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.

Modulation heating of the ionosphere using high-power high-frequency (HF) radio waves can generate an extremely low frequency (ELF) radiation source in the ionosphere, such as amplitude-modulation (AM) heating of the auroral electrojet or using two HF radio waves to illuminate the same region of the ionosphere to form beat-wave (BW) modulation. We first present observations of ELF waves received on the China seismo-electromagnetic satellite (CSES) and on the ground simultaneously, which were excited in the European Incoherent Scatter Scientific Association (EISCAT) modulation heating experiment carried out alternately in the AM and BW modes. When the CSES passed over the EISCAT heater, ELF waves excited in the AM and BW modes were received successfully by an electric field detector on board. The comparison of the received results between the satellite and the ground demonstrates that although the intensity of the ELF signals excited in the BW mode is far lower than that in the AM mode on the ground, the ratio of ELF wave intensities between the satellite and the ground in the BW mode is significantly larger than that in the AM mode. According to the experimental results, the propagation characteristics are simulated using the full-wave method when the ELF radiation sources are located in the D and F regions in the ionosphere.

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 radiation efficiency of VLF/ELF waves in AM ionospheric heating experiments. Based on numerical simulation, ranges of parametric selectivity in optimal duty cycle and heating frequency (fHF) are derived. The IRI model and 2-parameter model are used to predict background electron density profiles and optimized ranges of duty cycle for different density profiles are analyzed and compared. Influences of wave polarizations on optimal duty cycle are also discussed. It is shown that intensity of the VLF/ELF equivalent radiation source ( ) firstly rises and then falls with increase of duty cycle. When using the IRI model, peaks at a duty cycle of 50%, optimally ranged from 40% to 70%. For the 2-parameter model case, an optimal duty cycle is 40% and the optimized ranges vary from 30% to 60%. Heating with an X-mode polarization is more efficient than O-mode case in VLF/ELF wave generation. Nevertheless, optimal duty cycle is almost independent of HF wave polarizations. To obtain better VLF/ELF generation, optional fHF may be 0.8 to 0.9 times of foE for O-mode heating and 0.75 to 0.85 times for X-mode polarization case. Finally, the variations of these two parameters in different latitudes are discussed.

During the course of ionospheric heating experiments, an apparent electron density enhancement was observed at the European Incoherent Scatter Scientific Association (EISCAT). The enhancement extended over a wide range of altitudes, above the reflection altitude of the high-frequency pump wave. However, it remains open whether this enhancement actually corresponds to a true enhancement in electron density. Following the dispersion relation of ion acoustic waves, the frequency ratio of the enhanced ion line to the background ion line suggests that the profile of the effective ion mass might remain unchanged. Furthermore, the solar radio flux and ion drift velocity indicated no significant changes in the ion species and their densities. In conclusion, the electron density enhancement observed at EISCAT, might not in fact be a true enhancement.