focusing effect of terrestrial ionosphere on the solar radiation unmasks itself in ground-based observations of the Sun by radio telescopes in meter and decameter wavelengths. Its manifestations are called as Spectral Caustics (SCs). They emerge in the solar dynamic spectra and exhibit unusual morphology and specific time-frequency features. The interpretation of SCs in terms of diffraction and focusing of solar radiation on ionospheric irregularities has been given by @Meyer-Vernet1980 [@Meyer-Vernet1981]. Keen interest on this subject was demonstrated in following observational works by @Bougeret1981, @Genova1983, @Mercier1986a [@Mercier1986b], and @Mercier1989. In particular, spectral and heliographic methods for further exploration of SCs were used. In later years, there appeared a long lull in the investigation of the focusing effect of solar radio emission.

From a careful review of earlier studies we concluded that the SC topic has not been fully explored yet. Therefore, we have revisited this subject by carrying out a statistical analysis of SCs in the solar dynamic spectra observed by the Nançay Decameter Array [NDA; @Lecacheux2000] covering 10-80 MHz band [@Koval2017]. Over a 17-year period (1999-2015) we identified SCs in 129 observational days. For the first time, SCs have been classified into several types according to their spectral manifestations. There 