On the VLF wave generation by beating of two HF heaters

Abstract

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.

Determine the Physical Mechanism and Source Region of Beat Wave Modulation by Changing the Frequency of HF Waves

“…Furthermore, according to 2) and 3), the modulated VLF radiation is stronger when the HF waves were reflected in the region not too far below the foF2 layer rather than well below the foF2 layer, while the VLF radiation of underdense situation is weakest. This is consistent with the conclusion of [28].…”

“…thermal nonlinearity), which means BW is an electrojet-dependent modulation method whose source region is located in the D-E region. Whereas [16,[24][25][26][27][28] report that BW is an electrojet-independent modulation method, and the source region and mechanism of BW are F region ponderomotive nonlinearity. In this paper, BW modulation efficiency as a function of HF frequency is simulated for the two competing scenarios.…”

“…Whereas Refs. [16,[24][25][26][27][28] report that BW is an electrojet-independent modulation method, and the source region and mechanism of BW are F region ponderomotive nonlinearity.…”

The Generation of ULF/ELF/VLF Waves in the Ionosphere by Modulated Heating

“…Vartanyan et al [20] proposed two points: 1) The VLF waves observed at the LH frequency are due to the interaction of the LH waves with meter-scale field-aligned striations, and 2) the VLF waves at twice the LH frequency are due to the interaction of two counter propagating LH waves. This mechanism was also verified experimentally by Kuo and Lee [76]. Furthermore, Gigliotti et al [77] carried out an experiment using Large Plasma Device (LAPD) at the University of California, Los Angeles (UCLA), to generate polarized SAW using a rotating magnetic field (RMF) source created via a phased orthogonal twoloop antenna.…”

ELF/VLF Wave Radiation Experiment by Modulated Ionospheric Heating Based on Multi-Source Observations at EISCAT

“…Kuo et al experimentally determined that beat-wave-generated ELF/VLF signals could be strong even when AM-generated ELF/VLF signals were weak, and proposed a possible mechanism that included an ELF/VLF source induced by the ponderomotive nonlinearity of the BW transmission in the F region [13,14]. Subsequently, Kuo and Lee found that the intensity of VLF signals could be dramatically enhanced in the presence of irregularities in the ionosphere [15]. Meanwhile, Cohen et al predicted that BW generated in the D-region could be much stronger than that of AM, depending on the location of the receiver [16].…”