The height of the HF-enhanced plasma lines (HFPLs) has been observed to be greater than that of the photon-enhanced natural plasma lines by 1 km or so at Arecibo, Puerto Rico. Besides the development of HFPLs affected strongly by that of the short-scale field-aligned irregularities (SFAIs) has been indicated in the recent Arecibo heating experiments. These outstanding features of HFPLs can be reasonably understood in terms of the proposed instability process which is elucidated in the present paper. This instability involves a four-wave interaction process whereby a parallel propagating (i.e., k↘//B↘0) Langmuir pump wave excites two obliquely propagating daughter Langmuir waves together with a purely growing field-aligned mode (density irregularities), The field-aligned mode can further scatter the propagating Langmuir waves to render even larger propagation angles leading to the detection of HFPLs. The parallel propagating Langmuir waves originate from the excitation of the parametric decay instability by the HF heater wave near its reflection height. Since the HFPLs were detected by the 430-MHz radar, while the SFAIs by a portable 49.92-MHz backscatter radar, the thresholds and growth rates of the instability are calculated separately for these two cases. One case is for the excitation of the Langmuir sidebands having a scale length of 42 cm and a propagation angle of 23¿ (which leads to HFPLs after being scattered off the 1.1-m field-aligned density irregularity), and the other case for the generation of the 3-m SFAIs. The threshold field of the instability leading to the generation of HFPLs is about 0.15 V/m. It is also found that the growth rate of the HFPLs is slightly greater than twice that of the 3-m SFAIs and that it takes a few milliseconds for the HFPLs to develop. Further, the proposed instability process occurs at an altitude about 1.2 km higher than the matching height determined by the dispersion relation of the HFPLs when the heater wave at 5.1 MHz is employed. Based on these results, the puzzling height difference between the HFPLs and the photon-enhanced natural plasma lines as well as the correlation between the HFPLs and SFAIs can be explained consistently. ¿ American Geophysical Union 1993