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.

Abstract. This paper examines the ability of empirical and physical models to reproduce the peak electron density of the midlatitude ionospheric F2 region (NrnF2) from 1976 to 1980. The data from all midlatitude stations show a tendency toward a semiannual variation in noon NrnF2 with peaks at the equinoxes for all levels of solar activity. The Southern Hemisphere semiannual variation is more pronounced than in the Northern Hemisphere primarily because the winter density is relatively low in the Southern Hemisphere. At most locations the equinox density peaks are approximately equal. However, the September peak is much weaker than the March peak at most

Abstract. The equatorial plasma fountain and equatorial anomaly in the ionospheres over Jicamarca (77øW), Trivandrum (77øE), and Fortaleza (38øW) are presented using the Sheffield University plasmasphere-ionosphere model under magnetically quiet equinoctial conditions at high solar activity. The daytime plasma fountain and its effects in the regions outside the fountain lead to the formation of an additional layer, the F 3 layer, at latitudes within about plus or minus 10 ø of the magnetic equator in each ionosphere. The maximum plasma concentration of the F 3 layer, which occurs at about 550 km altitude, becomes greater than that of the F2 layer for a short period of time before noon when the vertical E x B drift is large. Within the F 3 layer the plasma temperature decreases by as much as 100 K. The ionograms recorded at Fortaleza on January 15, 1995, provide observational evidence for the development and decay of an F 3 layer before noon. The neutral wind, which causes large north-south asymmetries in the plasma fountain in each ionosphere during both daytime and nighttime, becomes least effective during the prereversal strengthening of the upward drift. During this time the plasma fountain is symmetrical with respect to the magnetic equator and rises to over 1200 km altitude at the equator, with accompanying plasma density depletions in the bottomside of the underlying F region. The north-south asymmetries of the equatorial plasma fountain and equatorial anomaly are more strongly dependent upon the displacement of the geomagnetic and geographic equators (Jicamarca and Trivandrum) than on the magnetic declination angle (Fortaleza). IntroductionThe horizontal orientation of the geomagnetic field at the geomagnetic equator is known to be the basic reason for the active nature of the low-latitude ionosphere, which is characterized by the equatorial electrojet, equatorial plasma fountain, equatorial anomaly, plasma bubbles, and spread F. The equatorial plasma fountain and equatorial anomaly arise from the vertical upward drift of plasma across the geomagnetic field lines at equatorial latitudes due to E

One of the greatest uncertainties in modeling the ionospheric densities and temperatures lies in the neutral winds that affect the vertical ion drift. Wind measurements are difficult to make, and current wind models are not adequate. An alternative is to deduce an equivalent meridlonal neutral wind from measurements of the height of the F2 peak electron density. This method has been effective in allowing ionospheric modelers to reproduce the observed height with reasonable precision when the peak height does not vary rapidly. However, when there are rapid movements of the peak height, this method underestimates the magnitude of the necessary wind changes, and it is also complicated and computer intensive to do accurately. This paper introduces an improved algorithm for use in time dependent ionospheric models that reduces the complexity of the procedure while more accurately reproducing the observed heights. The improvement is achieved by estimating the wind that is needed to reproduce the observed height at the next time step using the calculated height and wind at the current time step. Thus the winds are continuously being a•usted to bring the calculated height into better agreement with the observed height during the time dependent simulation. The winds from the new algorithm agree well with both optical and radar measurements made at Arecibo on August 17-18, 1982. 1.density (nmF9.) and the height at which the peak density occurs (hmF•). Normally, the wind is poleward during the day, enhancing the downward diffusion to a region of increased loss and thereby reducing the peak density (nmF2) and lowering the height of the layer. At night the wind becomes equatorward, inhibiting the downward diffusion, raising the height of the layer, and helping maintain the nighttime F• layer density. Thermospheric neutral winds have been measured from satellites with mass spectrometers [Spencer et al., 1981] and by Fabry-Perot spectrometers [Hays et al., 1981]. Currently, no satellite is making wind measurements. Ground-based measurements continue to be made with both Fabry-Perot instruments and incoherent scatter radars. However, neither of these methods can supply wind measurements on the global scale needed to model the ionosphere. The optical measurements are linfited by such factors as clouds, daylight, and the phase of the Moon. The radar measurements require expensive equipment and are often unavailable for routine ionospheric measurements. The lack of suitable measurements on a global scale makes it difficult to accurately model the ionospheric electron density. This in turn makes it difficult to accurately model the many emissions, densities, and temperatures that are sensitive to electron density. Richards and Tort [1986] to suggest using the height of the peak F•. region electron density (h,,.F2) to deduce neutral winds. The relationship between h,,,F2 and the meridional wind has been shown to be approximately linear for small winds under steady state conditions 17,839 17,840 RICHARDS: WINDS FROM HEIGHT OF F2 PEAK ...

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