506 



BELL SYSTEM TECHNICAL JOURNAL 



always quite obscure, which may account for its escaping observation 

 in previous work. It appeared to consist of a band of frequencies, 

 the midpoint of which could usually be determined with an accuracy 

 of approximately ± 50 c.p.s. The resonance was usually observed 

 during the evening and morning twilight periods when the damping 

 of tweeks was high, and appeared to be closely connected with the 

 tweeks themselves, although ordinarily showing a somewhat higher 

 frequency. During the hours of total darkness the resonance was 

 either absent or obscured by tweeks. At evening, resonance some- 

 times appeared at sunset or a short tim.e before. Usually the first 

 highly damped tweeks were observed at about the same time. In the 

 early morning the resonance was observed sometimes several minutes 

 after the last tweek. 



2400 



2200 



2000 



1800 



1600 



7 8 

 RM. 



9 10 11 12 1 2 3 

 EASTERN STANDARD TIME 



4 5 

 A.M. 



Fig. 5 — T, T, lower limiting tweek frequencies. 



R. R, evening and morning resonance frequencies. 



Fig. 5 shows frequencies of the resonance tone and the lower limiting 

 frequencies of individual tweeks as determined by aural observations 

 made in the latter part of August, 1932. The tones began with fre- 

 quencies well above 2000 c.p.s. and decreased to approximately 1650 

 c.p.s. in a period of 2| hours. The resonance disappeared as the 

 tweeks approached the usual night intensity. Approximately 1/2 

 hour before sunrise the resonance reappeared and a rapid frequency 

 increase began. The last definite tweek observed in the morning was 

 still under 2000 c.p.s., although the resonance rose well above this 

 frequency before disappearing. In approximate figures, the effective 

 reflecting surface for audio frequencies is indicated by the data of 

 Fig. 5 to be located at an altitude of 61 km. at sunset and to rise to 

 88.5 km. in a period of 2\ hours. Half an hour before sunrise the 

 indicated altitude is 87 km. and at 15 minutes after sunrise it has 

 returned to 61 km. 



