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Featured observations in high-frequency (HF) heating experiments conducted at Arecibo, EISCAT, and high frequency active auroral research program are discussed. These phenomena appearing in the F region of the ionosphere include high-frequency heater enhanced plasma lines, airglow enhancement, energetic electron flux, artificial ionization layers, artificial spread-F, ionization enhancement, artificial cusp, wideband absorption, short-scale (meters) density irregularities, and stimulated electromagnetic emissions, which were observed when the O-mode HF heater waves with frequencies below foF2 were applied. The implication and associated physical mechanism of each observation are discussed and explained. It is shown that these phenomena caused by the HF heating are all ascribed directly or indirectly to the excitation of parametric instabilities which instigate anomalous heating. Formulation and analysis of parametric instabilities are presented. The results show that oscillating two stream instability and parametric decay instability can be excited by the O-mode HF heater waves, transmitted from all three heating facilities, in the regions near the HF reflection height and near the upper hybrid resonance layer. The excited Langmuir waves, upper hybrid waves, ion acoustic waves, lower hybrid waves, and field-aligned density irregularities set off subsequent wave-wave and wave-electron interactions, giving rise to the observed phenomena.

[1] Theory of a beat-wave mechanism for very low frequency (VLF) wave generation in the ionosphere is presented. The VLF current is produced by beating two high power HF waves of slightly different frequencies through the nonlinearity and inhomogeneity of the ionospheric plasma. Theory also shows that the density irregularities can enhance the beat-wave generation. An experiment was conducted by transmitting two high power HF waves of 3.2 MHz and 3.2 MHz + f, where f = 5, 8, 13, and 2.02 kHz, from the HAARP transmitter. In the experiment, the ionosphere was underdense to the O-mode heater, i.e., the heater frequency f 0 > foF2, and overdense or slightly underdense to the X-mode heater, i.e., f 0 < fxF2 or f 0 ≥ fxF2. The radiation intensity increased with the VLF wave frequency, was much stronger with the X-mode heaters, and was not sensitive to the electrojet. The strongest VLF radiation of 13 kHz was generated when the reflection layer of the X-mode heater was just slightly below the foF2 layer and the spread of the O-mode sounding echoes had the largest enhancement, suggesting an optimal setting for beat-wave generation of VLF waves by the HF heaters.

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.