The Langmuir waves responsible for the HF-wave-induced plasma lines observed during ionospheric heating experiments at Arecibo are generated and propagate to the point of detection in field-aligned ionization ducts. These Langmuir waves grow parametrically in the ducts at the height in the ionosphere at which the HF wave electric field is strongest. If it is assumed that there is a positive density gradient upward along the duct, then the observed ion acoustic frequency asymmetry between the upshifted and downshifted plasma lines can be explained. Agreement can be obtained between experimental and theoretical values for the frequency asymmetry and for the growth rates of the plasma line for a scale length along the magnetic field direction of about 40 km. A 40-km-scale length is less than that which would be expected during the day in the ambient plasma. However, it is probably not an unreasonable value for a duct under the influence of HF heating. A 40-km scale length also leads to a height differential of about 55 m between the points at which the waves responsible for the upshifted and downshifted plasma lines are detected by the Arecibo radar. The model also implies that the intensity of the upshifted plasma line would be greater than that for the downshifted plasma line.