Abstract
Ground-based high-frequency modulated waves can periodically heat the ionosphere and create “virtual antennas”, which can radiate extremely low frequency (ELF, 0.3–3 kHz) or very low frequency (VLF, 3–30 kHz) waves for long-distance communication. Ionospheric X-mode and O-mode heating experiments using amplitude and beat-wave (BW) modulations were conducted on 21 November 2019. Experimental results were analyzed from multiple perspectives based on data from Dynasonde, a magnetometer, stimulated electromagnetic emissions, an ELF/VLF signal receiver, and ultra-high-frequency radar. The strongest excited ELF/VLF signals in previous BW modulation heating experiments were around 8–12 kHz; however, in this experiment, no signal excited in this frequency range was observed, and the signal with the highest signal/noise ratio was at the frequency of 3517 Hz, which will aid in understanding the best communication frequency under different ionospheric backgrounds. It is well-accepted that the electron temperature changes periodically with the modulation frequency. However, we noted that the electron temperature had insufficient cooling during the O-mode modulated heating process and then increased again, resulting in a continuous electron temperature increase. We found that this was related to the change in ion composition after analyzing ion-line spectra, which will be helpful in studying the effect of modulation heating on the ionosphere background.
Highlights
Ground-based facilities using high-power high-frequency (HF) radio waves inject energy into the ionosphere through wave–particle and wave–wave interactions; this can artificially and temporarily change the local state of the ionosphere and cause various perturbation phenomena, such as electron-temperature enhancements, electron density fluctuations, stimulated electromagnetic emissions (SEEs), optical emissions, and extremely low frequency (ELF)/very low frequency (VLF) “virtual antennas” [1,2,3,4,5]
In the previou6s oBfW12 modulation heating experiments, the strongest signals of the excited ELF/VLF waves were observed around 8–12 kHz; in this experiment, no signal excited in this freoqbuseenrcvyedraanroguenwda8s–1o2bskeHrvze; dhotwhreovuegr,hinththeisfreexqpueernimcyenstw, neoepsipgnatatleerxnc.itTehdeinsitghniaslfrweqituhenthcye rhaignhgeswt saisgnoabls/enroviesed rtahtriouwgahsthate tfhreq3u5e1n7cHy zswfreeeqpuepnactyte.rTnh. iTs htreesnidgnwalhewrieththteheELhFig/VheLsFt signal/intoeinsesirtyativoarwieass watitthheth3e51p7reHsezt fmreoqduuelnactyio. nThfriesqtureenndcywwhaesreinthcoenEsLisFte/nVtLwFisthignthael irnetseunltssitoyfvparreievsiowuisthBtWhemporedsueltamtionduhleaatitoinngfreexqpueernimcyenwtassbiyncYoannsgistaenndt wKiutho t[h14e,1re9s,3u0lt]s
Multi-source data were used to observe the process of amplitude modulation (AM) and BW modulated heating to generate ELF/VLF waves
Summary
Ground-based facilities using high-power high-frequency (HF) radio waves inject energy into the ionosphere through wave–particle and wave–wave interactions; this can artificially and temporarily change the local state of the ionosphere and cause various perturbation phenomena, such as electron-temperature enhancements, electron density fluctuations, stimulated electromagnetic emissions (SEEs), optical emissions, and extremely low frequency (ELF)/very low frequency (VLF) “virtual antennas” [1,2,3,4,5] Among these phenomena, the generation of virtual antennas, which can radiate ELF/VLF waves over long ranges with minimal attenuation in the Earth–ionosphere waveguide, is important for communication with submarines and ground stations [6,7]. It is necessary to explore the frequency dependencies and ionospheric background conditions in VLF waves generated by the AM and BW approaches
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