Production of theE-Layer in the Oxygen Dissociation Region in the Upper Atmosphere

Abstract

Starting with the generally accepted theory that the $E$-layer is formed by ionization of ${\mathrm{O}}_{2}$ in the region of its dissociation, the probable value of the absorption cross section of ${\mathrm{O}}_{2}$ for this ionization is calculated by utilizing the height distribution of ${\mathrm{O}}_{2}$ (in the transition region) as recently obtained by Moses and Wu. It is found that, depending upon the temperature gradient and the boundary temperature chosen for the region of dissociation, the necessary absorption cross section varies from 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}19}$ ${\mathrm{cm}}^{2}$ to 9\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}18}$ ${\mathrm{cm}}^{2}$. Hence, it follows that for the two current hypotheses of $E$-layer ionization, namely, pre-ionization by solar rays in the wavelength range 900A-1000A (Nicolet) and ionization by high energy photons emitted from the solar corona (Hoyle and Bates), the ionization cross section of ${\mathrm{O}}_{2}$ should also lie within this range. For the former, the rate of ion production (as obtained by application of simple Chapman formula and assuming the sun to be radiating like a blackbody) appears to be one hundred times more than the observed rate. The discrepancy is removed if it is assumed that of the molecules excited to the pre-ionization levels by absorption, only a small fraction (one in a hundred) undergoes ionization. For the high energy photons it is found that in order to have the necessary absorption cross section, the energy should be 181 ev rather than 325 ev as obtained by Hoyle and Bates. It is suggested that both the pre-ionization process and the ionization by high energy photons are operative in producing $E$-layer ionization. The former produces the normal $E$-layer and the latter (of different frequencies) intensifies the ionization at different levels producing the fine structure of the $E$-layer as reported recently.