Statistical results for the ionospheric outflows indicate that the ionosphere is an important source of plasma to the magnetosphere. However, the exact consequences on the dynamics of the magnetosphere from this ionospheric outflow have yet to be determined. This issue is taken up in multifluid modeling of the 24--25 September 1998 magnetic cloud event for which strong heavy ionospheric outflows have been previously reported. It is demonstrated that one of the key influences of heavy ionospheric outflows is to lower the cross-polar cap potential due to the mass loading it produces on the magnetosphere; i.e., the heavy ions provide a major sink for momentum that is transferred from the solar wind to the magnetosphere. The derived values for the cross-polar cap potential are shown to converge to that attained by assimilated mapping of ionospheric electrodynamics (AMIE) as the O+ concentration at the ionospheric boundary is increased to ~50% of the H+ concentration. The mass outflows produced in the model are comparable to the reported statistical values for the same level of Kp observed during the event. The position of the open-closed boundary is shown to be located progressively poleward with increasing O+ concentration, and that the derived position at the higher O+ concentrations best fits the Polar UVI images during the event. In addition, the model results shown that the heavy ionospheric ions provide a substantial sink for the energy input into the auroral oval as calculated by either UVI and AMIE. Thus ionospheric outflows are not just important in determining the source of the plasma in the magnetosphere, but have a clear role in controlling the global dynamics of the magnetosphere and the transport of solar wind energy and momentum through the magnetosphere.