Optimized analysis of ionospheric amplitude modulated heating parameters for excitation of very/extremely low frequency radiations

Abstract

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.

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

“…Larchenko et al [27] found that there is a strong correlation between the strength of ELF/VLF waves generated by AM and the equivalent current, which is an infinitely thin sheet of current located at an altitude of 100 km. Yang et al [28] pointed out that AM with an X wave is more efficient than that with an O wave; in order to obtain better ELF/VLF generation, the optimal frequency of HF waves was found to be 0.8~0.9 and 0.75~0.85 times for the O wave and X wave, respectively.…”