Morphology of NmF2 nighttime increases in the Eurasian sector

A statistical analysis of two peaks (pre-midnight and post-midnight) occurr ence in NmF2 daily variations was made on a latitudinal chain of four ionos onde stations in the Eurasian longitudinal sector. Overall 6182 cases of th e first and 5600 cases of the second peak occurrence were analyzed using al l available foF2 observations for the years of solar maximum and minimum. W ell-pronounced and systematic variations with season and solar activity wer e revealed in the occurrence probability of the peaks, their amplitude and timing. The pattern of both peaks occurrence is similar during winter and e quinoxes for midlatitude stations implying one and the same mechanism of th eir formation. The pre-midnight summer peak shows specific variations in pa rticular during solar maximum pointing to a different mechanism controlling its appearance. Possible mechanisms of both peaks formation are discussed.