It is known that the solar wind has significant effect on the magnetospheric convection and that magnetospheric electric fields can penetrate into the midlatitude and low-latitude ionosphere. In this paper, we present observations of midlatitude ionospheric plasma perturbations during two events. In one event, the interplanetary magnetic field (IMF) oscillated between southward and northward with a period of ~2 hours, while the solar wind pressure was relatively stable. In the other event the solar wind dynamic pressure oscillated with a period of ~1 hour, while the IMF remained strongly northward and stable. In both events the Millstone Hill incoherent scatter radar detected quasiperiodic perturbations in all F region plasma parameters (electron density, ion velocity, electron temperature, and ion temperature) in the midlatitude dayside ionosphere. The midlatitude ionospheric perturbations were well correlated with the IMF or solar wind pressure variations. Near the F peak altitude the electron density could be increased or decreased by ~30% within 30--70 min. We suggest that the midlatitude ionospheric electron density perturbations were caused by the penetration of magnetospheric electric fields which were controlled or modulated by the oscillations in the IMF or solar wind pressure.