CHAPTER 21 - VERY LOW FREQUENCY RADIO WAVE PROPAGATION AT HIGH LATITUDES

Abstract

Diurnal, seasonal and yearly variations of the normal 16 kc/s signal strength from the Rugby (GBR) transmitter (ø= 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 (ø=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.