VLF propagation effects of aD-region layer produced by cosmic rays

The single-layered D-region model based only on Lyman α photoionization of nitric oxide is inadequate to explain certain aspects of VLF radio propagation. Empirical evidence indicates the existence of a second and lower D layer which is important for VLF propagation. It is shown that normal cosmic rays produce sufficient ionization for such a layer. The electron-production and -depletion processes effective below 100 km and the latitudinal distribution of cosmic-ray primaries due to the geomagnetic field are discussed. A series of electron-density profiles obtained from photoionization and cosmic-ray ionization processes is compared with the two-layered D-region model deduced from electromagnetic measurements. The calculated phase and amplitude characteristics of VLF signals reflected from the lower cosmic-ray-induced layer during the presunrise hour are in good agreement with those measured.