Propagation of ELF waves generated by an HF ionospheric heater in the earth's plasma environment [electronic resource]
Keywords: Electrical Engineering

Electromagnetic waves in the Extremely Low Frequency range (ELF, 30-3000 Hz) have broad application in physics and engineering such as ionospheric and underground remote sensing and global submarine communications. Additionally, ELF waves can resonantly interact with energetic electrons, an important process that results in the removal of trapped electrons from the radiation belts. ELF waves can be generated by lightning discharges and by natural processes in the Earth's magnetosphere. However, it is extremely difficult to generate ELF waves artificially due to their long wavelengths. In this work, the High Frequency Active Auroral Research Program (HAARP) transmitter array is used to generate ELF waves. The HAARP array generates ELF waves by heating the lower ionosphere with a powerful (3.6 MW) high frequency (2.75-10 MHz) beam. The heating is modulated at an ELF frequency resulting in modulation of the natural auroral electrojet current, which in turn radiates at the ELF frequency. For four years, a set of experiments was conducted in which ELF signals generated by HAARP were detected by the DEMETER satellite at an altitude of 670 km. In addition to observations, the distribution of ELF power is examined with several modeling techniques to explain the observed features. In the experiments and modeling, three distinct regions of ELF radiation are identified. Region 1, the most important region, is a column of radiation propagating upward into space with a horizontal extent of about the size of the heated region (50-100 km) and average field strengths of 100-150 uV/m at 2 kHz. In Region 2, which can extend up to 300 km laterally from HAARP, it is believed that the waves reach the satellite by propagating directly from the source in the ionosphere without reflection from the ground. In contrast, in Region 3, which can extend to 1000 km from HAARP, the generated waves first propagate in the Earth-ionosphere waveguide and partially leak through the ionosphere to be detected on the spacecraft. During the nighttime, the intense column of radiation (Region 1) is displaced by about 100 km horizontally to the south from the HAARP field line. During the daytime, there is no substantial north-south displacement from the HAARP field line. A horizontally homogeneous full-wave model is used to facilitate the physical understanding of the wave propagation. The model accurately predicts the extents of the three regions during daytime and nighttime conditions as well as the location of Region 1 during daytime. However, during the nighttime the model predicts that the column should be up to 100 km north of the HAARP field line. It is proposed that the displacement in observations during the nighttime is caused by a horizontal electron density gradient within the main ionospheric trough. Using ray tracing simulations, we estimate that the gradient of this trough should be an order of magnitude change over a latitude range of 3-5 degrees. It is also demonstrated that the main ionospheric trough is an important parameter of the medium above HAARP not only for ELF observations but also for other types of experiments too. It is found to occur over HAARP during the nighttime in at least 50% of our cases. The first satellite observations of one-hop and two-hop ELF waves generated via HF heating are reported. Among the important new understandings is the fact that daytime is preferential for this type of ELF generation and propagation to the conjugate region. The signal during the daytime is observed almost two times more often than during the nighttime, and triggered emissions are observed only in the daytime. We also find that the region with the strongest signal is displaced about 300 km toward the equator, and the signal is overall higher toward the equator than toward the pole. It is hypothesized that this can be the result of plasmapause guiding. Another important result is the fact that one/two-hop signals are observed over a long range of distances (> 1000 km) and over a wide range of L-shells, although always with roughly constant time delay. This observation suggests that the propagation in the magnetosphere is within the narrow range of L-shells or within a duct, and wide range in the observations is the result of ELF wave backscattering from the ionosphere.