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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.
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.
In this paper, the experimental results of the extremely/very low frequency (ELF/VLF) radiation from the ionosphere by amplitude modulation (AM) mode and dual-beam beat-wave (BW) mode modulation heating using the European Incoherent Scatter Scientific Association (EISCAT) heating facility are presented. By comparing the dependence of the ELF/VLF radiation sources formed by AM and BW models on the geomagnetic field disturbance and the differences in different periods, this paper aims to examine some issues such as source regions and mechanisms addressed in the theoretical study of ionospheric high frequency modulated heating. The results show that (1) in the AM mode, the intensity of the ELF/VLF radiation source depends on the intensity of geomagnetic disturbance and thus on the magnitude of natural currents in the ionosphere, while in the BW mode, the intensity of the ELF/VLF radiation source is less dependent on the intensity of geomagnetic disturbance; (2) during the relatively quiet period of geomagnetic disturbance, the intensity of the ELF/VLF radiation source formed by AM mode modulation heating decreases and by BW mode modulation, heating increases when the ionization of the lower ionosphere decreases. The results of our EISCAT modulation experiments presented in the current paper and the earlier one by Yang et al. [“The polarization characteristics of ELF/VLF waves generated via HF heating experiments of the ionosphere by EISCAT,” Phys. Plasmas 25, 092902 (2018)] support that the BW mode heating mechanism mainly results from the ponderomotive nonlinearity, and the modulation region is located in the F layer.
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