On frequency variations of ionospherically propagated HF radio signals

A technique for measuring the frequency variations of ionospherically propagated radio waves by spectrum analysis of the received signal is described, together with some theory needed to interpret such measurements. Examples of the types of variations observed with both vertical and oblique propagation are presented. On “quiet” days in middle latitudes, frequency variations of the order of ± 0.2 c/s occur, having periods of the order of 10 min. During disturbed periods the variations may be as much as 2 to 3 c/s on 5 Mc/s with vertical propagation. The frequency stability is generally greater for oblique propagation than for vertical propagation; however, the received spectrum is broader for oblique propagation. The frequency stability of E echoes is, in general, much greater than that of F2 echoes. At night and during some geomagnetic storms, the discrete (line) spectra give way to diffuse spectra. During periods of flutter fading at low latitudes, the discrete spectra are transformed into continua with frequency spreads of up to ± 5 c/s about the mean value. Distinctive frequency deviations are caused by solar flares, geomagnetic disturbances, and traveling ionospheric disturbances. Of these, the solar-flare-induced frequency variations are the most striking. Deviations of up to 50 c/s have been observed on WWV −10 Mc/s received at Boulder during flares. However, frequency deviations associated with solar flares have a most probable duration of about 4 min, and the majority of them have magnitudes less than 1 c/s.