Optimized analysis of ionospheric amplitude modulated heating parameters for excitation of VLF/ELF radiations

It is now well known that amplitude modulated (AM) high frequency (HF) radio wave transmissions into the ionosphere can be used to generate very/extremely low frequency (VLF/ELF) radio waves using the so-called 'electrojet antenna'. Duty cycle and heating frequency are analyzed and discussed with the lower-ionosphere modulated heating model, so as to improve the radiation efficiency of VLF/ELF waves in AM ionospheric heating experiments. Based on numerical simulation, the ranges of parametric selectivity in optimal duty cycle and heating frequency (f(HF)) are derived. The International Reference Ionosphere 2015 (IRI-2015) model and two-parameter model are used to predict background electron density profiles, and optimized ranges of duty cycle for different density profiles are analyzed and compared. The influences of wave polarizations on optimal duty cycle are also discussed. It is shown that intensity of the VLF/ELF equivalent radiation source (M) firstly rises and then falls with the increase of duty cycle. When using the IRI model, M peaks at a duty cycle of 50%, optimally ranging from 40% -70%. For the two-parameter model case, an optimal duty cycle is 40% and the optimized ranges vary from 30%-60%. Heating with an X-mode polarization is more efficient than with the O-mode case in VLF/ELF wave generation. Nevertheless, an optimal duty cycle is almost independent of HF wave polarizations. To obtain better VLF/ELF generation, optional f(HF) may be 0.8-0.9 times of f(o)E for the O-mode heating and 0.75-0.85 times for the X-mode polarization case. Finally, the variations of these two parameters in different latitudes are discussed.