On the strong influence of the F1 layer on medium to high latitude HF propagation

Abstract

During the recent sunspot minimum of the F1 layer critical frequency was close to that of the F2 in the medium to high latitudes. This considerably modified HF propagation conditions over long distance paths. At times the F1 layer appeared to block off the propagation of signals in the very oblique F2 modes. High order F2 mode signals were frequently strong and were reflected from large virtual heights. This retardation was produced by the dense, thick F1 layer which existed in the medium to high latitudes during the summer of 1964. The maximum radio frequency for the path was frequently determined by the F1 layer; although this effect has been noted by various workers, it has not been taken into account for communications predictions. One particularly strong and highly delayed type of signal is not readily explainable as a higher order F2 mode because it occurred singly. If it were an ordinary F2 mode trace, its occurrence would lead to the conclusion that the only F2 layer signal propagating was a third or fourth order mode signal on paths of 3500 to 5000 km, and such a conclusion is hard to justify. As an alternative, an F1-F2 ducted mode hypothesis is presented.