504 BELL SYSTEM TECHNICAL JOURNAL 



frequency, as determined from the oscillograms, is taken as the 

 reciprocal of the time spacing of successive impulses. Due to the 

 difficulty in accurately measuring these short time intervals, especially 

 in the presence of other forms of atmospherics, there is a possibility 

 of error which might account for the irregularities in the location of 

 points. However, irregularities in effective height of the reflecting 

 layer might be expected to produce a like result. 



With one possible exception, ^ tweeks have never been observed by 

 us during daytime except near sunrise and sunset. In the usual case, 

 the intensity of static impulses increases during the early evening with 

 no indication of tonal quality. At twilight certain of the impulses 

 are observed to be accompanied by a slight indication of a highly 

 damped frequency. Shortly thereafter the characteristic tweek tone 

 appears, often trailing a good share of the static impulses. Both 

 tweek rate and intensity ordinarily increase for some two hours. For 

 the remaining hours of darkness the tweeks, usually of low damping, 

 continue with many irregular variations in intensity. Just previous 

 to the approach of daylight a brief increase in tweek rate often occurs 

 followed by a rapid reduction in both intensity and rate of occurrence. 

 The last highly damped tweek is usually observed several minutes 

 before sunrise. 



H. Barkhausen ^ in attempting to explain the type of atmospheric 

 tone known as the "swish" or the "long whistler" considers the 

 multiple reflection of an impulse. While our observations indicate 

 this theory to fail in explanation of the swish, it appears to be applicable 

 to tweeks. According to this theory a tweek may be produced by 

 energy, from a source of momentary static disturbance, arriving at a 

 receiving point as a series of impulses. The first impulse arrives by 

 direct transmission. Shortly thereafter a second impulse arrives 

 after having suffered one reflection at an ionized layer in the upper 

 atmosphere. The third impulse arrives after two reflections from the 

 ionized layer and one from the earth's surface. Other impulses follow 

 in like manner. In case the origin of the disturbance is not near the 

 observation point, the time spacing of the observ^ed impulses results 

 in a reducing frequency, initially varying rapidly and finally approach- 

 ing an asymptotic value. The initial frequency is dependent upon 

 the distance from source to observer and the reflecting layer height, 

 while the lowest frequency depends upon the height alone. The 

 failure of tweeks to appear in daytime may be attributed to damping 

 by sunlight ionization at low altitudes. Occasional highly damped 



* E. T. Burton and E. M. Boardnian, " Effects of Solar Eclipse on Audio Frequency 

 Atmospherics," Nature, 131, p. 81, January 21, 1933. 



» H. Barkhausen, Proc. I. R. E., 18, p. 1155, July, 1930. 



