6 BELL SYSTEM TECHNICAL JOURNAL 



Observations on GBR at 16 kilocycles at Houlton during the latter 

 part of April, 1934, showed little evidence of a minimum. These 

 measurements were made on telegraph traffic and if a minimum 

 occurred its amplitude must have been less than the observational 

 errors of this method of measurement. These results seem to be 

 confirmed by the observations of Yokoyama and Tanimura * which 

 show a pronounced decrease in the amplitude of the sunset cycle at 

 frequencies below 17 kilocycles, while frequencies slightly above this 

 value display the characteristic effects. This apparent difference in 

 the behavior of 18 and 16-kc. transmission, seems rather unusual and 

 if real may have some geophysical significance. 



Results 



An examination of the data taken on the Rocky Point-Houlton 

 60-kc. transmission path as plotted on Figs. 2 and 3 discloses that of 

 the ten cases plotted, nine show that a minimum in measured field 

 occurs 22-26 minutes or an average of 23 minutes after surface sunset 

 at Rocky Point, corresponding to an altitude of the sun of 4 to 5^ 

 degrees below the horizon, and in a single case at 18 minutes after 

 sunset on August 29, 1934. The results in the winter months seem less 

 regular than in summer and the cases of 2/25/34 and 1/21/34 are 

 especially noteworthy in that they show multiple minima, the 23- 

 minute minimum being subsidiary to a minimum occurring about an 

 hour after Rocky Point sunset on 1/21/34. The fact that minima 

 occur at a nearly constant interval after Rocky Point surface sunset, 

 which occurs at the same time that the sun's rays are tangent to con- 

 centric elevated layers above this point, rather than sunset at some 

 other path point, is believed to be the result of a fortuitous combination 

 of circumstances rather than a rational relationship between these 

 times. This hypothesis is strengthened by data shown in Table I 

 taken over the same path at 18 kilocycles and data in Table II taken 

 at 18 kilocycles over the 900-km. Tuckerton-Houlton path which do 

 not show this same constant relationship with the time of sunset at 

 the transmitting station. Of three observations obtained over the 

 Rocky Point-Houlton 18-kc. transmission path the minimum in field 

 occurred 34 minutes after sunset at Rocky Point in two cases but on 

 the other day it occurred 22 minutes after sunset at Rocky Point. 

 On all three occasions, however, the minimum was between 35 and 37 

 minutes (— 6° to — 6.5° altitude of sun) after surface sunset at the 

 mid-path point. As in the case of the 60-kc. path for which data are 

 shown on Figs. 2 and 3, the field began to fall at mid-path surface 

 sunset. 



