HF radio wave absorption in the nighttime. Part I. Measurements of the field intensity of HF radio waves on board the ship for the 2nd(1958)-8th(1966) Japanese Antarctic Research Expedition

Abstract

Observations of the nighttime ionospheric absorption have been carried out since 1958 on board the ship for the Japanese antarctic research. Field intensities of the HF standard frequency signals at different frequencies were measured during hours around midnight. In this paper, techniques of the measurement for extracting a relatively small quantity of the nighttime absorption are described. Existence of the absorption is confirmed from the fact that the rate of decrease of field intensities against the ground range at a lower frequency is much larger than that at a higher frequency. Moreover, the dependence of absorption upon the solar activity is found out from the measurement over nine years in the declining epoch of the last solar cycle. A provisional expression for estimating the nighttime absorption is proposed.