HF-induced airglow at magnetic zenith: Thermal and parametric instabilities near electron gyroharmonics

We analyze airglow induced by intense radio waves at the High‐frequency Active Auroral Research Program (HAARP) facility for pump frequencies f0 near the second and third electron gyroharmonic (fc). While passing through the double resonance in the weakening ionosphere during heating at f0 ∼ 2fc, airglow intensities rose abruptly and remained elevated during four consecutive pulses. During this period both the thermal and parametric instabilities coexisted. Subsequently, the thermal parametric instability was hampered. The effectiveness of heating near the second gyroharmonic is ascribed to specific dispersion characteristics near the double resonance. The observations suggest that Langmuir waves participate in cyclotron acceleration when the local plasma and second gyroharmonic frequencies match. With f0 ∼ 3fc the airglow terminated due to the suppression of cyclotron acceleration and parametric decay instability.