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 theOmode 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 public