We report observations of a direct ionospheric plasma outflow response to the incidence of an interplanetary shock and associated coronal mass ejection (CME) upon the earth's magnetosphere. Data from the WIND spacecraft, 185 RE upstream, document the passage of an interplanetary shock at 23:20 UT on 24 Sept. 1998. The polar cap plasma environment sampled by the POLAR spacecraft changed abruptly at 23:45 UT, reflecting the compressional displacement of the geopause relative to the spacecraft. POLAR left the polar wind outflow region and entered the mantle flows. Descending toward the dayside cusp region, POLAR later returned from the mantle to an enhanced polar wind flux dominated by O+ plasma and eventually containing molecular ions. The enhanced and O+-dominated outflow continued as the spacecraft passed through the high altitude cleft and then the southern cleft at lower altitude. Such a direct response of the ionosphere to solar wind dynamic pressure disturbances may have important impacts on magnetospheric dynamics. ¿ 1999 American Geophysical Union