A second‐order four‐wave interaction process including two pump photons, an upper hybrid plasmon, and an electron Bernstein plasmon is studied. The pump is the second harmonic of the HF heater in the plasma. It is found that, when the heater wave frequency f0 is above a harmonic of the electron cyclotron frequency fc, frequency‐upshifted upper hybrid waves and frequency‐downshifted electron Bernstein waves can be excited above the upper hybrid resonance layer via the considered process. The process occurs in a local region where the heater wave frequency is about the mean of the upper hybrid wave frequency and the electron Bernstein wave frequency. Moreover, in this interaction process, a low‐frequency electrostatic oscillation in the frequency range of the lower hybrid wave is generated through nonlinear coupling of the HF heater wave with the excited high‐frequency electrostatic waves. However, this wave does not satisfy the linear dispersion relation of the lower hybrid wave and is thus a driven wave. The excited frequency‐upshifted upper hybrid waves can then scatter off field‐aligned density irregularities to generate O ‐ mode emissions with frequencies around 2f0 − nfc This is consistent with observations of the broad upshifted maximum (BUM) feature in the stimulated electromagnetic emission (SEE) spectrum. The concomitantly excited frequency‐downshifted electron Bernstein waves are found to have much smaller amplitudes; hence their scattering products are also relatively weak. This explains why only BUM lines are detected. Furthermore, the driven low‐frequency fluctuations can also be the scatterers to convert the upper hybrid waves into emissions with frequencies aroundy fBUM + fLH and fBUM − fLH, where fLH is the lower hybrid wave frequency. This is suggested to be the generation mechanism of the second BUM feature which appears when the shoulder of the BUM feature is not very high.

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Parametric decay of an upper hybrid/electron Bernstein pump wave into an upper hybrid/electron Bernstein sideband wave and a lower hybrid decay wave in the long-wavelength regime is studied. It is found that the process associated with the electron Bernstein pump wave has a lower threshold field than that of a similar decay process of the upper hybrid pump wave when the instability is excited in the region away from the double resonance layer. Near the double resonance layer, where the upper hybrid resonance frequency equals a harmonic of the electron cyclotron frequency, the upper hybrid wave and the electron Bernstein wave become linearly coupled, and the threshold field of the parametric decay process changes back to a similar functional dependence as that of the upper hybrid decay process. Thus, their threshold fields approach each other. When incorporated with appropriate nonlinear scattering processes, this instability process along with its cascading is proposed to be the generation mechanism for the downshifted maximum (DM), 2DM, 3DM, ... etc. features as well as the upshifted maximum (UM) feature in the stimulated electromagnetic emission spectrum observed in ionospheric heating experiments.

Parametric decay of an electron Bernstein pump wave into an electron Bernstein sideband wave and a nearly perpendicularly propagating ion acoustic (or electrostatic ion cyclotron) decay wave is studied. When incorporated with appropriate nonlinear scattering processes, this instability process along with its cascading is proposed to be the generation mechanism for the downshifted peak (DP) and 2DP features as well as the upshifted peak (UP) feature in the stimulated electromagnetic emission spectrum observed in ionospheric heating experiments. It is found that the instability can be excited most favorably when the heater wave frequency f0 is in a very narrow frequency range near the third harmonic of the electron cyclotron frequencYfc of the ambient ionosphere. Moreover, the frequencies associated with the low-frequency decay mode decrease as f0 is increased toward 3f c, consistent with experimental observations.

A thermal oscillating two‐stream instability as the generation mechanism of electron Bernstein/upper hybrid waves by the o‐mode HF heater wave in the high‐latitude ionosphere is investigated. The purely growing decay mode is described by the fluid equations, while the kinetic equations are employed to derive the coupled mode equations of the electron Bernstein/upper hybrid sidebands. It is found that the instability can be excited in the altitude regions both above and below the upper hybrid resonance layer. The parametric excitation of electron Bernstein waves is in general not effective. On the other hand, the upper hybrid sidebands of the instability excited in the “below” region have been considered by Leyser (1991) to be the pumps for the parametric excitation of downshifted maximum (DM) lines of the stimulated electromagnetic emissions (SEEs) observed in the ionospheric heating experiments. The detail analysis of the proposed work shows that the instability zone of upper hybrid waves below the upper hybrid resonance layer becomes very small when the heater frequency fnof;0 is operated near three times of the local electron cyclotron frequency fnof;c. Thus the source wave of the DM lines is effectively suppressed. This result explains the phenomenon of quenching of the DM feature in the SEE spectrum as fnof;0 is adjusted near 3fnof;c in the ionospheric modification experiments.