on the solar dynamic spectra. The SCs modeling on the basis of geometric optics has been performed in our latest study [@Koval2018].

In the study we have simulated the propagation of a plane electromagnetic wave through the perturbed ionosphere with MSTIDs which was approximated by the electron density model given by @Hooke1968, using a two-dimensional ray-tracing method. The method can reveal the trajectories of radio rays through the ionosphere irregularities, and thus the formation of caustics in space under different conditions. We could simulate four types of SCs among the five ones from our classification. This covers the majority of the observed SC morphologies. Also, we gave an explanation of the seasonal behavior in the SC emergence. According to our simulations SCs are generated mainly at relatively low solar elevation angles ($<30^\circ$). This corresponds to late fall, winter, and early spring in the Northern Hemisphere, in line with our earlier statistical study.

Despite these latest studies and earlier ones [@Meyer-Vernet1980; @Meyer-Vernet1981; @Koval2017; @Koval2018], a major challenge remains, which is to capture observationally a specific TID perturbation caused a specific SC. In other words, can we identify a one-to-one – MSTID-SC – correspondence using available data? So far, direct observations of 