| The radio spectrum between 300 Hz and 10 kHz (ELF/VLF) has broad applications to global communication, remote sensing of the ionosphere and magnetosphere, and subterranean prospecting. While lightning is a dominant source of these radio waves, artificial generation of these waves has posed an enduring challenge to scientists and engineers, due to the extremely long wavelengths (30-1000 km) and the lossiness of the Earth's surface at these frequencies.;Recently, ELF/VLF waves have been successfully generated by high frequency (HF, 3-10 MHz) heating of the lower ionosphere (60-100 km altitude), which changes the atmospheric plasma conductivity. In the presence of natural currents such as the auroral electrojet, ON-OFF modulation of this HF energy can impose an ELF/VLF alternating current onto those natural currents. This technique turns the lower atmosphere into a large antenna, which radiates energy downward into the Earth-ionosphere waveguide and upward into the magnetosphere.;While this technique remains one of the few means of reliable ELF/VLF wave generation, HF to ELF/VLF conversion efficiencies remain quite low. Utilizing the 3.6 MW HAARP HF heating facility in Alaska, we show that proper utilization of motion of the HF beam can boost the generated ELF/VLF wave power by as much as tenfold. Furthermore, as a result of having effectively created the world's first controllable large-element ELF/VLF phased array, directional launching of this energy becomes possible. We utilize theoretical models of the HF heating and cooling process, and of ELF/VLF wave propagation, to illuminate the observations and identify the physical mechanisms underlying the wave generation, particularly as it relates to motion of the HF beam. |
