International audienceThe current status of the large decameter radio telescope UTR-2 (Ukrainian T-shaped Radio telescope) together with its VLBI system called URAN is described in detail. By modernization of these instruments through implementation of novel versatile analog and digital devices as well as new observation techniques, the observational capabilities of UTR-2 have been substantially enhanced. The total effective area of UTR-2 and URAN arrays reaches 200 000 m2, with 24 MHz observational bandwidth (within the 8–32 MHz frequency range), spectral and temporal resolutions down to 4 kHz and 0.5 msec in dynamic spectrum mode or virtually unlimited in waveform mode. Depending on the spectral and temporal resolutions and confusion effects, the sensitivity of UTR-2 varies from a few Jy to a few mJy, and the angular resolution ranges from ~ 30 arcminutes (with a single antenna array) to a few arcseconds (in VLBI mode). In the framework of national and international research projects conducted in recent years, many new results on Solar system objects, the Galaxy and Metagalaxy have been obtained. In order to extend the observation frequency range to 8–80 MHz and enlarge the dimensions of the UTR-2 array, a new instrument – GURT (Giant Ukrainian Radio Telescope) – is now under construction. The radio telescope systems described herein can be used in synergy with other existing low-frequency arrays such as LOFAR, LWA, NenuFAR, as well as provide ground-based support for space-based instruments

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.