are inverted V-like, V-like, X-like, fiber-like, and frindge-like types of SCs, with rare exceptions (i.e., unclassified events). Also, we established seasonal and solar cycle dependencies in the occurrence rate of SCs. In particular, most SCs appear during late fall, winter, and early spring [see details in @Koval2017].

It is suggested that ionospheric irregularities accounting for SCs are traveling ionospheric disturbances (TIDs) of medium scale (MSTIDs), however, the origin of SCs may still cause disputes among solar radio observers. TIDs are spatial quasi-periodic electron density structures produced by acoustic gravity waves (AGWs) [@Hines1960]. MSTIDs are characterized by spatial and temporal periods of 100-600 km and 0.25-1 h, respectively [@Hunsucker1982]. They can propagate as the guided wave mode in a mirror-like layer at heights 250-300 km above the ground [@Fedorenko2013]. Because of its nature the density perturbation consists of alternating structural cells with high and low magnitudes of electron concentration. Since a plasma depletion cell holds higher value of refractive index as compared to its adjoining cells, the cell can cause refraction and focusing of incident electromagnetic wave if plasma density gradients are large enough. As a result, the caustic surfaces produced by MSTIDs can appear in space, and transform into SCs 