Nighttime ionospheric D region: Equatorial and nonequatorial

Nighttime ionospheric D region parameters are found to be generally well modeled by the traditional H′ and β as used by Wait and by the U.S. Navy in their Earth‐ionosphere VLF radio waveguide programs. New comparisons with nonequatorial, mainly all‐sea VLF path observations reported over several decades are shown to be consistent with the previously determined height H′ ∼ 85.0 km and sharpness β ∼ 0.63 km−1. These paths include NPM (Hawaii) to Washington, D. C., Omega Hawaii and NLK (Seattle) to Japan, NWC (N.W. Australia) to Madagascar, and NBA (Panama) to Colorado. In marked contrast, transequatorial path observations (even when nearly all‐sea) are found to be often not well modeled: for example, for Omega Japan and JJI (Japan) to Dunedin, New Zealand, the observed amplitudes are markedly lower than those which would be expected from H′ ∼ 85.0 km and β ∼ 0.63 km−1, or any other realistic values of H′ and β. Other transequatorial observations compared with modeling include NWC to Japan, Omega Hawaii to Dunedin, and NPM (Hawaii) to Dunedin. It is suggested that the effects of irregularities in the equatorial electrojet may extend down into the nighttime D region and so account for the observed equatorial VLF perturbations through scattering or mode conversion.