Very Low Frequency Radio Wave Propagation at High Latitudes

Diurnal, seasonal and yearly variations of the normal 16 kc/s signal strength from the Rugby (GBR) transmitter (ϕ = 52·4° N; λ = 1·2° W) have been studied on the basis of data accumulated during approximately five years of continuous recording (September 1958 to May 1963) at Kiruna Geophysical Observatory (ϕ = 67·8° N; λ = 20·4° E). The principal experimental findings are as follows:

1. For the first two years of observation the average monthly signal strength was of much greater intensity during the day than at night. More than 90 per cent of all individual days during this period show this behaviour. Between August 1961 and December 1962 the average ratio between the nocturnal and daytime signal level was approximately one, but this ratio varied considerably from month to month. Since January 1963, the average monthly signal strength has shown a markedly higher signal level during the night-time than during daylight hours. This holds true for about 80 per cent of all individual days during the months of January to May, 1963.

2. The best propagation conditions for v.l.f. communications between Rugby and Kiruna have been found during the summer months, and the lowest signal strength has always been found during the winter months. But while the average summer-time signal strength was approximately twice as high as in winter for the years 1959 and 1960, the corresponding figure for 1962 is only about 1·2.

3. A maximum of the 16 kc/s signal strength was observed in 1960 about two years after the sunspot curve reached its maximum. The propagation conditions deteriorated substantially in 1962 and 1963, the average signal level being less than two-thirds of that for the years 1959 and 1960.

The observed day-night variation of the 16 kc/s signal strength is discussed in terms of the mode theory of propagation, in a first-order approximation. For the case when the night-time signal level is lower than the daytime level ( as for the years 1958 to 1961), an increase of apparent reflection heights from 70 km at day to 80-85 km at night can explain the observation. An increase in reflection heights of say 5-10 km both during night and day, may explain the observations obtained during the more quiet ionospheric conditions in 1961 to 1963; namely that the night-time signal strength was of greater intensity than that during the daytime. The reception of 16 kc/s Rugby transmissions at Uppsala (ϕ = 59·8° N; λ = 17·6° E) supports this conclusion. The marked decrease in ionospheric absorption for the periods when the night-time level of the 16 kc/s signal strength exceeded the daytime level, can be understood if the region, where the main part of the ionospheric absorption, as well as the reflection of very low frequency waves takes place, was some kilometres higher than during the remainder of the observation period. From these results it seems reasonable to conclude that the height of the D-region is lowest during years of high solar activity and highest during years of more quiet ionospheric conditions. The average increase in reflection heights from 1959 to 1963 may be of the order of 5-10 km.