Ion gyroharmonic structures in stimulated radiation during second electron gyroharmonic heating: 1. Theory



[1] Stimulated electromagnetic emissions (SEEs) may provide important diagnostic information about space plasma composition, energetics, and dynamics during active experiments in which ground-based high-powered radio waves are transmitted into the ionosphere. The nonlinear plasma processes producing this secondary radiation are not well understood particularly for some recent observations where the transmitter (pump) frequency is near the second harmonic of the electron gyrofrequency. New, more comprehensive, experimental observations of spectral features within 1 kHz of the pump wave frequency are reported here to begin more careful comparisons of the experimental observations and a possible theoretical underpinning, which is also provided. The experimental observations typically show two distinct types of secondary radiation spectra, which are (a) discrete narrowband harmonic spectral structures ordered by the ion gyrofrequency and (b) broadband spectral structure with center frequency near 500 Hz and similar spectral bandwidth. A theoretical model is provided that interprets these spectral features as resulting from parametric decay instabilities in which the pump field ultimately decays into high-frequency upper hybrid/electron Bernstein and low-frequency neutralized ion Bernstein and/or obliquely propagating ion acoustic waves at the upper hybrid interaction altitude. Detailed calculations of the threshold level, growth rate, unstable wave number, and frequency bandwidth of the instabilities are provided for comparisons with experimental observations. An assessment of the effect of the critical instability parameters are provided including pump electric field strength, proximity of the pump frequency to the electron gyrofrequency and pump electric field geometry. The model shows quite reasonable agreement with the experimental observations. Further discussions are provided of connections with past observed SEE spectral features and potential new diagnostic information provided by these newly categorized spectra.