The plasma-line spectrum observed during on-off cycling of the HF heater at Arecibo contains a broad component extending about 50 kHz from the first decay line. Shortly after HF turn-on this spectrum can have a maximum intensity 10--20 kHz from the heater frequency (broad bump) or the intensity can decrease monotonically with frequency separation from the first decay line. One possibility is that Langmuir waves (L waves) generated by parametric-decay of the heater wave are trapped in small-scale field-aligned ducts and are responsible for the first decay line. The heater electromagnetic wave can also become ducted near reflection in large-scale field-aligned ducts. If the heater wave is not ducted near reflection, its electric field is very nearly parallel to the earth's magnetic field B. The L waves then have wavenormals restricted to a range of angles about B. When these L waves decay parametrically into other L waves that can be detected by the radar, a broad-bump spectrum results. If the heater wave becomes ducted near reflection (which is probable during spread-F conditions), its electric field can make a considerable angle to B near reflection and the unstable L waves can propagate at all angles to B (including perpendicular). When these L waves decay parametrically into other L waves that can be detected by the radar, a monotonic spectrum results. ¿American Geophysical Union 1991