The temporal development of the so-called Hf-enhanced plasma line (HFPL) has been studied in detail using the high-power high-frequency (HF) facility located near Arecibo, Puerto Rico. Observations were made with the Arecibo 430-MHz radar. In the current work, attention is focused on the evolution of the HFPL spectrum subsequent to the onset of the HF wave in the F region plasma. The development of the HFPL spectrum is examined for several different HF pulse lengths and HF duty cycles. In this regard, low duty cycles with long HF-off periods tend to consistently produce a different spectral evolution than high duty cycles with short HF-off periods.

Broad, diffuse HFPL spectra are observed with low HF duty cycles, whereas sharply defined nonlinear Landau damping cascades develop at high duty cycles. Under a variety of observing conditions, the Arecibo HFPLs are found to develop dominant decay line peaks within a few milliseconds of HF turn-on in the plasma. In addition, at early times during periods of slow HFPL growth, and also for very low HF power levels, weak flat-top and/or double-humped spectral features are observed that can be attributed to radio wave scattering off ion acoustic waves. During periods of strong HFPL excitation, weak spectral signatures are occassionally observed in the band 430 MHz¿(fHF+△), where fHF is the HF frequency and △=10--100 kHz. The origin of this structure is currently not known. Finally, the temporal development of the 430-MHz spectra is closely examined during the so-called main plasma line overshoot. A diffuse spectral cascade is evident within a few milliseconds of the onset of the HF wave in the plasma, and dominant decay line peaks quickly develop thereafter. Processes involving direct conversion of the HF wave into Langmuir waves and/or soliton formation do not appear to be the dominant processes responsible for the 430-MHz HFPLs observed in the Arecibo F region. However, the formation of solitons may influence the development of the HFPLs.

The measured HFPL spectra are in qualitative agreement with the predictions of nonlinear Landau damping theory, but the experimental observations do not match the theoretical predictions in every detail. It is likely that HF-induced small-scale cavities and/or HF-induced geomagnetic field-aligned irregularities play a central role in determining the amplitude, spectral signature, and temporal development of strongly excited HFPLs at Arecibo.