Generation Of Extremely Low Frequency Waves Via Modulated Ionospheric Heating For Communications Applications

Extremely low frequency (ELF) electromagnetic waves (0.3--3 kHz) have many applications including communications with submerged submarines, remote sensing of the lower ionosphere, and active wave-particle interaction experiments involving energetic particles trapped in Earth's radiation belts. However, waves in this frequency range are difficult to generate due to their large wavelengths (~100--1000 km) and therefore the large antennas required for efficient radiation. One relatively new and unconventional technique for ELF generation uses modulated heating of the lower ionosphere. The modulated ionospheric heating technique utilizes a high frequency beam (HF, 3--10 MHz) amplitude modulated at ELF frequencies to create a time-varying change in the conductivity of the lower ionosphere. When performed in regions where natural, large-scale horizontal ionospheric currents exist, such as the auroral electrojet at polar latitudes, the conductivity change results in a time-varying current that radiates at the ELF modulation frequency. While sidestepping the challenges of constructing a conventional ELF antenna, the modulated heating technique introduces new difficulties. The amplitude of the ELF waves depends strongly on the strength of the electrojet and the conversion between HF power and the ELF field is highly nonlinear. The purpose of this work is to characterize the dependence on electrojet strength and mitigate or utilize the nonlinearity between HF power and ELF fields so that experimenters and communication systems operators can optimize the generation of ELF waves using modulated ionospheric heating. Higher ionospheric density results in a large increase in the electrojet current strength but only a small increase in the generated ELF amplitude while lower ionospheric density can result in a weaker electrojet but only a small decrease in ELF generation. Harmonic content can be reduced by transmitting a predistorted HF power envelope that results in a sinusoidal ELF current at a particular altitude. This technique depends on a correct model of the ionosphere, which is highly variable in practice. Other modulation envelopes explored, such as square wave modulation, create stronger harmonics but can also result in higher generated ELF power or greater efficiency. Finally, we conduct experiments using quaternary phase shift keying (QPSK) to transmit digital data and examine how the bit error rate (BER) varies with generation conditions and changes in transmitter parameters.