Numerical models of the E-region ionosphere

Abstract

Although the ambient E-region of the ionosphere has been fairly well characterized in the non-auroral regions with respect to density, ion composition, and variability, it is not as well understood as might be expected from a theoretical point of view. This is due primarily to uncertainties in the solar irradiance that produces E-region ionization (mainly the H Lyman-β line and the soft X-ray region), and to the variability of odd-nitrogen species that control ion composition. Recent measurements from the SNOE, TIMED, and SORCE satellites have greatly improved our quantitative understanding of both solar spectral irradiance and lower thermosphere nitric oxide, which in turn improves our ability to accurately model the E-region. The new measurements indicate higher solar soft X-ray irradiance but lower flux in the H Lyman-β line than earlier estimates by Hinteregger et al. [Hinteregger, H.E., Fukui, K., Gilson, B.R., Observational, reference, and model data on soalr EUV, from measurements on AE–E. Geophys. Res. Lett. 8 (1981) 1147] based on measurements from Atmosphere Explorer and from rocket flights. However, preliminary results from the Solar EUV Experiment (SEE) on the TIMED satellite are in better agreement with the EUVAC model [Richards, P.G., Fenenelly, J.A., Torr, D.G., EUVAC: a solar EUV flux model for aeronomic calculations. J. Geophys. Res. 99 (1994) 8981–8992]. This raises the question of whether either of these representations of the solar spectrum can adequately represent E-region electron density and variation. Model simulations of the E-region using these two solar model inputs are compared to the International Reference Ionosphere (IRI) empirical model, which has been validated using measurements from the Boulder ionosonde. Calculations using the Hinteregger et al. solar inputs significantly underestimate the peak electron densities at all levels of activity, while using EUVAC solar inputs produces better agreement but slightly more variability than the empirical representations. As improved estimates of variability from TIMED/SEE over a complete solar cycle become available, these remaining discrepancies may be resolved.