Evidence is uncovered that plasmaspheric drainage plumes flowing into the dayside reconnection site mass load the reconnection rate and thereby reduce the coupling of the solar wind to the Earth's magnetosphere. Solar-wind/magnetosphere coupling is statistically analyzed with the 1963–2003 OMNI2 data set matched up with the AE, AU, and PCI geomagnetic indices. Times when plasmaspheric drainage plumes are flowing are discerned using multiple spacecraft with plasma detectors in geosynchronous orbit. It is found that for a given value of −vBz of the solar wind, the geomagnetic indices AE, AU, and PCI are statistically lower when plasmaspheric drainage plumes are present than when plumes are not present. This is taken as a measure of a weakened coupling of the solar wind to the Earth's magnetosphere caused by the plumes. An Addendum examines the effects of polar-cap saturation on the auroral electrojet index.