Propagation of Very Low Frequency Transmitter Signals in the Inner Magnetosphere




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Signals from ground-based very low frequency (VLF) transmitters can leak through the ionosphere and propagate in the inner magnetosphere as whistler-mode waves. They interact with energetic electrons from the Earth’s radiation belts, and precipitate them into the ionosphere. The effect of wave-particle interactions is affected by signal propagations, which depend on the spatial variation of the cold plasma population (as the propagation medium). Therefore, a further understanding of transmitter signal propagation and cold plasma medium is essential for investigating wave-particle interactions. First, a case study is performed on Russian Alpha transmitter signals observed by the Van Allen Probes. The signals are in ducted propagation, experience multiple reflections, and excite triggered emissions. The ducted propagation is justified by a ray-tracing technique, and the nonlinear cyclotron resonance theory is tested by the observed triggered emissions. Second, we perform a statistical study on the distribution of the two propagation modes, ducted and nonducted, by the use of observed Russian Alpha transmitter signals. The statistics show the dominance of nonducted signals in the plasmasphere in terms of both occurrence and power. The proportion of ducted signals is enhanced at higher L-shells and during active geomagnetic conditions. Finally, we statistically analyze the spatial and temporal distributions of inner-magnetospheric cold plasma density irregularities, which are responsible for ducted propagation. The density irregularities deep inside the plasmasphere are dominant in the night and dusk sectors and show no significant variation with geomagnetic conditions. In contrast, the density irregularities in and near the plasmasphere boundary layer occur at post-midnight during quiet times and expand throughout the night sector during active times.



Physics, Atmospheric Science, Physics, Fluid and Plasma, Physics, Electricity and Magnetism