Large-scale density modification was optimally achieved by electromagnetic waves whose frequency matched the plasma frequency at a height where the ionospheric density profile was flat. The density in this region was dramatically clamped during the morning when it normally increases from solar ionization. Electromagnetic wave propagation in the polar magnetic field geometry and strongly resonantly enhanced electrostatic fields over a large region of constant density account for the observation.PACS 52.25.Sw, 52.70.Gw We report the observation of large-scale modification of the polar ionosphere by electromagnetic (EM) waves at modest power density (0.1