Observations of HF-induced plasma lines in blanketing sporadic E at Arecibo have been reported in the literature. The purely growing instability was found to be the probable source of the plasma lines, although the parametric decay instability could not be ruled out. It was also observed that the upshifted plasma lines tend to be much stronger than the downshifted plasma lines. A calculation of the purely growing and decay thresholds, using typical ray paths of the type thought to be responsible for the Arecibo observations, indicates that the purely growing instability is most likely responsible for the observed E8 plasma lines. The purely growing threshold is smallest when it is determined by electron collisions rather than by the density gradient and when the wave normal of the unstable waves is nearly parallel to the earth's magnetic field. It is thus likely that the unstable Langmuir waves are generated where these conditions are satisfied and subsequently propagate to where they are observed by the radar. In order that these waves not be highly damped, the density gradient must be greater than about 10¿ off the vertical. This indicates that these waves propagate in ionization irregularities embedded in the E8 layer. For irregularities with an upward component of density gradient directed north of the antimagnetic field direction, upshifted E8 plasma lines would occur; otherwise, downshifted E8 plasma lines would result. Since the former case covers a much larger range of angles than the latter case, it is not surprising that normally the upshifted E8 plasma line is stronger than the downshifted E8 plasma line.