Occurrence features of Intermediate descending layer and Sporadic E observed over the higher mid-latitude ionospheric station of Moscow

Abstract Nine years of ionograms from a higher mid-latitude ionospheric station (Moscow) are analyzed, by applying the ‘height–time–intensity’ (HTI) technique along with Spectrum (Lomb periodogram) analysis with the aim to investigate the daily and seasonal variability of sporadic E (Es) and intermediate descending layers (IDLs). Es and IDL traces are observed over Moscow which are characterized by a 12-hour periodicity prevailing throughout the year. Shorter periodicities in IDL and Es occurrence are also observed. A 6-hour periodicity in Es and IDL dominates during winter, while an 8-hour periodicity is found mainly in winter for IDL and in summer for Es. These periodicities are most probably induced by the semi-, quarter- and terdiurnal thermospheric tides respectively. Our results also establish the systematic and widespread manifestation of shorter-scale (4.8- and 4-hour) periodicities observed mainly for IDL and less frequently for Es only during winter, in the nine years considered, which is most probably linked to higher-order solar tides.