Ionospheric electron densities calculated using different EUV flux models and cross sections: Comparison with radar data

Abstract

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 (Ne) calculated using them compare with actual measurements collected by the incoherent scatter radar at Millstone Hill (42.6°N, 288.5°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 Ne 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 Ne 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 β (1026 Å) flux, but gives an unrealistically deep E‐F1 valley. The ionospheric models predict that the O2+ 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 EUV94X is significantly larger than that given by the EUVAC and earlier models. This is due to larger EUV fluxes in EUV94X compared to EUVAC over the entire 300‐1050 Å 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 Å. The differences between ionospheric model results for the different cross‐section sets are generally much smaller than the differences with the data.

Model results for the ionospheric E region: solar and seasonal changes

“…Ion composition measurements from rockets indicate that NO> is the major ion near the peak of the E-layer (Keneshea et al, 1970;Danilov and Semenov, 1978;Swider and Keneshea, 1993). Most modelling calculations show that NO> becomes less important than O> at heights below 130-140 km, as discussed by Buonsanto et al (1995). Results from the present model, with the improvements described in Sect.…”

Model results for the ionospheric E region: solar and seasonal changes

“…Ion composition measurements from rockets indicate that NO> is the major ion near the peak of the E-layer (Keneshea et al, 1970;Danilov and Semenov, 1978;Swider and Keneshea, 1993). Most modelling calculations show that NO> becomes less important than O> at heights below 130-140 km, as discussed by Buonsanto et al (1995). Results from the present model, with the improvements described in Sect.…”

“…Results obtained for the peak density N K E are, however, commonly about 30% below observed values, while the calculated width and depth of the valley between the E and F1 regions are perhaps two times too large (e.g. Titheridge, 1990;Buonsanto, 1990;Buonsanto et al, 1992;Tobiska, 1993). Because of the square-law loss process, an increase of 40% in N K E requires an increase of 100% in the ionisation rate at the peak of the E layer (at 105-110 km).…”

“…Comparisons between model results and observed electron densities in the E region have also been made by Buonsanto (1990) and Buonsanto et al (1992Buonsanto et al ( , 1995. These use data obtained with the Millstone Hill radar, and so relate to individual occasions rather than the mean ionosphere used in the present work.…”

“…Ion chemistry includes all the reactions normally considered (e.g. Buonsanto et al, 1992) Allowance is made for the direct production of N> ions, through ionisation of N or of N, since this is significant under some conditions. H> calculations are included, along with a full allowance for the effects of diffusion, winds, exospheric fluxes and temperature gradients (as summarised by Titheridge, 1993) to ensure a realistic F-layer as the upper bound of the valley region.…”