Ionospheric D Region: Characteristics Near Dawn and Dusk

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.