NIGHTTIME ENHANCEMENTS OF THE F2-LAYER IONIZATION OVER HAVANA, CUBA

Night-time enhancements (NE) of the F2-layer ionization observed by the Faraday rotation and vertical sounding techniques are studied by statistical methods for low and high solar activity conditions at the low latitude station Havana. The enhancements in ionospheric electron content (N(F)) and critical frequency (foF2) indicate a strong seasonal variation which is controlled by solar activity effects. Thus the occurrence rate is a maximum during wintertime for low solar activity conditions, whereas it is a maximum in summer during high solar activity periods. Other parameters, such as the duration or intensity of the night-time enhancements, are sensitive to solar activity in a similar way. It is suggested that downward plasma transport from the plasmasphere is the main process which causes the enhancements in ionospheric ionization during the night-time. Thus both NE's and the so-called night-time winter anomaly (NWA) effect are generated by the same mechanism in this region. This conclusion is confirmed by theoretical modelling of the night-time ionospheric behaviour which is sensitive to the interplay of electric fields and neutral winds.