On the VLF wave generation by beating of two HF heaters

Nighttime very low frequency (VLF) wave generation high frequency (HF) heating experiments were performed under three distinctive ionospheric situations, employing two x-mode transmissions with a frequency difference of 3.5-9.5 kHz. The three situations were HF heater transmissions: (1) reflecting well below, (2) reflecting slightly below, and (3) penetrating through the F-peak (i.e., foF2 layer). The results reveal that (1) the N-S component of the VLF wave magnetic field dominated in all measurements; (2) the VLF radiation intensity is the strongest when the HF heaters are reflected slightly below the foF2 layer, i.e., in situation (2); (3) the VLF radiation intensity does not vary strongly with frequency; however, there is an anomaly in situation (2), the 5.5 kHz radiation intensity is approximately 7 dB above the intensity trend with frequency in situations (1) and (3). In situation (2), foF2 was decreasing with time, while a density cusp appeared slightly below the heater reflection height. Theoretical analysis indicates that the density cusp instigates the enhancement of VLF radiation intensity in situation (2) and the stimulated electromagnetic emission via the generation of 5.5 kHz lower hybrid waves by the HF beat wave is a likely mechanism for 7 dB intensity anomaly.