Ionospheric <i>D</i> Region: Characteristics Near Dawn and Dusk

Abstract The characteristics of very low frequency (VLF) radio wave propagation in the Earth‐ionosphere waveguide are determined particularly through dawn and dusk using phase and amplitude measurements of man‐made signals propagating below the ionospheric D region. For the first time variations of “Wait” height and sharpness parameters, H' and β, have been determined for dawn and dusk conditions. These measurements provide observational data to constrain D region modeling efforts, extending the capabilities of VLF propagation monitoring for geophysical phenomena such as lightning, solar flares, and energetic particle precipitation. At mid‐latitudes, H' varied from ∼85 km at night, then, starting from solar zenith angle (SZA) ∼ −97.5°, rapidly down to ∼73 km at dawn (SZA = −90°), then back up to ∼78 km at SZA ∼ −75° and then down to the appropriate noon value for the latitude (and season). In contrast, from noon through dusk to night, H' varied essentially monotonically from ∼70 to 75 km through ∼80 to ∼85 km. At low latitudes no dawn minimum in H' was observed, due to the reduced effect of galactic cosmic rays (GCR). Sharpness, β , varied from its nighttime value of ∼0.6 km −1 down to a minimum of ∼0.25 km −1 at SZA ∼85° near dusk or ∼75° near dawn, rising again to (SZA‐dependent) noon values of ∼0.35–0.5 km −1 . The results are interpreted through the geophysical effects controlling D region electrons, including the daytime dominant role of solar Lyman‐α from low to mid‐latitudes, and the greater role of GCR at increasingly higher mid‐latitudes.