Determine the physical mechanism and source region of beat wave modulation by changing the frequency of high-frequency waves

This paper introduces a new approach for the determination of the source region of beat wave (BW) modulation. This type of modulation is achieved by transmitting high-frequency (HF) continuous waves with a frequency difference f, where f is the frequency of modulated ELF/VLF (extremely low frequency/very low frequency) waves from two sub-arrays of a high power HF transmitter. Despite the advantages of BW modulation in terms of generating more stable ELF/VLF signal and high modulation efficiency, there exists a controversy on the physical mechanism of BW and its source region. In this paper, the two controversial theories, i.e., BW based on D-E region thermal nonlinearity and BW based on F region ponderomotive nonlinearity are examined for cases where each of these two theories exists exclusively or both of them exist simultaneously. According to the analysis and simulation results presented in this paper, it is found that the generated VLF signal amplitude exhibits significant variation as a function of HF frequency in different source regions. Therefore, this characteristic can be utilized as a potential new approach to determine the physical mechanism and source location of BW.