VLF wave generation by amplitude-modulated HF heater waves at Gakona, Alaska

Abstract

[1] Experiments conducted at Gakona, Alaska, using the intensity-modulated HF heating waves to interact with electrojet currents for the generation of VLF waves, are reported. An unexpected large increasing rate from 4 to 8 kHz in the frequency dependency of the VLF radiation intensity was observed. The peak value at 8 kHz was intense (about 7.5 dB above that of the 2 kHz signal used as a marker) and the wave intensity from 5 to 17 kHz appeared to be abnormally high (i.e., stronger than that at 2 kHz). In the experiments, we also observed the enhancement of spread-E irregularities at electrojet current altitudes due to the amplitude-modulated heater wave. These results and theoretical analyses suggest that temporally modulated electrojet currents mix with heater wave-excited density irregularities to form whistler mode currents, which generate VLF whistler waves directly with much larger intensities and better directivity than a Hertzian dipole can.