Incoherent scatter measurements of the topside ionosphere above Arecibo on August 15--16, 1976, are presented and compared with a time-dependent mathematical model of the L=1.4 magnetic flux tube. In the mathematical model the ions are O+ and H+, and their concentrations and field-aligned velocities are calculated from time dependent plasma continuity and momentum equations. As far as possible, the values of the atmospheric parameters used in the mathematical model are appropriate to August 15--16, 1976. However, to help explain the processes that occur in the topside ionosphere above Arecibo, calculations have been performed with some modified atmospheric parameters. The observed values of electron concentration and percentage H+ abundance can be reproduced by the mathematical model if we permit an interhemispheric flow of H+. There does not appear to be any other physically realistic way to achieve agreeement. At times, the model indicates that interhemispheric flow of H+ is from the hemisphere conjugate to Arecibo, i.e., from the winter hemisphere to the summer hemisphere. However, the results of the calculations suggest a net 24-hour transfer of H+ into the conjugate hemisphere i.e., from the summer hemisphere to the winter hemisphere. Typically, it amounts to 55% of the 24-hour mean values of the total plasma content of the L=1.4 magnetic flux tube between an altitude of 963 km and the equatorial plane. This is an excellent agreement with the estimate of net integrated H+ flux given by Vickrey et al. (1979b), which was based on comparing Arecibo observations from summer and winter days. However, the model indicates that there is no simple relationship on a moment-by-moment basis between the magnitude and direction of the interhemispheric H- flux and the magnitude and direction of the H+ field-aligned flux at an altitude around 1000 km. We also show that provided the plasma concentration remains unchanged at F2-region altitudes, the concentration of H+ in the topside ionosphere is more sensitive to changes in the neutral atomic hydrogen concentration, n(H), than to changes in the neutral atomic oxygen concentration. However, the concentration of O+ in the topside ionosphere is only marginally affected by n(H).