162 



ANNUAL REPORT SMITHSONIAN INSTITUTION,, 1942 



We thus see that radio becomes a sensitive and extremely useful 

 tool in recording changes of degree of ionization of the upper atmos- 

 phere. When we observe radio field strengths at long distances we 

 are in a way tracing an integrated effect throughout the whole trans- 

 mission path for a given frequency. 



FiGURK 3. — Trend of radio field strengths of WBBM received at Boston compared 

 with sunspot activity 1936-1940. Top curve (&) represents sunspot numbers 

 over entire disk of sun; middle curve (c) represents variation in areas of sun- 

 spots of zones of solar latitude 0° to ±15° either side of the sun's equator; low- 

 est curve (a) represents variation in radio field strengths corrected for sea- 

 sonal and diurnal variations. The fact that the lowest curve parallels closely 

 sunspot activity in the 0° to ±15° zones suggests effect of solar disturbances 

 is greatest when spots are near solar equator. 



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 in sending up a radio pulse of known frequency 

 and recording its return from the reflecting layer. The time elapsed 

 while the wave was traveling this path to the ionosphere and back 

 is measured with high precision on an oscillograph. Assuming that 

 the radio wave travels with the velocity of light, one can calculate 



