Abstract
Modulation of electrojet currents by amplitude-modulated HF heating waves creates ionospheric antenna currents to generate ELF/VLF waves. The dependence of the generation efficiency on the HF heating wave modulation scheme and frequency is studied experimentally and numerically. Three heating wave modulation schemes: (1) rectangular wave, (2) sine wave, and (3) half-wave rectified wave are examined. The experimental results show that the half-wave rectified wave modulation scheme is the most efficient one to generate signals at the modulation frequency and its second harmonic, corroborating theoretical predictions. The equations modeling the radiation process and experimental measurements include the contribution from the near field, in addition to the far field, of the antenna current and the resonant effect on the wave intensity generated in the resonator formed by the conducting ground and the plasma layer in the lower ionosphere. The numerically calculated dependency of the radiation intensity on the modulation frequency is shown in good agreement with that measured experimentally for all three modulation-schemes.
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