TheD-layer of the ionosphere

This paper discusses in two parts the lowermost region of the ionosphere, namely, the D-layer. In Part I, a résumé of the present state of our knowledge is given. In Part II, new theoretical studies are made on the formation and structure of the layer, based on contemporary knowledge. It is assumed that the D-layer is produced by ionization of O2 at the first ionization potential, as first suggested by S. K. Mitra, et al. The number densities of electrons and ions are calculated for various heights, taking due account of the variations of atmospheric temperature and effective recombination coefficient with height. The number density distribution of electrons does not show any maximum; that of ions, however, gives a maximum at nearly the same height as that of the rate of ion production. The distributions obtained have been used to give (i) the variation of reflection height with solar zenith angle (for long and very long waves), and (ii) values of the coefficient of reflection for various frequencies at the low frequency range.