SC at one observing place. We suppose that this scenario is correct for pure TID valleys. Presumably, if a TID valley itself contains fluctuations of electron density, more than one caustic surface may be generated. On dTEC maps such fine structure may be poorly distinguishable or considered as not-significant. In TID structures in cases $\#$2 and $\#$3, the valleys do comprise some minor and isolated irregularities with the crests bordering the valleys. Likely, these irregularities account for double occurrences of spatial caustics resulted in partial overlap in time of spectral caustics.

We should also emphasize the importance of the paper for solar radio observations in light of realization of new ambitious projects of grand building of radio astronomy facilities. Thus, in the recent years the global trend in the deployment of radio astronomy instruments tends towards broadband antenna arrays (NenuFAR, LOFAR, GURT, MWA, LWA, SKA), operating in meter-decameter wavelength range. In this connection, the subject of transients of various origins in HF and VHF solar observations is regarded as of current relevance.

This research is motivated by our previous works [@Koval2017; @Koval2018]. With this series of studies on SCs, we pursue a better understanding of this intriguing phenomenon. Further investigations 