ANALYSIS OF THE IONOSPHERE 



459 



i.e. millions of cycles per second) corresponding to wave-lengths of 

 300, 30 and 3 metres. Here already the reader meets the fact that 

 the height at which such a signal is reflected, or the question whether 

 it shall be reflected at all, depends on the frequency of the waves and 

 the density of the ions. For every frequency there is what I shall 

 call a "mirror-density": signals are reflected as soon as they reach 

 the lowest level in the ionosphere where that mirror-density is attained. 

 The higher the frequency, the higher the mirror-density. The formula 



will soon be derived and shown; but for the moment, let us inquire 

 how the altitude of either layer can be measured, in terms of the 

 simple model of Fig. 1. 



One way of measuring the altitude is very obvious. Suppose the 

 observer to go a known distance away from the aerial, and measure 

 there the angle which the reflected wave or "sky wave" makes with 

 the horizontal as it comes down to him. If this can be done, then 

 clearly he can get the altitude by the simplest trigonometry — the 

 altitude of the E-layer or the F-layer, according to the frequency 



