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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1961 



100 

 ANGSTROMS 



Figure 1. — Solar spectral energy distribution below 2,000 A. Solid lines represent rocket 

 measurements. Dashed curves labeled with temperatures represent black-body dis- 

 tributions. Dot-dash curve is Elwert's (1960) theoretical continuum. 



X-ray fluxes with the observed variation in critical frequency of 

 the E-region. According to the theory of the formation of an iono- 

 spheric layer, the critical frequency is proportional to the fourth root 

 of the intensity of ionizing radiation. The observed sevenfold varia- 

 tion in total flux from minimum to maximum would therefore produce 

 a factor of 7 ^/*, or 1.6 in the critical frequency for E-region if X-rays 

 were the sole source. Using values of the critical frequency corre- 

 sponding to the times of rocket flights, the variations were from about 

 2.7 mc./sec. to 4.1 mc./sec., a factor of 1.5, in good agreement with 

 the variation expected from the X-ray observations. Actually, there 

 is an important contribution to the ionization of E-region by solar 

 ultraviolet radiation, and it is theoretically difficult to evaluate the 

 relative importance of X-ray and ultraviolet contributions. How- 

 ever, an analysis based on the best information available at the pres- 

 ent time indicates that the X-ray influence is predominant. 



