Experimental comparisons between AM and BW modulation heating excitation of ELF/VLF waves at EISCAT

Abstract

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.

Experimental ELF/VLF Wave Communication With Excitation by Ionosphere Modulated Heating

“…The HF heating wave frequency was 4.04 MHz, polarization was an X wave, and the effective radiation power was 75.5 ∼ 83.4 MW. As in previous experiments (Yang et al, 2018(Yang et al, , 2019(Yang et al, , 2020, the ELF/VLF receiver was located at Breivikeidet, 15 km east of heating facility, and the ionosonde and magnetometer for ionospheric background detection were located at the EISCAT station and at the University of Tromsø, respectively. The transmitted data packet consisted of synchronization and equalization sequences, as well as communication and random symbols and contained a total of 256 QPSK symbols.…”

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

“…However, Cohen et al [10] also pointed out that the F layer BW modulation theory proposed by Kuo et al [47,48] could not be ruled out at present, and needs to be further tested and verified. Therefore, it is still controversial whether the source region of ELF/VLF waves generated by BW modulation is located in the D layer [52][53][54][55][56] or F layer [57][58][59][60].…”

A Novel Method to Identify the Physical Mechanism and Source Region of ELF/VLF Waves Generated by Beat-Wave Modulation Using Preheating Technique

“…Experiment [23] carried out on December 4, 2018 at Tromsø, EISCAT is chosen as an example to illustrate the importance of D region absorption to BW in F region. Figure 5 (a) and (b) are ionograms on 1000 UT and 1210 UT respectively.…”

“…Some scientists [12][13][14][15][16][17][18] believe that the source region of BW modulation is in the D region and the mechanism of beat-wave modulation is in essence consistent with AM (i.e., thermal nonlinearity) which is an electrojet-dependent modulation method. Others [19][20][21][22][23] hold the idea that the source region of BW modulation is in the F region, which means BW modulation is an electrojet-independent modulation method whose mechanism is ponderomotive nonlinearity.…”