Observations of traveling ionospheric disturbances on the basis of vertical and near-vertical sounding data

We have conducted a morphological analysis of vertical and near - vertical sounding ionograms. The reported experimental results were produced by vertical incidence ionosonde located in Tory settlement (52°53 N, 103° 16 E) and by near-vertical ionospheric sounding from the town of Usolie (51°48 N, 103°50 E) to Tory (ground range of 120km). The observations were carried out from 2011 till 2016 years continuously. Apart from themain trace, some ionograms feature an additional trace (called “cusp”). Cusp-shaped traces on ionograms have been associated with off-angle reflections from tilted surfaces of constant ionization density caused by the passage of traveling ionospheric disturbances (TID). Simulation revealed typical scales and amplitudes of TIDs that had caused cusps. Conventional cusps (so-called U-shaped cusps) have been considered to reveal their occurrence rate, average duration dependent on the time of the day. The ionosonde records were taken once each minute over a period of six years, which allowed us to trace the dynamics and evolution of cusps as well as seasonal characteristics of TID occurrence. Normally, cusps in the ionograms were found to move from the region of high frequencies and large group delays toward the region of lower frequencies and smaller group delays (downward and from right to left along the main trace of the ionogram). There were also cases when cusps moved in the opposite direction or changed their direction, but such cases were quite rare. We also studied dependence of the average cusp occurrence and duration of cusps onthe time of the day, seasons of year and geomagnetic activity.