ELEMENTS OF THE SOLAR-TERR1 



Figure 1-4 — THE IONOSPHERE 



Geometric Altitude (in Kilometers) 



DAYTIME 

 (max.) 



10 

 OZONE LAYER 



10 2 10 3 10" 



Number of Electrons per cubic centimeter 



The unfiltered ultraviolet and X-rays of the sun ionize many molecules, producing the 

 ionosphere. The ionosphere has several layers, each characterized by a more or less 

 regular maximum in electron density. The difference between the day and night 

 profile is due to the availability of solar radiation. 



Our understanding of the forma- 

 tion and behavior of the ionosphere 

 is considerably more advanced than 

 in the areas discussed previously. 

 Major breakthroughs have been 

 made, particularly in the past two 

 decades, when direct probing through 

 rockets and satellites has been pos- 

 sible. Nevertheless, as is usually the 

 case, increasing knowledge has raised 

 new and previously unsuspected 

 questions, some of which have con- 

 siderable practical importance. 



The ionosphere is conventionally 

 divided into three fairly distinct re- 

 gions: 



1. The D region, lying between 

 about 60 and 95 kilometers al- 

 titude; 



2. The E region, extending from 

 95 to about 140 kilometers; and 



3. The F region, containing the 

 bulk of the ionization and ex- 

 tending upward from 140 kilo- 

 meters. 



The E and F regions are capable of 

 reflecting medium- and short-wave 

 radio waves and thus permit long- 

 distance communication. The D re- 

 gion plays an important role in 



propagating long waves, but it has 

 an undesirable effect on radio propa- 

 gation at the higher frequencies 

 through absorption of the radio-wave 

 energy. 



The F Region — In the case of the 

 F region, where the concentrations 

 of free electrons reach their peak, 

 ionization is now known to be created 

 by EUV radiation from the sun. The 

 contributions of the various portions 

 of the solar spectrum within this 

 band are quite well understood. The 

 principal unknowns arise basically 

 from the fact that the atmosphere 

 at these altitudes is so rarified that 

 collisions between electrons, ions, 

 and neutral particles are extremely 

 rare, so that an individual electron 

 has a very long lifetime and can move 

 considerable distances from the re- 

 gion in which it is formed. As a 

 result, the electron concentration at 

 any given time and place is strongly 

 influenced by motions, including 

 winds, atmospheric waves, and dif- 

 fusion. Many of the anomalies in 

 the behavior of the F region, which 

 have been recognized since the early 

 days of radio propagation, are almost 

 certainly based on motions of this 

 kind. 



Much of the current interest in the 

 F region is focused on the explanation 

 of these anomalies and the informa- 

 tion they can provide on the winds 

 of the outer atmosphere. One out- 

 standing anomaly is that the daytime 

 F region is usually denser in winter 

 than in summer, despite the decreased 

 sunlight available. Another is that 

 the F region tends to be maintained 

 throughout the long polar night when 

 the sun disappears completely for 

 long periods of time. This latter 

 phenomenon seems at least partly 

 due to bombardment of low-energy 

 particles from the outer magneto- 

 sphere, but movement of electrons 

 from lower latitudes probably also 

 plays a role. 



The most powerful tool to emerge 

 in recent years for studying the F 

 region is the so-called incoherent 



