In an ionospheric modification experiment at Arecibo in July 1976 it was discovered that the height of the enhanced plasma line due to photoelectrons does not agree with the height of the enhanced plasma line due to the HF heating wave. The properties of the Barker decoder, used in the experiment, indicate that the photoelectron enhanced plasma line occurs at the height expected by theory for a uniformly varying ionosphere, whereas the observed HF enhanced plasma line occurs a few kilometers above this height. The Langmuir waves responsible for the observed HF plasma line at Arecibo probably exist near the largest or first maximum of the Airy function, which describes the standing HF electric field. This is about 200 m below the heater wave reflection height. This observation requires that the Langmuir waves responsible for the HF-induced plasma line be generated in ionization irregularities and subsequently propagate in the irregularities to the appropriate plasma frequency for detection by the Arecibo radar.