By using the data from the July 1976 ionospheric modification experiments at Arecibo it was discovered that the Langmuir waves responsible for the enhanced plasma line are generated at the height in the ionosphere at which the Airy function, which describes the standing electric field of the high-frequency (HF) modifying wave near reflection, has its main maximum. It would be impossible for the enhanced Langmuir waves which backscatter the 430-MHz radar signal to exist at this height in a uniformly varying ionosphere. This implies that underdense ionization irregularities exist in the ionosphere that allow the Langmuir waves to be generated and to propagate to the appropriate ionization density for detection by the incoherent backscatter radar. This discovery puts a new light on the generation mechanism of the HF-enhancd plasma line, since up until now it has been implicitly assumed in theoretical work that the parametric decay process occurs in a uniformly varying ionosphere, that is, an ionosphere without irregularities. The ionization irregularities responsible for the HF-enhanced plasma line are probably field aligned with an electron density deviation decrease of about 4% or more and with a diameter between about 1 m and a few hundred meters. It is proposed that Langmuir waves are amplified parametrically as they propagate in the duct to where their wave normals are parallel to the radar beam. The observed frequency displacement from the radar frequency (430 MHz) of the parametric decay line at Arecibo is about Fhf-3.5 kHz, where Fhf is the frequency of the HF heating wave (4-10 MHz). Numerical calculations for typical daytime conditions show that the maximum coupling between the HF and Langmuir waves occurs for a frequency offset between the two waves of about 3.4 kHz for the field-aligned irregularity or duct model, which is in good agreement with observations. The observed frequencies of the peaks in the HF-enhanced plasma line spectrum, which are displaced from 430 MHz¿Fhf by approximately odd harmonics of the ion acoustic or offset frequency, can be explained with the duct model. The 'broad bump' observed in some plasma line spectra probably results from cascading of intense Langmuir waves which are produced parametrically and propagate approximately along the axis of the duct. It is proposed that the backgound or 'immature' spectrum may result from a four-wave decay process. The 'overshoot' phenomenon (intense plasma line signals occurring immediately after transmitter turn-on) may be due to the effects of HF heating on the ducts and on the ambient plasma.