Research progress on VLF radio wave propagation and observation in the Earth-ionosphere waveguide

Very low frequency (VLF) waves have the advantages of long-distance low-loss propagation and good phase stability, and have important applications in submarine communication, terrestrial navigation, timing, lower ionospheric remote sensing, and space weather monitoring. This paper reviews the research progress on the propagation and observation of ground-based VLF waves in the Earth-ionosphere waveguide. First, the development of global ground-based VLF transmitting stations and receiving observation systems is summarized. Then, based on the theory of Earth-ionosphere waveguide propagation, lower ionospheric medium models, reflection characteristic calculation methods, and field strength prediction methods, including waveguide-mode analysis, finite difference time domain (FDTD)/finite element method (FEM) methods, and data-driven approaches, are reviewed. On this basis, the effects of special space environment events, such as solar flares, geomagnetic storms, sunrise and sunset transitions, and earthquake-related lower ionospheric anomalies, on VLF propagation observations are analyzed. Finally, future research directions for VLF propagation and observation are discussed from the perspectives of refined lower ionospheric modeling, high-accuracy prediction in complex propagation environments, multi-source joint observation, and the incorporation of disturbance factors into prediction models.