High frequency induced enhancements of the incoherent scatter spectrum at Arecibo

Observations are reported of enhancements of the incoherent scatter spectrum excited by strong high frequency (HF) radio waves. A number of enhancement characteristics, observed during an ionospheric heating experiment in January at Arecibo, are described. The enhancement spectrum includes: a strong narrow line displaced below the HF (5.62 MHz) by the frequency of the ion acoustic waves fi in the plasma (about 4 kHz); a broader line (about 30-kc width) and other weak lines near the HF; and a line near fi. Enhancements above thermal levels range above a factor of 104 and may be taken as verification of the HF excitation of parametric instabilities in the ionosphere. Significant time variations over scales of tens of µsec through hours were seen. Upshifted and downshifted enhanced plasma lines display asymmetries in intensity, width, power dependence, and decay rates (of the order of msec). The O-mode (but not the X-mode) excitation of these enhancements is of significance to heating experiments.