Night-time enhancements of F region ionization over Cuba: relation to solar activity

Night-time enhancements of ionosphere electron content (NF) and of critical frequencies of F2-layer were studied in Cuba from June 1974 up to December 1980. It was determined, that possibility of occurrence and structure parameters of ionosphere enhancements NF and f0F2 depend essentially on season and on solar activity level. At low solar activity the occurrence frequency, duration and intensity of these events have maxima in winter. At high solar activity season dependence varies: duration decreases, while intensity -increases. Besides, clear differences of event occurrence in NF and f0F2 take place