We report on a direct, prompt effect of the solar wind on the midlatitude ionosphere. On November 7, 1997, an interplanetary shock impinged on the magnetopause around 1614 UT. The solar wind dynamic pressure dropped by 70% across the shock, and the interplanetary magnetic field (IMF) changed from southward to northward. The Millstone Hill incoherent scatter radar measured a significant decrease of 25--30% in the dayside ionospheric F region plasma density over 37¿ and 44¿ geographic latitudes within 1.5 hours and a drop of 23% in the total electron content (TEC). We propose that the decrease of the ionospheric density is associated with the decompression of the magnetosphere and/or the direct penetration of interplanetary electric field into the inner magnetosphere. ¿ 2001 American Geophysical Union