LONG-WAVE RADIO TRANSMISSION PHENOMENA 15 



2. The altitude of the sun as measured from the most easterly path 

 apex of an hypothetical three-reflection path varies from approximately 

 — 6° in summer to — 13° in winter at the instant of minimum field, 

 and computation shows that there is no point on the path at which the 

 altitude remains constant. 



3. Daily irregularities of considerable magnitude are often noted. 



Ongar-Houlton Path 



1. Multiple minima of irregular distribution seem characteristic 

 of this path. 



2. Averages of field strength as a function of the sun's altitude at 

 three path apices show separate minima when the sun is at about — 6° 

 during May and June. 



Nauen-Houlton, Table III 



1. The time of minimum field varies as much as 40 minutes within 

 a few days. 



2. An average of the March-April data indicates that the minimum 

 takes place when the sun's altitude is approximately — 5.6° at the first 

 of three path apices, or 37 minutes after sunset at the earth's surface 

 under this point. 



Conclusions 

 Based solely upon the rather meagre data gathered during this 

 study, and therefore subject to further confirmation before being 

 considered generally applicable to all long-wave transmission paths, 

 our conclusions may be recapitulated as follows: 



1. A characteristic cycle of events accompanies the cessation of the 

 sun's active rays, consisting of an initial increase in field during the 

 reduction of solar intensity, followed by a minimum received field 

 after the sun's rays are cut ofT at the earth's surface. This cycle, 

 occurring both at sunset and during total solar eclipses, is typical of 

 long-wave transmission in the 18-70-kc. band over paths in excess of a 

 few hundred kilometers. 



2. The time interval between sunset at any point on the transmission 

 path and the instant of minimum field has an annual and an apparently 

 fortuitous daily variation. As a result of these variations the minimum 

 does not occur when the sun is at a fixed angular altitude at any point 

 on the transmission path. 



3. The time interval between sunset at some fractional path point 

 and the instant of minimum field, and likewise the angular altitude of 

 the sun referred to the plane of the horizon at such a point, increases 

 with both the path length and the wave-length, and is maximum during 

 the winter months. 



