ANALYSIS OF THE IONOSPHERE 



m 



In all of these photographs the curves are repeated several times 

 along the vertical direction. This signifies echoes which have traveled 

 four, six, eight or more times between ground and ionosphere, being 

 reflected by both. Figure 8 shows a wonderful multitude of such 

 echoes. 



The curves of Fig. 9 are sketches generalized from many data. 

 Contrasting those on the left with those on the right, we see the 



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JUNE 1937 



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 FREQUENCY IN MEGACYCLES PER SECOND 



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Fig. 9 — Dependence of (/t', /) curve on season and on the sunspot cycle (Smith, 

 Gilliland and Kirby: National Bureau of Standards). 



crinkle prominent in the ones, missing or feeble in the others. This 

 is the difference between summer and winter. All of the data were 

 taken near noon, but though the District of Columbia is not exactly 

 in polar latitudes, the sun in December does not rise far enough in 

 Washington's sky to bring out that feature of the ionosphere of which 

 the crinkle is the sign. Running the eye along either column, one 



