Continuous measurement of stimulated electromagnetic emission spectra from HF excited ionospheric turbulence

Stimulated electromagnetic emissions of the auroral ionosphere in the vicinity of Fairbanks, Alaska, have been observed during high-power HF heating experiments. Asymmetric spectra around the heater frequency with dominant lower sidebands are observed under conditions of very long density scale lengths corresponding to f0F1 transition of the heater frequency. Continuous recording of these spectra indicate their strong dependence on controlled cycling of the HF power density and weak dependence on ionospheric fading of the HF wave. Possible physical mechanisms for generating these spectra include stimulated backscattering of the HF pump wave from parametric instabilities and cavitons.