A study of the daytime E‐F1 region ionosphere at mid‐latitudes

Abstract

A photochemical equilibrium daytime model is used to study the ionosphere between 110 and 180 km at mid‐latitudes. The model includes the latest photoionization and photoabsorption cross sections, extreme ultraviolet (EUV) fluxes in 37 wavelength bands, and all reactions believed to be important in this region. Model results are compared with (1) noon‐time E layer critical frequency (foE) at Boulder and Wallops Island over a full solar cycle; (2) Millstone Hill incoherent scatter radar observations of electron density at 180 km (N180) for a wide variety of seasons and solar geophysical conditions; (3) selected Millstone Hill incoherent scatter profiles of electron density between 110 and 180 km which included E‐F1 valley minima; and (4) the ratio of the molecular ion concentration to the total ion concentration at 180 km for noon throughout the solar cycle as given by both the IRI‐86 ion composition model and the semiempirical ion composition model of Oliver. Best agreement between the photochemical model documented in this paper and the observations and ion composition models is generally obtained if (1) the EUV fluxes in the photochemical model are increased by 25–30% above values derived from published reference spectra; (2) neutral densities used in the photochemical model are decreased by 25% below those given by MSIS‐86 at equinox, with larger decreases in winter, and smaller or no decreases in summer. The results show that this region of the ionosphere can be modeled with reasonable success given our current state of knowledge. Modeling this region of the ionosphere is important for resolving ambiguities in true height analysis of ionograms and reduction of incoherent scatter spectra. Improved modeling requires more accurate values of aeronomical parameters, i.e., ionizing fluxes, cross sections, reaction rates, composition and temperature.