Estimates are calculated for the storm time reduction of solar wind/magnetosphere coupling by the mass density ρm of the magnetospheric plasma. Based on the application of the Cassak-Shay reconnection-rate formula at the dayside magnetopause, a numerical factor M is developed to quantify the effect of ρm on the dayside reconnection rate. It is argued that the mass loading of dayside reconnection by ρm also makes reconnection more susceptible to shutoff by magnetosheath velocity shear: a formula is developed to estimate the shortening of the dayside reconnection X-line by ρm. Surveys of plasmaspheric drainage plumes at geosynchronous orbit during high-speed-stream-driven storms and coronal mass ejection (CME)-driven storms are presented: in the surveys the CME-driven storms are separated into sheath-driven portions and magnetic-cloud-driven portions. The storm time mass density of the warm plasma cloak (ionospheric outflows into the electron plasma sheet) is obtained from Alfven-wave analysis at geosynchronous orbit. A methodology is developed to extrapolate geosynchronous-orbit plasma measurements to the dayside magnetopause. For each of the three plasmas, estimates of the fractional reduction of the total dayside reconnection rate vary, with typical values of tens of percent; i.e., solar wind/magnetosphere coupling is reduced by tens of percent during storms by oxygen in the ion plasma sheet, by the plasmaspheric drainage plume, and by the plasma cloak. Dependence of the reduction on the F10.7 solar radio flux is anticipated. Via these ionospheric-origin plasmas, the magnetosphere can exert some control over solar wind/magnetosphere coupling. Pathways to gain a fuller understanding of the physics of the solar wind-driven magnetosphere-ionosphere system are discussed.