Remote Sensing of the Lower Ionosphere during the April 8, 2024 Solar Eclipse by VLF Transmitter Signal Variations Recordings

Solar eclipses provide opportunities to study the short-time scale dynamics of the physical and chemical processes of the ionospheric D region. Measurements have been made previously using rocket-borne instruments and by receiving signals crossing the region of totality from man-made VLF transmitters. Whereas rockets tend to provide snapshot measurements of the electron density of the lower ionosphere, monitoring VLF transmissions allows for continuous indirect sensing with high time resolution. Radio waves from VLF transmitters propagate inside the waveguide formed by the lower ionosphere and the Earth’s surface. Significant variations in the received amplitude and phase arise from changes in the D region ionosphere. We use the full solar eclipse to collect new multi-point ELF and VLF observations. The map of the transmitters and detector locations in relation to the ionosphere level totality is shown in Figure 1. We utilize the ELF and VLF mobile stations to achieve strategic transmitter-receiver geometries relative to the eclipse trajectory to characterize the D region structure and dynamics in and near the totality zone, improving the spatial resolution of past works to characterize (1) how steep the electron density gradients in the vicinity of totality; (2) the observed ionospheric perturbations when the region of totality is near a powerful VLF transmitter; (3) the lower ionosphere characteristic reaction time to a point like perturbation from the eclipse