MODELING STUDIES OF THE CONJUGATE-HEMISPHERE DIFFERENCES IN IONOSPHERIC IONIZATION AT EQUATORIAL ANOMALY LATITUDES

The relative importance of the equatorial plasma fountain (caused by v ertical E x B drift al the equator) and neutral winds in leading to th e ionospheric variations at equatorial-anomaly latitudes, with particu lar emphasis os conjugate-hemisphere differences, is investigated usin g a plasmasphere model. Values of ionospheric electron content (IEC) a nd peak electron density (Nmax) computed at conjugate points in the ma gnetic latitude range 10-30 degrees at longitude 158 degrees W reprodu ce the observed seasonal, solar activity, and latitudinal variations o f IEC and Nmax, including the conjugate-hemisphere differences. The mo del results show that the plasma fountain, in the absence of neutral w inds, produces almost identical effects at conjugate points in all sea sons; neutral winds cause conjugate-hemisphere differences by modulati ng the fountain and moving the ionospheres at the conjugate hemisphere s to different altitudes. At equinox, the neutral winds, mainly the zo nal wind, modulate the fountain to supply more ionization to the north ern hemisphere during evening and night-time hours and, at the same ti me, cause smaller chemical loss in the southern hemisphere by raising the ionosphere. The gain of ionization through the reduction in chemic al loss is greater than that supplied by the fountain and causes stron ger premidnight enhancements in IEC and Nmax (with delayed peaks) in t he southern hemisphere at all latitudes (10-30 degrees). The same mech anism, but with the hemispheres of more flux and less chemical loss in terchanged, causes stronger daytime IEC in the northern hemisphere at all latitudes. At solstice, the neutral winds, mainly the meridional w ind, modulate the fountain differently at different altitudes and lati tudes with a general interhemispheric flow from the summer to the wint er hemisphere at altitudes above the F-region peaks. The interhemisphe ric flow causes stronger premidnight enhancements in IEC and Nmax and stronger daytime Nmax in the winter hemisphere, especially at latitude s equatorward of the anomaly crest. The altitude and latitude distribu tions of the daytime plasma flows combined with the longer daytime per iod can cause stronger daytime IEC in the summer hemisphere at all lat itudes.