A new method to study dynamic behavior of the ionosphere-protonosphere coupling within the plasmasphere is developed and used to calculate ion distributions above 500 km using observed electron densities at 500 km. The method is based on the relation between the total magnetic flux tube content of H+ above some reference height (e.g., 3000 km) and the H+ flux at that height, which is uniquely determined by the coupled momentum and continuity equations for the quasi-steady state. The O+ profile is perturbed from diffusive equilibrium only by ion drag with H+. The time dependency is taken into account by applying self-consistent boundary conditions which are obtained from the solution of the time-dependent equation for the tube content of H+. Calculations are carried out for two models. The first model deals with the ion dynamics in a tube of force which rotates with the earth. In the second model, effects of the cross-L plasma drifts are considered. Both models produce diurnal variations of H+ and O+ densities near 1000 km similar to observational results. While the corotating model yields upward flow of H+ in the daytime and downward flow in the nighttime, the inclusion of the cross-L plasma motion gives rise to significant changes in the tube content and alters the diurnal variation of the H+ flux as well as the direction of flow in the evening sector. Although this effect is not very significant in the inner portion of the plasmasphere, the cross-L motion of plasma will play a more important role in the outer portion of the plasmasphere.