STATISTIC OF SEASONAL AND DIURNAL VARIATIONS OF DOPPLER FREQUENCY SHIFT OF HF SIGNALS AT MID-LATITUDE RADIO PATH

Purpose: An experimental study of signatures of traveling ionospheric disturbances (TID) observed in diurnal-seasonal variations of the parameters of probe HF signals propagating on theoblique single-hop RWM–LFO radio path, derived from the yearlong monitoring data. Design/methodology/approach: A long term digital recording of the HF radio signals’ waveforms of the Exact time and frequency service station (RWM, Moscow, Russia) was made at the Low Frequency Observatory of the IRA NAS–Ukraine (LFO, Martove, Kharkіv reg., Ukraine). The Doppler frequency shift (DFS) was derived from the power spectra of the recorded signals. The DFS quasiperiodic variations were interpreted as the result of passage of traveling ionospheric disturbances associated with the acoustic-gravity waves (AGW) at the height of the F-layer of ionosphere. The value of the DFS variation period was determined as the sum of the time intervals between neighboring zero crossing of two consecutive half-periods, and the amplitude was determined as the range of variations. The cases of F region shielding by the underlying ionospheric layers Es and E were taken into account as well. Findings: The data on the periods and amplitudes of the DFS variations were used for statistical analysis. The probability of DFS variations’ observation was determined for each month. This value lies within 81 to 91 % in winter and spring and decreases to within 52 to 80 % in summer and autumn seasons. It is shown that the rise of electron density in the lower layers of the ionosphere Es and E makes it difficult to detect TIDs in the F region. This results in a significant underestimation of the probability of observation in the summer and partially in the spring-autumn seasons. The diurnal-seasonal dependences of the probability of DFS observation, as well as their periods and amplitudes were determined. The forms of daily distributions of both amplitude and period are generally similar for all the seasons. They show two peaks, one in the morning and the second one in the evening, and the minimum in the afternoon. As respects the seasonal distributions of periods and amplitudes, in summer, a higher median value of period and more even distribution of amplitude are observed. In addition, we evaluated the influence of the level of geomagnetic storminess on the characteristics of DFS variations. It was determined that a rise of geomagnetic activity (K-index ≥2) is accompanied by decreasing of the observation probability and increasing of the amplitudes and periods of DFS variations. Conclusions: The techniques developed for the analysis of the data of Doppler ionospheric sounding by non-special type HF signals can be used for diagnostics and analysis of the ionospheric disturbances.