Wave behaviour of sporadic E-layer variations at the latitudes 30-70°N

Wave behaviour of ionospheric sporadic E-layer variations is investigated by analysing Northern hemisphere ionosonde data for a continuous interval of 20 years. Wavelet analysis is used to derive time-frequency amplitude spectra of foEs oscillations. Dominant periodicities of 4-6 and 14-16 days in Es variations were detected. A spatio-temporal picture of quasi-16-day Es-layer oscillations during 1970-1989 is created by interpolation of wavelet amplitude values, calculated for each of more than one hundred stations. Several typical movement directions of quasi-16-day waves are determined using an original pattern recognition algorithm. A zonal component of 16-day wave direction is found to be dominant practically everywhere in the latitudes 45-75°N. Zonal distributions of quasi-16-day oscillation amplitude at 30-70°N are derived; their seasonal and longitudinal features are significant.