IONOSPHERIC ELECTRON-DENSITIES CALCULATED USING DIFFERENT EUV FLUX MODELS AND CROSS-SECTIONS - COMPARISON WITH RADAR DATA

The recent availability of the new EUVAC (Richards et al., 1994) and EUV94X (Tobiska, 1993b, 1994) solar flux models and new wavelength bin averaged photoionization and photoabsorption cross section sets led us to investigate how these new flux models and cross sections compare with each other and how well electron densities (N-e) calculated using them compare with actual measurements collected by the incoherent scatter radar at Millstone Hill (42.6 degrees N, 288.5 degrees E). In this study we use the Millstone Hill semiempirical ionospheric model, which has been developed from the photochemical model of Buonsanto et al. (1992). For the F2 region, this model uses determinations of the motion term in the N-e continuity equation obtained from nine-position radar data. We also include two simulations from the field line interhemispheric plasma (FLIP) model. All the model results underestimate the measured N-e in the E region, except that the EUV94X model produces reasonable agreement with the data at the E region peak because of a large Lyman beta (1026 Angstrom) flux, but gives an unrealistically deep E-F1 valley. The ionospheric models predict that the O-2(+) density is larger than the NO+ density in the E region, while numerous rocket measurements show a larger NO+ density. Thus the discrepancy between the ionospheric models and the radar data in the E region is most likely due to an incomplete understanding of the NO+ chemistry. In the F2 region, the photoionization rate given by EW94X is significantly larger than that given by the EUVAC and earlier models. This is due to larger EUV fluxes in EW94X compared to EUVAC over the entire 300-1050 Angstrom wavelength range, apart from some individual spectral lines, In the case of EUVAC, this is partly compensated for by larger photoelectron impact ionization due to the larger EUV fluxes below 250 Angstrom. The differences between ionospheric model results for the different cross-section sets are generally much smaller than the differences with the data.