This paper presents a new solar EUV flux model for aeronomic calculations (EUVAC), which is based on the measured F74113 solar EUV reference spectrum. The model provides fluxes in the 37 wavelength bins that are in widespread use. This paper also presents cross sections to be used with the EUVAC flux model to calculate photoionization rates. The flux scaling for solar activity is accomplished using a proxy based on the F10.7 index and its 81‐day average together with the measured solar flux variation from the EUVS instrument on the Atmosphere Explorer E satellite. This new model produces 50‐575 Å integrated EUV fluxes in good agreement with rocket observations. The solar cycle variation of the chromospheric fluxes agrees well with the measured variation of the Lyman α flux between 1982 and 1988. In addition, the theoretical photoelectron fluxes, calculated using the new EUV flux model, are in good agreement with the solar minimum photoelectron fluxes from the Atmosphere Explorer E satellite and also with the solar maximum photoelectron fluxes from the Dynamics Explorer satellite. Its relative simplicity coupled with its ability to reproduce the 50‐575 Å solar EUV flux as well as the measured photoelectron spectrum makes the model well suited for aeronomic applications. However, EUVAC is not designed to accurately predict the solar flux variability for numerous individual lines.

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Abstract. This paper is primarily concerned with the causes of the large density and temperature enhancements that are often observed during magnetically quiet periods on winter nights at midlatitudes in the North American sector. Measurements from a network of Digisondes and an incoherent scatter radar are compared with the field line interhemispheric plasma (FLIP) model for January 6-12, 1997, in order to examine the temporal evolution and geographical extent of the enhancements in eastern North America. Postsunset measurements at Millstone Hill show high electron temperatures accompanied by rapid density decay until midnight followed by a rapid temperature decay accompanied by a pronounced density enhancement in the early morning hours. The FLIP model reproduces the nighttime density enhancement well, provided the model is constrained to follow the topside electron temperature and also that the overlying plasmaspheric flux tube is full. The dramatic reduction in plasmaspheric heat flux near midnight results in a sharp decrease in ionospheric temperature, inducing a large downward flow of plasmaspheric ions which creates the nighttime enhancement in ionospheric density. We find that the nighttime plasmaspheric heat flux variation drives the nighttime ionospheric density variation, which is opposite to conclusions in previously published work. Although the plasmaspheric heat flux variation can explain the ionospheric density variation, the reasons for this heat flux variation are not understood. Convection of plasma from higher magnetic latitudes is now included in the FLIP model but is not needed to produce the observed nighttime density maximum. We have found that the fraction of light ions in the topside ionosphere at 500 km altitude in the model is very close to that obtained from chemical equilibrium and agrees well with the measured fraction. The model generally reproduces the daytime electron density very well at all stations except Bermuda, where the difference is as much as 50%.

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.

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