X-RAYS FROM THE SUN — FRIEDMAN 253 



especially at the shorter wavelengths. In the 2 to 8 A. band, the mini- 

 mmn-to-maximmn variation was a factor of several hundred; from 

 8 to 20 A., at least a factor of 45 ; in the 44 to 60 A. band, the variation 

 was approximately sevenfold. Assmning that the X-ray spectrmn 

 had a gray body distribution, it was not possible to fit the measure- 

 ments in these three wavelength intervals by a single temperature. 

 The longer wavelength emission could be adequately described by a 

 temperature between 0.5 and 1 X 10^ degrees K., but the shorter wave- 

 length range, below 20 A., required a temperature closer to 2X10^ 

 degrees K. At the higher temperature, the gray body emission needed 

 to supply the observed counting rate at 8 to 20 A. contained only 1 

 percent of the flux deduced for the 20 to 100 A. range from the longer 

 wavelength measurements. It was concluded, therefore, that the 

 shortest wavelength X-ray emission was associated with local, hotter 

 regions occupying no more than 1 percent of the volume of the corona, 

 in which the temperature was of the order of 2 million degrees K. 

 These hotter regions were presumably distributed within a corona 

 whose general temperature did not exceed 1 million degrees K. Fig- 

 ure 1 is a plot of the solar spectral energy distribution illustrating the 

 results of these measurements. The curve marked "A-16" is for 

 1953 and "A-43" for 1956. They represent the minimum and maxi- 

 mum fluxes observed during the past sunspot cycle. The shaded 

 region added to the A-16 curve is the increment of flux measured be- 

 low 20 A. and attributed to localized hot spots at 2 million degrees K. 



Practically all our knowledge of the ionosphere before direct rocket 

 measurements were available was based on radio soundings. A pulse 

 of radio waves entering a cloud of electrons is reflected when the 

 density of the electrons reaches a critical value proportional to the 

 square of the frequency. The time required for the pulse to travel to 

 the ionosphere and back to ground is a measure of the height of 

 the reflecting region. At certain critical frequencies there appear 

 abrupt discontinuities in reflection heights as though the electron 

 density were distributed in several well-defined layers. These layers 

 are named "E," "Fi," and "Fg." In the lowest region of the iono- 

 sphere, named "D," the electron density is too small to reflect mega- 

 cycle-per-second frequencies. The lower ionosphere normally acts 

 as an absorbing region for these short waves and a good reflector for 

 very long waves, such as the static generated by thunderstorms. 



The variation of intensity with altitude showed that solar X-rays 

 were absorbed in the E-region of the ionosphere between 100 and 

 140 km. Furthermore, the X-ray energy absorbed there appeared 

 adequate to account for a major portion of the ionization. A direct 

 check on the relationship between X-rays absorbed in the E-region 

 and the resulting electron density there can be obtained by comparing 



