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.

通过应用“高度-时间-强度”（HTI）技术以及频谱（Lomb 周期图）分析，研究了来自三个欧洲中纬度电离层站（雅典、尼科西亚和普鲁霍尼采）的长电离图序列，旨在调查零星的 E (Es) 和中间降层 (IDL) 的每日和季节性变化。Es 和 IDL 轨迹在所有三个站点上都被观察到，其特点是全年普遍存在 12 小时的周期性。在所有站点中也观察到 IDL 和 Es 出现的较短周期。Es 和 IDL 的 6 小时周期性在所有站点的冬季占主导地位，而 Es 的 8 小时周期性主要出现在中低纬度站点（雅典和尼科西亚）的夏季。这些周期性很可能是由半、分别为四分之一和昼夜热层潮汐。一个值得注意的发现是主要在 IDL 中观察到的 4 小时周期性，而 Es 在三个高纬度站点（Pruhonice、Juliusruh 和 Dourbes）中的出现频率较低，仅在冬季、五年内出现，这可能也与太阳潮汐有关。