A model for ionospheric density structures at the O+ ion gyroradius induced by the Arecibo heater during the recent El Coqui campaign has been developed. We find that (1) these density structures can be accounted for by the parametric decay of the high frequency high power Arecibo heater wave into a low frequency wave with frequency at or near the ion cyclotron frequency and high frequency sideband modes (2) the growth rate of this process peaks at scale sizes corresponding to the O+ ion gyroradius (3) computed density fluctuations, using a strong turbulence model, associated with the ion cyclotron modes, in the nonlinear regime, are in the range of 2--12% and (4) anisotropic ion-conic-like heating will result from this process. The results of this model are in good agreement with recent El Coqui observations. ¿ American Geophysical Union 1995