Intermediate descending layer and sporadic E tidelike variability observed over three mid-latitude ionospheric stations

Abstract Long sequences of ionograms from three European mid-latitude ionospheric stations (Athens, Nicosia, and Pruhonice) are examined, 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 all three stations which are characterized by a 12-h periodicity prevailing throughout the year. Shorter periodicities in IDL and Es occurrence are also observed in all stations. A 6-h periodicity in Es and IDL dominates during winter over all stations, while an 8-h periodicity in Es is found mainly in summer in the lower mid-latitude stations (Athens and Nicosia). These periodicities are most probably induced by the semi-, quarter- and terdiurnal thermospheric tides respectively. A notable finding is a 4-h periodicity observed mainly in IDL and less frequently in Es occurrence in three higher mid-latitude stations (Pruhonice, Juliusruh and Dourbes) only during winter, in five years, which maybe also associated with solar tides.