." Although the dark lines reduce the intensity of higher tem- 

 perature radiation from the photosphere, they are (^nly relatively dark 

 and radiate toward the earth with inttMisities appropriate to a lower tem- 

 perature source at about 4800° K. 



The Fraunhofer absorption is stronger in the ultraviolet than in the 

 visible solar spectrum. Pettit (1940) mapped the spectral energy 

 between the Fraunhofer lines, using a high-dispersion spectrograph and 



2900 2800 2700 2600 2500 2400 2300 2200 2100 



WAVE LENGTH, A 

 Fig. 3-2. Ultraviolet solar spectrum obtained during a rocket flight of June 14, 1949, 

 by Johnson, Purcell, and Tousey (1952). 



sensitive photocells, and, from his measurements, estimated the attenua- 

 tion of radiation from the photosphere by the overlying layers of the sun. 

 Comparison of his results for integrated radiation from the sun and for 

 radiation between the Fraunhofer lines indicates that the total intensity 

 between 3200 and 4000 A is about 70 per cent of the radiation which 

 would be emitted by the unobscured photosphere, whereas between 4000 

 and 7000 A the ratio is about 91 per cent. 



The chromosphere is a region consisting principally of hydrogen, helium, 

 and calcium, located immediately above the reversing layer; it extends 

 from the top of the reversing layer at about 1500 km above the photo- 

 sphere to a height of 12,000 km which is the greatest height at which H„, 

 the first Balmer line (6563 A), occurs. Other lines of the Balmer series 

 of hydrogen and line spectra of other un-ionized atoms fade out at lower 

 heights. The fla.sh spectrum of the chromosphere as seen during a solar 

 eclip.se contains emission lines of helium and ionized helium together with 

 lines of ionized metals. Menzel points out the existence of ionized helium 

 in the chromosphere and Hoyle gives results obtained from studies of line 

 widths in the flash spectrum as evidence that the temperatures in this 

 region are 20,000-30, 000°K. Prominences originating in the chromo- 

 sphere and sometimes rising several hundred thousand miles also contain 

 matter at these temperatures. The chromosphere also emits continuous 

 radiation, but none of the.se emissions — the line spectra, the spectra of 



