Generation of a broad downshifted spectrum of HF wave enhanced plasma Lines (HFPLs) in ionospheric heating experiments is explored. Langmuir waves are first excited within a cone around the geomagnetic field by the HF wave, in the region near its reflection height, through the oscillating two stream instability (OTSI). These Langmuir waves then cascade through a secondary parametric instability, whereby an obliquely propagating Langmuir pump wave decays into an obliquely propagating Langmuir sideband and a lower hybrid decay mode, which propagates in a direction perpendicular to the Langmuir pump wave. The excited Langmuir sidebands have a broad downshifted frequency spectrum and large propagation angles. Their propagation angles are further widened via the filamentation instability or scattering off short-scale field-aligned density irregularities. Thus they become detectable by backscatter radars as HFPLs with a broad downshifted frequency spectrum. The results of our analysis show that it requires HF field amplitude of 3.6 V/m to cascade the OTSI-excited Langmuir waves, for example, 8 times to produce a downshifted spectral width of 50 KHz in the Arecibo heating experiments. ¿ 1999 American Geophysical Union