We present an in-depth study into spectral perturbations appearing in solar dynamic spectra and being manifestations of the focusing effect of low-frequency solar emission by the Earth's ionosphere. Such perturbations are considered to be the result of radio waves focusing by medium scale traveling ionospheric disturbances (MSTIDs). Using the Nançay Decametric Array (NDA) data set, we have conducted a statistical analysis of the spectral structures in solar dynamic spectra within 10-80 MHz. We have detected the spectral structures in the NDA spectral data for 129 observation days from 1999 to 2015. On spectrograms they appear as intensity variations different from well-known solar radio bursts. The sharp edges with enhanced intensity are distinctive characteristics of the structures for most events. Due to this spectral feature, they are termed as Spectral Caustics (SCs). We have classified the SCs observed by the NDA as several types, based on their spectral morphology, namely: inverted V like, V like, X like, fiber like, and fringe like. We have found that the rate of occurrence of SCs in dynamic spectra depends on the phase of the solar cycle. About 81% of all days with detected SCs fall on active phases of solar cycles 23 and 24 (48% and 33%, respectively). We have also established the seasonal dependence in occurrence of the SCs. It was found that about 95% of days with SCs belong to autumn-winter months, whereas only near 5% of days with SCs belong to spring-summer months. This is well correlated with the reported dependence in MSTID occurrence rate.