Beating HF waves to generate VLF waves in the ionosphere

Details Beating HF waves to generate VLF waves in the ionosphere Beating HF waves to generate VLF waves in the ionosphere

Mar 2012

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012jgra..11703318k&link_type=abstract

Journal of Geophysical Research, Volume 117, Issue A3, CiteID A03318

Physics

Plasma Physics

Ionosphere: Active Experiments, Ionosphere: Wave Propagation (0689, 3285, 4275, 4455, 6934), Radio Science: Nonlinear Phenomena (4400, 7839), Space Plasma Physics: Active Perturbation Experiments, Space Plasma Physics: Wave/Wave Interactions

Scientific paper

Beat-wave generation of very low frequency (VLF) waves by two HF heaters in the ionosphere is formulated theoretically and demonstrated experimentally. The heater-induced differential thermal pressure force and ponderomotive force, which dominate separately in the D and F regions of the ionosphere, drive an electron current for the VLF emission. A comparison, applying appropriate ionospheric parameters shows that the ponderomotive force dominates in beat-wave generation of VLF waves. Three experiments, one in the nighttime in the absence of D and E layers and two in the daytime in the presence of D and E layers, were performed. X mode HF heaters of slightly different frequencies were transmitted at CW full power. VLF waves at 10 frequencies ranging from 3.5 to 21.5 kHz were generated. The frequency dependencies of the daytime and nighttime radiation intensities are quite similar, but the nighttime radiation is much stronger than the daytime one at the same radiation frequency. The intensity ratio is as large as 9 dB at 11.5 kHz. An experiment directly comparing VLF waves generated by the beat-wave approach and by the amplitude modulation (AM) approach was also conducted. The results rule out the likely contribution of the AM mechanism acting on the electrojet and indicate that beat-wave in the VLF range prefers to be generated in the F region of the ionosphere through the ponderomotive nonlinearity, consistent with the theory. In the nighttime experiment, the ionosphere was underdense to the HF heaters, suggesting a likely setting for effective beat-wave generation of VLF waves by the HF heaters.

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