A quantitative explanation is given for the observed (Hansen et al., 1992) large-scale (several kilometers in extent, ΔTe/Te0~3,‖Δne‖/ne0~25%) ionospheric modifications produced with the Arecibo HF heating facility. The process of nonlinear refraction is introduced and shown to be the key nonlinearity which makes possible the generation of the observed effects. Nonlinear refraction consists of the self-consistent reorientation of the HF reflection surface by heating-induced density modifications from a gravitationally aligned density gradient to a geometry with ∇n perpendicular to the geomagnetic field. It causes the O mode radiation to refract across the geomagnetic field and results in focused heating along a narrow flux tube colinear with the modified reflection surface. A two-dimensional transport model that incorporates the effects of nonlinear refraction on F region modification is used to establish a detailed comparison with experimental results. ¿ American Geophysical Union 1992