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Generation of energetic electrons at second harmonic cyclotron resonance in ionospheric HF heating experiments

The theory of electron acceleration by upper hybrid waves at second harmonic cyclotron resonance is presented. The results show that the meter‐scale upper hybrid waves can incorporate the finite Larmour radius effect to make a second harmonic cyclotron resonance interaction effective. The finite Larmour radius effect provides a positive feedback to the interaction, thus the energies of the accelerated electrons increase in time exponentially, rather than linearly as in the case of fundamental cyclotron resonance. Consequently, energetic electrons (having energies larger than 10.7 eV) can be generated even at very low upper hybrid wave intensities. The threshold field for parametric excitation of meter‐scale upper hybrid waves by O‐mode HF heating wave is shown to be very low. The theory can be a reasonable basis for explaining the enhancement of airglow at 777.4 nm observed in recent low‐heating‐power experiment at HAARP.