162 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



in the Kennelly layer. When we observe field strengths at long dis- 

 tances, we are in a way tracing the integration of the effect throughout 

 the whole transmission path. 



Another way in which we gain important information as to the sun's 

 effect upon the upper atmosphere is by making radio soundings from 

 day to day. This method which has been in use for some years at 

 the National Bureau of Standards, at the Department of Terrestrial 

 Magnetism of the Carnegie Institution in Washington and elsewhere, 

 consists of sending up a radio pulse, letting it bounce off the reflecting 

 layer, come back nearly vertically into a sensitive receiver, and 

 measuring the time elapsed while the wave was traveling this path 

 to the ionosphere and back. Assuming that the radio wave travels 

 with the velocity of light, one can calculate from the elapsed time the 

 height to which the pulse ascended before it was turned back by the 

 ionosphere. The method, you see, is very similar to that used at sea 

 when a sound wave is sent from an oscillator in the ship's hull to the 

 ocean bottom and is received back at the microphones of the ship's 

 navigating equipment. Knowing the speed with which|sound travels 

 through water, the ship's captain can thus determine the ocean depth. 

 Soundings made in the ionosphere, therefore, reveal different con- 

 ditions at various times, showing marked changes in the ionic density 

 dependent upon the hour of the day and the season of the year. This 

 method of radio soundings is sometimes modified by changing the 

 frequency at which the radio pulse is emitted. If the frequency is 

 sufficiently increased, these more penetrating waves may pass com- 

 pletely through the ionized layer and not return. When such a 

 frequency is attained, it is known as the critical frequency. Its value 

 is an important index for studying cosmic effects. 



During the last few years of sunspot activity, there have been 

 occasions when remarkable fadeouts have occurred in radio com- 

 munication. In several of these instances extraordinary explosions 

 have occurred on the sun simultaneously with the interruption of all 

 radio communication in general. A record has been kept by Dr. 

 Dellinger, at the National Bureau of Standards, of such radio fadeouts 

 as are reported. In many of these instances observations at the 

 Mount Wilson Observatory have shown violent solar eruptions taking 

 place simultaneously. It would appear that the intense ionizing radia- 

 tion from the region of the sun where these eruptions occur reaches 

 the earth with the velocity of light and of sufficient intensity to 

 disturb immediately the ionized layer, confusing the reflection of 

 radio waves, and thereby resulting in these fadeouts which sometimes 

 last for an hour or more. Records at magnetic observatories show 

 that during such instances characteristics of the earth's magnetic 

 field are likewise suddenly altered. 



