AUDIO FREQUENCY ATMOSPHERICS 501 



frequencies, when observed on a submarine cable, are of comparatively 

 high amplitude, appearing as a deep rumble intermittently broken by 

 noises variously described as splashes and surges. The range from 

 500 to 1500 c.p.s. generally consists largely of clicks and crackling 

 sounds which accompany the low-frequency surges. At times sub- 

 stantial amplitude increases occur accompanying quasi-musical sounds, 

 which may dominate this frequency range. In the upper voice range 

 intermittent hissing or frying sounds are observed, often accompanying 

 surges in the low-frequency range. Above 1800 c.p.s. occur at least 

 two ranges which at times possess slight tonal characters. In addition 

 to the slightly musical sounds, two varieties of distinct musical 

 atmospherics have been observed and given the onomatopoeic names 

 "swish" and "tweek." Particular interest attaches to these because 

 of their extraordinary character. 



Diurnal and Seasonal Characteristics 



The daytime non-musical atmospherics consist ordinarily of inter- 

 mittent low-amplitude impulses. As a general rule the night-time 

 intensities are considerably higher; the impulses being more frequent 

 and more prominent than during the daylight hours. The night 

 intensity is further increased by the presence of the type of musical 

 atmospheric known as tweek. 



During a usual day, the intensity of audio-frequency atmospherics 

 from sunrise until mid-afternoon is comparatively low. During the 

 afternoon, a slow rise may or may not occur. Shortly following 

 sunset, a gradual increase of intensity is usual. This rise continues 

 for two hours or more after which a high level is maintained rather 

 consistently until shortly before daybreak. A brief increase some- 

 times occurs at this time followed by a steady decrease, the daily 

 minimum being reached usually shortly after sunrise. 



Fig. 2 shows examples of summer and winter audio-frequency 

 atmospheric intensities over 24-hour periods. While these curves 

 show the usual characteristics, extraordinary conditions may result in 

 wide variations. The occurrence of local electrical storms or intense 

 disturbances of the earth's magnetic field usually contribute markedly 

 to these anomalies. 



The diurnal amplitude variations of certain types of atmospherics 

 may be reasonably explained by assuming the continued presence of 

 an audio-frequency reflecting layer in the upper atmosphere, and 

 assuming a low lying ionized attenuating region ^ to be present during 



^ Such a region affecting radio frequencies is described by R. A. Heising, Proc. 

 I. R. E., 16, p. 75, January, 1928. 



