DIRECT ALLOWANCE FOR THE EFFECT OF PHOTOELECTRONS IN IONOSPHERIC MODELING

Model calculations for the ionospheric E and F1 regions yield electron densities which are much too small, if no allowance is made for the p roduction of secondary ionization by primary photoelectrons. Full calc ulations of this secondary production are quite difficult, since the u pward and downward photoelectron fluxes must be determined as a functi on of energy at each height. Early studies showed that secondary ioniz ation increases the total production rate by about 30% in the F2 regio n, while more recent studies show increases of around 100% in the E an d F1 regions. Use of a fixed correction factor (for a given height and zenith angle) is not satisfactory, however, since the amount of secon dary production varies greatly for different radiation bands. This pap er describes a new approach in which a secondary production factor is determined for each ion and each radiation band. These factors (n(s)) are defined by the initial photon energy, and the mean energy of the f inal secondary electrons. For each radiation band the effective produc tion efficiencies, for each ion, are increased by the factor 1 + n(s). Modeling of the ionosphere then proceeds normally, with no other chan ges and no increase in computer time. All results automatically includ e a full allowance for secondary production, for any assumed values of zenith angle, atmospheric model, or EUV fluxes. Comparison with recen t, full photoelectron calculations shows that this procedure gives rel iable results, with errors which are less than those due to current un certainties in the solar fluxes, the photoionization cross sections an d the electron collision cross sections.