Parametric instabilities in ionospheric heating experiments

Parametric instabilities excited in ionospheric heating experiments are studied. The primary processes excited directly by HF heating (pump) waves include parametric decay instabilities, which decay the HF heating wave to a frequency-downshifted Langmuir/upper hybrid sideband together with an ion acoustic/lower hybrid wave as the decay mode, and oscillating two stream instabilities, which decay the HF heating wave to two oppositely propagating Langmuir/upper hybrid sidebands and a purely growing mode/field-aligned density irregularities. These instabilities provide effective channels to convert electromagnetic heating waves to electrostatic plasma waves in the F region of the ionosphere. The instability thresholds, growth rates, angular distribution, and regions of excitation are determined. The high frequency sidebands (Langmuir waves and upper hybrid waves) of primary parametric instabilities can be driven to large amplitudes. These waves then become new pump waves to excite secondary parametric instabilities, which provide cascade channels to broaden the spectra of plasma waves and generate short-scale density irregularities. The secondary parametric instabilities include cascades of Langmuir pump waves into Langmuir sidebands and ion acoustic waves/lower hybrid waves, and decay of upper hybrid waves to Langmuir sidebands and ion decay modes, as well as the filamentation of those high frequency electrostatic waves to generate field-aligned density irregularities. Again, the thresholds and growth rates of these instability processes are determined.