Experimental investigations of large-scale modifications created by a high-power HF beam (effective radiated power of ~100 MW, frequency of 3--5 MHz) in the nighttime ionosphere above Arecibo Observatory are presented. The modifications consist of field-aligned temperature enhancements (ΔTe/Te0~2--4) and density depletions (‖Δne‖/ne0~25--50%) and have length scales along the geomagnetic field of tens of kilometers. A two-stage time evolution of the modifications is documented; they start from a broad and symmetric perturbation and develop into a narrow, northward shifted universal steady state on a time scale of 15--30 min. It is found that nonlinear refraction, i.e., the self-consistent bending of the O mode HF beam across the geomagnetic field lines and the reorientation of the HF reflection surface to a geometry parallel to the geomagnetic field, is the key process involved in the generation of the large modifications. Preconditioning results showing the immediate onset of the narrow late-stage behavior provide dramatic confirmation of the underlying nonlinear mechanism sustaining the large modifications. Highlights of quantitative comparisons of these observations with a transport model including nonlinear refraction have been given in a previous publication (Hansen et al., 1990). ¿American Geophysical Union 1992