Using observations from the FAST small explorer spacecraft, we present fields and plasma observations above the dayside auroral oval showing the erosion of ionospheric plasmas from the topside ionosphere by the action of Alfv¿n waves. Using interferometric techniques, the waves are shown to approximately obey the expected dispersion for Alfv¿n waves with transverse scales extending from greater than electron inertial lengths down to ion gyroradii. Measurements of the plasma density where these waves are observed show that over latitudinal widths exceeding 100 km total depletion of the cold ionospheric plasma can occur. These depleted regions or cavities are populated by magnetosheath plasmas, upgoing transversely accelerated ionospheric ions, and downgoing field-aligned electrons. The ionospheric ions and field-aligned electrons are distributed as conics and beams, respectively. Poynting flux observations on the density gradients comprising the cavity walls show that these waves are directed downward and focused inward toward regions of lower density. Wave phase velocity measurements, while subject to significant uncertainty, show that the wave vector is directed transversely outward from the cavity. These observations suggest a feedback model for Alfv¿n wave focusing and ion heating on density gradients that can lead to intense ion outflow from the ionosphere and subsequent depletion of ionospheric plasmas.