Analysis of the Ionosphere * 



By KARL K. DARROW 



The ionosphere is a region in the very high atmosphere from 

 which radio signals are reflected, a fact which is adequately ex- 

 plained by assuming that region to be populated with free electrons. 

 In exploring the ionosphere, signals of a wide range of frequencies 

 are successively sent upward, and the time elapsing before the re- 

 turn of the echo is measured. The delay of the echo mulitplied by 

 ^c is called the virtual height of the ceiling for the signal. The data 

 appear in the form of curves relating virtual height of ceiling to 

 frequency of signal. These curves are peculiar in shape and vary 

 remarkably with time of day, time of year and epoch of the solar 

 cycle. By theory they can be translated into curves relating elec- 

 tron-density to true height above ground. The theory is approxi- 

 mative, but the results are accurate enough to be of value. The 

 magnetic field of the earth affects the data remarkably, making it 

 possible to test the theory and to evaluate the field-strength at 

 great heights. The free electrons are supposed to be liberated from 

 the air-molecules by ionizing agents, of which the chief but not the 

 only one is ultra-violet light from the sun. 



THE very title of this article embodies the assumption that in 

 the upper reaches of the atmosphere there is a host of ions. 

 By "upper reaches" here is meant, a region of the atmosphere so high 

 that no man ever entered it, nor even a balloon with instruments. 

 The ions therefore have never been observed by normal electrical 

 means. They are postulated as the explanation of two things mainly: 

 the echoing of radio signals from the sky, and that small portion of 

 the earth's magnetic field which fluctuates with time. 



The idea that these things require explanation, and the idea of the 

 sort of postulate that is required to explain them, can both be followed 

 back for many years. What was lacking in the early days was the 

 notion of mobile electrified particles, that is to say, of "ions," in the 

 air. That notion did not even exist, when in the eighties Balfour 

 Stewart desired to imagine a conducting layer in the upper air for 

 explaining magnetic fluctuations. It was only just being formed, 

 when in 1902 Kennelly and Heaviside independently desired to imagine 

 a conducting layer in the upper air for explaining why wireless signals 

 can travel around the world. To speak of a conducting layer in the 



* This paper, in abbreviated form, appears in the current issue of Electrical En- 

 gineering. 



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