ELF/VLF wave generation using simultaneous CW and modulated HF heating of the ionosphere

[1] Experimental observations of ELF/VLF waves generated using the dual‐beam heating capability of the High frequency Active Auroral Research Program (HAARP) HF transmitter in Gakona, Alaska, are compared with the predictions of an ionospheric HF heating model that accounts for the simultaneous propagation and absorption of multiple HF beams. The model output is used to assess three properties of the ELF/VLF waves observed on the ground: the ELF/VLF signal magnitude, the ELF/VLF harmonic ratio, and the ELF/VLF power law exponent. Ground‐based experimental observations indicate that simultaneous heating of the ionosphere by a CW HF wave and a modulated HF wave generates significantly lower ELF/VLF magnitudes than during periods without CW heating, consistent with model predictions. Further modeling predictions demonstrate the sensitive dependence of ELF/VLF magnitude on the frequency and power of the CW signal. The ratio of ELF/VLF harmonic magnitudes is also shown to be a sensitive indicator of ionospheric modification, although it is somewhat less sensitive than the ELF/VLF magnitude. Last, the peak power level of the modulated HF beam was varied in order to assess the power dependence of ELF/VLF wave generation under both single‐ and dual‐beam heating conditions. Experimental and theoretical results indicate that accurate evaluation of the ELF/VLF power law index requires high signal‐to‐noise ratio; it is thus a less sensitive indicator of ionospheric modification than either ELF/VLF magnitude or the ELF/VLF harmonic ratio.