Quantitative information on the flux of H+ ions flowing from the ionosphere to the plasmasphere is important for studying the refilling of emptied plasmaspheric flux tubes. A rigorous calculation of the flux is cumbersome and costly for some applications, so one purpose of this study is to evaluate quantitatively the usefulness of a new simple analytical expression for the H+ flux by comparison with results derived from a comprehensive calculation. We also present model calculations of the seasonal and solar cyclical variations of the daytime limiting upward H+ flux applicable to the period between 1974 and 1980. These calculations indicate a relatively small seasonal variation of a factor of 1.6. The calculated solar cycle variation depends strongly on the neutral atmosphere model. Using the 1977 version of the mass spectrometer/incoherent scatter (MSIS) model, we obtained a factor of 10 decrease in flux from solar minimum to solar maximum; however, the 1983 MSIS model produced only a factor of 3 variation, principally because of a factor of 3 less variation in the neutral hydrogen density. The various factors that affect the limiting H+ flux were examined by using the simple analytical expression. In agreement with previous work we have found the limiting flux to be most sensitive to the topside ionosphere neutral hydrogen density, which decreases with increasing solar activity. However, a combination of smaller increases in O+ density, the O+ scale height, and the O+ +H reaction rate acts to oppose the decrease in the limiting flux that would be expected from the decrease in neutral H density. The net result is that the limiting flux variation is only one third of the neutral hydrogen variation. The 1983 MSIS model also produces a greater solar cycle variation in the F2 peak electron density because of a factor of 2 less variation in the O2 density.