Effects of composition and fountain effect on the annual variation of the day-time F-layer at low latitudes

The annual variation of the daytime F2-layer peak electron density (NmF2) is studied at two low latitude stations, Okinawa and Tahiti (geomagnetic latitudes ± 15°) for the sunspot maximum years 1979–1981. Observed values are compared with those calculated using the MSIS model and a simplified version of the continuity equation for day-time equilibrium conditions. Summer-winter differences imply an intensification of the fountain effect on the winter side of the equator at the expense of the summer side. This could be explained by a summer to winter neutral wind. Semi-annual variations, however, appear to be mainly due to changes in neutral composition.