SUN AND THE EARTH'S MAGNETIC FIELD — FLEMING 195 



waves. Study of these records soon confirmed the fact that, instead 

 of only one "Kennelly-Heaviside" layer, there are several layers or 

 stratifications of ionization. 



Present convention has adopted the term "ionosphere" to include all 

 these regions. Low-frequency radio waves penetrate to relatively low 

 heights (possibly 60 miles above the earth). These low frequencies— 

 or the long waves — include the radio spectrum used by our local 

 broadcasting stations. The high frequencies — or the short waves — 

 are used for international broadcasting over great distances. Higher 

 frequency radio waves are more penetrating and seek greater densities 

 of electrical charges before being reflected. These may penetrate to 

 200 or 300 miles above the earth. Radio waves of still higher fre- 

 quencies do not encounter enougli concentration of electrical charges 

 in the ionosphere and they may penetrate completely through and be 

 lost in space. 



These penetration frequencies measure the apparent height from 

 which echoes are received. There are three important ionized layers 

 in the outer atmosphere under direct influence of the sun. One is at a 

 level of about 100 kilometers, another at about 225 kilometers, and the 

 third at some 350 kilometers. The two upper layers do not exist 

 separately everywhere, but merge when the sun is low and form a 

 single layer in the night. At noon the highest layer reaches maximum 

 height directly under the sun. The heights of the layers are not con- 

 stant but change with time of day and season, with latitude, and even 

 with time along the sunspot cycle. 



Radio exploration of the ionosphere is accomplished by automati- 

 cally shifting the wave frequency over a range about 500 to 16,000 kilo- 

 cycles per second. An automatically recorded reference line for the 

 surface of the earth provides the base from which to measure heights 

 of radio echoes. At wave frequencies up to 4,000 kilocycles per second, 

 radio echoes are returned from heights at about 100 kilometers. Fre- 

 quencies above 4,000 kilocycles show a sudden jump to a higher re- 

 flecting region. At greater frequencies the waves penetrate, in gen- 

 eral, farther into the rarefied atmosphere until at 12,000 kilocycles per 

 second there are no reflections from the ionosphere and the waves pass 

 out into space. 



Continuous operation of ionospheric apparatus reveals that there 

 are striking differences between normal day and night conditions. 

 In general, the number of electrical charges in the ionosphere builds up 

 during the day under the influence of the sun's rays. During the 

 night, when the sun's rays are cut off, the ionization decreases. There 

 are also extensive changes in the ionosphere, from one season to an- 

 other. Complete knowledge of the variations and characteristics of 



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