PREDOMINANT TRAVELING IONOSPHERIC DISTURBANCES OVER EASTERN EUROPE DURING LOW LEVELS OF SOLAR AND GEOMAGNETIC ACTIVITIES USING INCOHERENT SCATTER RADAR DATA

Purpose: Detection of wave processes of various temporal and spatial scales in the mid-latitude ionosphere over the Eastern Europe near the characteristic geophysical time periods (equinoxes and solstices) during magnetically quiet and weakly disturbed conditions at low solar activity; estimation and calculation of traveling ionospheric disturbances (TIDs) characteristics based on the analysis of variations in the incoherent scatter radar signal power corresponding to electron density disturbances; analysis of TIDs generation sources. Design/methodology/approach: The time dependences of the incoherent scattering signal power were processed, and further bandpass filtering of data in various ranges of periods dominant modes was made. To remove slow signal variations (trend) and fast oscillations which may be caused by noise, the initial time series were passed through a digital filter with the wide bandwidth of 5–125 min. The spectral analysis was further made to localize the predominant oscillations on the time and period axes. Further, this range was divided into three subranges: 15–30, 30–60 and 60–120 min. For each of these subranges, the dominant TIDs were determined and their characteristics were estimated. Vertical components of phase velocity and disturbance wavelength were determined by the cross-correlation analysis, and their horizontal components were evaluated using the dispersion equation for acoustic-gravity waves (AGWs). Findings: Large and medium-scale TIDs were identified at altitudes from 100 to 400 km. The spectral analysis showed that for all the seasons the predominant quasi-periodic disturbances with periods within 60 to 120 min had the highest energy. The TIDs with periods within 15 to 120 min lasted from 2 to 10 h. We identified 59 TIDs in total. Most of them (49 events) were most likely associated with the AGW propagating upward (their sources were located at lower heights). The average values of large-scale perturbations in the subranges of 30-60 min (the average oscillation period being 45 min) and of 60-120 min (82 min): the maximum relative amplitude of variations – 0.14 and 0.20, respectively; the vertical phase velocity – 105 and 56 m/s; the horizontal phase velocity – 495 and 473 m/s; the vertical wavelength – 285 and 282 km; the horizontal wavelength – 1358 and 2322 km. The average values of these parameters for the medium-scale AGWs/TIDs in the subranges of 15–30 min (the average period being 22 min) and of 30–60 min (41 min) were respectively 0.13 and 0.13; 127 and 64 m/s; 289 and 268 m/s; 166 and 157 km; 403 and 658 km. It has been demonstrated that the largest number of TIDs is observed near the winter solstices and autumn equinoxes. Conclusions: As a result of a long-term systematic monitoring of the ionosphere state with the Kharkiv incoherent scatter radar, the characteristics of TIDs being observed in the periods close to equinoxes and solstices at low levels of solar and geomagnetic activities have been determined. The presence of largescale TIDs even under magnetically quiet conditions is proved. The plausible sources of the detected TIDs are discussed. The results obtained will improve the knowledge of the mid-latitude TIDs characteristics, as well as contribute to improvement of the global and regional models of the ionosphere.