Long-term variation of ionospheric reflection heights for v.l.f. radio waves

Abstract

The long-term variation of apparent reflection heights for very low frequency radio wave propagation at high latitude is discussed. The experimental findings used for this purpose are mainly the observed monthly day-night variation of the 16 kc/s signal strength from the Rugby (GBR) transmitter ( φ = 52·4° N; λ = 1·2° W) based on data accumulated during approximately five years of continuous recording at Kiruna Geophysical Observatory ( φ = 67·8° N; λ = 20·4° E). This day-night variation of the 16 kc/s communication condition between January 1959 to July 1963 can be summarized as follows: For the first two and a half years of observation the average monthly signal strength was much greater during the day than at night. More than 90% 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 the daylight hours. This holds true for about 80% of all individual days during the months of January to May, 1963. When these observations are discussed in terms of the mode theory of propagation in a first-order approximation, the following conclusions may be drawn: For the case when the night-time signal level is lower than the daytime level (as for the years 1958 to 1961), an apparent reflection height of about 70 km at day and 80–85 km at night can explain the observation. An increase in reflection heights of say 5 to 10 km both during night and day, may explain the observations obtained during the years of more quiet ionospheric conditions (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 kilometers higher than during the remainder of the observation period. From these results the following conclusion can be drawn: 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 to 10 km. Also the seasonal and yearly variation of the 16 kc/s communication conditions between Rugby and Kiruna has been investigated, and the experimental results are as follows: 1. (1) The best conditions for v.l.f. communication 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. 2. (2) A maximum of the 16 kc/s signal strength was observed in 1960 about two years after the sunspot curve reached its maximum. The conditions deteriorated substantially in 1962 and 1963, the average signal level being less than 2 3 of that for the years 1959 and 1960.