The importance of diffusion, perpendicular electrodynamic drift, and neutral wind on the generation and modulation of the equatorial plasma fountain of the Earth's ionosphere is studied using the Sheffield University plasmasphere-ionosphere model for the ionosphere above Jicamarca under magnetically quiet equinoctial conditions at medium solar activity. The effects of the fountain, which include the equatorial anomaly, are also investigated. As expected, the F region electrodynamic (E¿B) drift generates the plasma fountain and the anomaly, which are symmetric with respect to the magnetic equator. The neutral wind introduces asymmetries, with larger plasma flow (toward the hemisphere of stronger poleward wind) and stronger anomaly crest occurring in opposite hemispheres. During daytime, when the drift is upward, the fountain rises to about 800 km altitude at the equator and covers about ¿30¿ magnetic latitude; outside the reach of the fountain, plasma flows toward the equator from both hemispheres. This convergence of plasma leads to the formation of an additional layer (called the G layer) within ¿10¿ of the magnetic equator during the prenoon hours when the drift is large.

At the magnetic equator, the maximum plasma concentration of the G layer can be greater than that of the F layer for a short period of time just before noon and when the drift starts to decrease. In the evening, soon after the drift turns downward, the fountain becomes a reverse fountain with supply of ionization from both hemispheres from regions outside the fountain. The reverse fountain acts as the main source for the nighttime increase in ionization at equatorial anomaly latitudes, with some contribution from the prereversal strengthening of the forward fountain. The importance of the prereversal strengthening of the forward fountain, which rises to about 1000 km altitude at the equator, and the following reverse fountain on the generation and propagation of plasma bubbles and spread F irregularities is discussed. It is also shown that the equatorial anomaly in the vertical ionospheric electron content need not be as pronounced as the interpretation of the observations suggests. ¿ American Geophysical Union 1995