A plasmasphere extension has been incorporated in the International Reference Ionosphere, IRI*, using the Russian standard model of the ionosphere, SMI, at altitudes from 1000km to the plasmapause (⩽36,000km). The IRI model has been improved using global space–time statistics of the half-width of the topside ionosphere estimated from ISIS1, ISIS2, and IK19 satellite observations. Chebishev orthogonal polynomial approximations of the topside half-width of the electron-density profile, normalized to the F2-layer peak height for the ranges of geomagnetic latitude, local time, and solar activity, are incorporated in the IRI electron-density profile. Comparison of the Chebishev model of the topside half-width and its standard deviation with IRI-Bent and ISIS/IK19 data demonstrates an improvement of the IRI-Bent model using the new anchor point in the topside ionosphere. Extension of IRI ionospheric electron temperature towards the plasmasphere is made with a revised model for the electron temperature in the upper ionosphere and plasmasphere. The previous model equation is modified to produce height variations in accord with recent theory and observations. Field-aligned profiles then depend on three parameters, which are determined as a function of latitude by fitting the modified equation to satellite data at heights of 400–10,000km. This leads to a new global model giving Te as a function of height, latitude, local time and season, with first-order corrections for changes with solar flux and magnetic activity. Estimates of the ion temperature Ti are also obtained. Results represent a mean of current experimental data, and give smooth, physically realistic variations under all conditions.

Experimental data on the width and depth of the valley above the ionospheric E layer are available for only a limited range of conditions. Theoretical models are reasonably well defined in this region, where time constants are short, and they now provide a good match for most observations. Calculations can therefore be used to study the changes that will occur under different conditions, due to known changes in atmospheric composition, EUV radiation and the solar zenith angle χ. Results are given for the shape of the E layer, and the E–F valley region, throughout the day and for different geophysical conditions. Valleys are generally 10–15km wide near noon, with a depth of 4–7%. Both width and depth increase at larger zenith angles, varying approximately as (secχ)0.6. At sunset the day production peak rises until it disappears near 150km, while the stable night peak becomes visible at 105km. This differs appreciably from the variations assumed in the International Reference Ionosphere. A numerical model (available as a FORTRAN programme) is derived to give a close approximation to the full theoretical calculations under all conditions. The model begins at 80km, to include some realistic D region ionisation, and extends well into the F1 region giving smooth, physically based variations as a function of height, local time, latitude, season and solar flux.

A full ionospheric model, including the four night-time sources identified by Strobel et al. (1980), is used to calculate electron density profiles throughout the E,F1 and F2 regions. Full allowance is made for the effects of secondary production, atmospheric winds, and a realistic NO model. Near midnight, results show an E-layer peak at 105km with a density of 2–2.6×103cm−3 under most conditions. The peak thickness corresponds to a scale height of ≈5km. A wide valley, with a mean density of typically 1.2–1.6×103cm−3, extends from ≈120km to the sharply defined base of the F2 layer at 190–225km. This agrees with rocket and backscatter observations, but differs considerably from the deep, narrow valley assumed in the International Reference Ionosphere. IRI also has a night E layer that is far too thick (and too dense, near sunset). Experimental data on densities in the night E and F1 regions are available for only a limited range of conditions. Model calculations are therefore used to study the changes that will occur under different conditions, due to known changes in atmospheric composition and EUV radiation. In particular we examine electron density profiles across sunrise and sunset, when changes are rapid and observations are difficult. A numerical model is derived to give a close approximation to the full theoretical calculations under all conditions. It begins at 80km, to include some realistic D region ionisation, and extends into the lower F2 region. Combined with the day model described previously, it is available as a FORTRAN program that gives smooth, physically based variations at all times of day, at all heights up to at least 230km. Results are obtained as a function of local time, height, latitude, season, solar flux and magnetic activity. Based on full theoretical calculations, they give a good approximation to such data as are available and provide a best guess for unobserved regions. Results avoid the unacceptable variations and gradient discontinuities evident in most IRI profiles for the E and F1 regions.

The global maps for the E layer peak density are considered to be adequately good for daytime and new formulations for nighttime have been included in existing models. However, there are doubts about the validity of the shape of the electron density profile in E region heights. J. Titheridge has developed an E region chemistry model which provides improvements of the profile shape. We have modeled this shape on the basis of polynomial expansions. The resulting E region profile can be combined with existing ionospheric models, e.g., with the International Reference Ionosphere (IRI) or with the ‘family’ of models recently developed at Graz and Trieste.

A recent study of data from Arecibo concluded that the night E layer must consist primarily of metallic ions produced by meteors. Full model calculations show that this is not necessary. Known EUV sources (from starlight, scattered sunlight, and F region recombination) will produce an E region that agrees well with Arecibo results throughout the night. Direct rocket observations confirm that metallic ions are insignificant in the E region at all times. Model and data also agree on the absence of any appreciable changes with season or solar activity in the night values of NmE. The large solar cycle change in the IRI-95 model seems to come from an incorrect application of F region theory and should be deleted.

A recent study of data from Arecibo concluded that the night E layer must consist primarily of metallic ions produced by meteors. Full model calculations show that this is not necessary. Known EUV sources (from starlight, scattered sunlight, and F region recombination) will produce an E region that agrees well with Arecibo results throughout the night. Direct rocket observations confirm that metallic ions are insignificant in the E region at all times. Model and data also agree on the absence of any appreciable changes with season or solar activity in the night values of N m E . The large solar cycle change in the IRI‐95 model seems to come from an incorrect application of F region theory and should be deleted.

Calculations with a full time-varying model are used to study changes in the height and density of the E-layer peak, caused by known changes in the neutral atmosphere. Agreement with mean observed values of NmE requires an increase of 10% in calculated ion densities, and an increase of 33% in the solar-maximum EUV model at λ<150 Å. At a fixed site, changes with the solar zenith angle χ agree well with the simple Chapman theory during most of the daylight hours. Simple modifications to the Chapman equations give improved accuracy near sunrise and sunset. When corrected for changes in χ, model results for summer and equinox show a decrease in the peak density NmE at increasing latitudes. The overall change agrees well with experimental data, as summarised in the IRI model. Known changes in the neutral atmosphere also reproduce the increase in NmE in winter, at latitudes up to 30°. The continuing increase at higher winter latitudes, in the IRI model, requires a major reduction in NO densities in winter. A suitable compromise is suggested. Equations fitted to the model results then provide a simpler and better behaved replacement for the IRI equations. Calculations at night show that known sources of ionisation, largely from starlight, can produce observed peak densities using current chemistry. There is an appreciable change with latitude, as starlight production increases in the southern hemisphere. The improbably large solar cycle change built into the IRI model, at night, cannot be reproduced and is not found in recent data. A new, simpler model is suggested. Changes in zenith angle and atmospheric composition cause the peak height (hmE) to vary between 105 and 120 km, as a function of time, latitude, season and solar flux. These changes are approximated by simple equations that should be definitely preferable over the single, fixed height used in the IRI models.

Modeling of the topside ionosphere requires a knowledge of the electron and ion temperatures (Te and Ti) as a function of height. Data in the International Reference Ionosphere (IRI) model are sometimes conflicting, and extend only up to 3000 km. An exact, analytic solution is found for the variation of Te along a magnetic field line, making full allowance for changes in the cross section and inclination of the tube of force. The downward heat flux is assumed to decrease smoothly with height, becoming zero at the top of the field line. Changes in the Coulomb cross section increase plasmaspheric temperatures by 6–8%. For a given field line, the temperature profile is defined by two parameters, taken as the temperature To and gradient Go at a reference height of 400 km. Results are fitted to satellite data, for mean day and night conditions, at intervals of 5° in latitude and heights from 400 to 8000 km. Diurnal changes are reproduced using sunrise and sunset transitions that are matched to observed values. Seasonal changes in Te are generally less than experimental errors. A solar activity variation of 30–40% is required, at low heights, to match the change in neutral temperature. Exospheric temperatures must also increase by 10–20%, near solar maximum. A first‐order correction is derived for high‐latitude heating during periods of magnetic activity. Values of Ti are calculated from the Te profile and the neutral temperature Tn, giving results that agree well with mean observations. The use of physically acceptable temperature profiles, fitted to the often conflicting satellite data, should give more consistent results for most purposes.

A new, empirical model for NO densities is developed, to include physically reasonable variations with local time, season, latitude and solar cycle. Model calculations making full allowance for secondary production, and ionising radiations at wavelengths down to 25 Å, then give values for the peak density NmE that are only 6% below the empirical IRI values for summer conditions at solar minimum. At solar maximum the difference increases to 16%. Solar-cycle changes in the EUVAC radiation model seem insufficient to explain the observed changes in NmE, with any reasonable modifications to current atmospheric constants. Hinteregger radiations give the correct change, with results that are just 2% below the IRI values throughout the solar cycle, but give too little ionisation in the E-F valley region. To match the observed solar increase in NmE, the high-flux reference spectrum in the EUVAC model needs an overall increase of about 20% (or 33% if the change is confined to the less well defined radiations at λ < 150 Å). Observed values of NmE show a seasonal anomaly, at mid-latitudes, with densities about 10% higher in winter than in summer (for a constant solar zenith angle). Composition changes in the MSIS86 atmospheric model produce a summer-to-winter change in NmE of about –2% in the northern hemisphere, and +3% in the southern hemisphere. Seasonal changes in NO produce an additional increase of about 5% in winter, near solar minimum, to give an overall seasonal anomaly of 8% in the southern hemisphere. Near solar maximum, reported NO densities suggest a much smaller seasonal change that is insufficient to produce any winter increase in NmE. Other mechanisms, such as the effects of winds or electric fields, seem inadequate to explain the observed change in NmE. It therefore seems possible that current satellite data may underestimate the mean seasonal variation in NO near solar maximum. A not unreasonable change in the data, to give the same 2:1 variation as at solar minimum, can produce a seasonal anomaly in NmE that accounts for 35–70% of the observed effect at all times.

We carry out a detailed comparison between winds derived from F2 peak heights and winds obtained from incoherent scatter radar (ISR) line‐of‐sight velocity measurements. A total of 34 incoherent scatter radar experiments at Millstone Hill spanning all seasons and levels of solar activity are included in this study. For two experiments we compare results from five different wind‐derivation techniques. According to work by Titheridge [1993, 1995a, b], neutral winds derived from the servo model are inaccurate during the sunrise and morning period because of a shift in the zero‐wind F peak downward from the balance height. To investigate this effect, we determine a correction factor cfac to be applied to the servo model c parameter as a function of time of day for summer, equinox, and winter at both solar maximum and solar minimum. Our results confirm that a sunrise decrease in cfac is necessary to bring about best agreement between the servo model winds and the winds derived from the ISR ion velocity data at Millstone Hill. However, the effect is not large, so that a constant cfac for each season/solar activity level usually introduces less error than other factors which may result in differences between the servo model and ISR winds. These factors include measurement errors in hmF2 and the ISR line‐of‐sight ion velocities, spatial variations in the wind field above the station, and the assumption that hmF2 is the peak in the O+ altitude profile.

Model calculations for the ionospheric E and F1 regions yield electron densities which are much too small, if no allowance is made for the production of secondary ionization by primary photoelectrons. Full calculations of this secondary production are quite difficult, since the upward and downward photoelectron fluxes must be determined as a function of energy at each height. Early studies showed that secondary ionization increases the total production rate by about 30% in the F2 region, while more recent studies show increases of around 100% in the E and F1 regions. Use of a fixed correction factor (for a given height and zenith angle) is not satisfactory, however, since the amount of secondary production varies greatly for different radiation bands. This paper describes a new approach in which a secondary production factor is determined for each ion and each radiation band. These factors (n(s)) are defined by the initial photon energy, and the mean energy of the final secondary electrons. For each radiation band the effective production efficiencies, for each ion, are increased by the factor 1 + n(s). Modeling of the ionosphere then proceeds normally, with no other changes and no increase in computer time. All results automatically include a full allowance for secondary production, for any assumed values of zenith angle, atmospheric model, or EUV fluxes. Comparison with recent, full photoelectron calculations shows that this procedure gives reliable results, with errors which are less than those due to current uncertainties in the solar fluxes, the photoionization cross sections and the electron collision cross sections.

Winds in the upper atmosphere, and their effect on the ionosphere, are reviewed with an emphasis on information useful to ionospheric studies. The winds are driven by pressure gradients from solar and auroral hearing, with some forcing by tidal energy from below. Simple calculations which balance the pressure gradient by ion drag and Coriolis forces are generally unreliable, so large-scale numerical models of the coupled atmosphere and ionosphere are required. The accuracy of these global models is limited by uncertainties in the energy inputs at high latitudes and at the lower boundary (about 90 km). The best current wind data come from incoherent scatter radar or airglow installations, at a few sites and for only a Few nights per month. Satellite data are also available for several years, and results to 1989 are incorporated in the global HWM90 model. This seems acceptable for determining mean winds at night, less good during the day, and least good in the southern hemisphere where few data were available. Plots are given to show the mean winds at different latitudes and longitudes, for use in ionospheric calculations.Meridional winds alter the height of the mid-latitude F layer, causing large changes in the effective loss rare. This is the major cause of observed seasonal changes, of differences between the hemispheres, and of changes ar different longitudes. An increased knowledge of the winds is essential for further progress in F region studies, Ionospheric data provide the most promising route, using routinely scaled parameters. The simplest calculations compare observed peak heights, obtained from M(3000)F2, with the value h(o) predicted by simplified ''servo'' equations. Errors occurring for some hours after sunrise can be overcome using model results to define h(o) this allows rapid and accurate wind calculations at dip latitudes of 23-62 degrees. Winds can also be obtained from full model calculations, designed to match observed values of peak height or density.

Meridional winds can be determined from measurements of the peak height of the ionospheric F2 layer (h(m)), the peak density (N-m), or the total electron content (N-1). At night, ionospheric changes follow the wind with a time constant of 30-40 min. During the day, this increases to 50-100 min for h(m) and 3-6 h for N-m and N-1. Thus, peak-height data are most suitable for the direct calculation of atmospheric winds, and daytime results should be advanced by 1 h. The wind calculations require an estimate of the peak height h(o) at zero wind. h(o) is close to the servo result (h(s)) at night, if the servo constant c is increased by 25% to agree with current theory. During the day, however, the long time constants prevent the F layer from reaching equilibrium before sunset. h(o) is well below h(s) from sunrise until afternoon, giving serious errors in any results based on servo theory.Results from a full ionospheric modelling program are used to obtain analytic expressions which reproduce the true, zero-wind peak height with an accuracy of a few kilometres, for all times of the day, all seasons and solar flux, and all latitudes in the useful range of 20-60 degrees geomagnetic. The wind W required to produce a given change in h,, varies closely as sin(1.4)I, where lis the magnetic dip angle, and the variation of h(m) with W is accurately reproduced by a modified servo equation. Use of these results with accurate peak height data should give horizontal winds with an accuracy of about +/- 25 m/s. Peak heights derived from scaled ionospheric data (M3000F2 and foF2) have an accuracy of typically 10-20 km, giving overall errors of about 40 m/s in calculated winds.

The relative importance of the equatorial plasma fountain (caused by vertical E x B drift al the equator) and neutral winds in leading to the ionospheric variations at equatorial-anomaly latitudes, with particular emphasis os conjugate-hemisphere differences, is investigated using a plasmasphere model. Values of ionospheric electron content (IEC) and peak electron density (Nmax) computed at conjugate points in the magnetic latitude range 10-30 degrees at longitude 158 degrees W reproduce the observed seasonal, solar activity, and latitudinal variations of IEC and Nmax, including the conjugate-hemisphere differences. The model results show that the plasma fountain, in the absence of neutral winds, produces almost identical effects at conjugate points in all seasons; neutral winds cause conjugate-hemisphere differences by modulating the fountain and moving the ionospheres at the conjugate hemispheres to different altitudes.At equinox, the neutral winds, mainly the zonal wind, modulate the fountain to supply more ionization to the northern hemisphere during evening and night-time hours and, at the same time, cause smaller chemical loss in the southern hemisphere by raising the ionosphere. The gain of ionization through the reduction in chemical loss is greater than that supplied by the fountain and causes stronger premidnight enhancements in IEC and Nmax (with delayed peaks) in the southern hemisphere at all latitudes (10-30 degrees). The same mechanism, but with the hemispheres of more flux and less chemical loss interchanged, causes stronger daytime IEC in the northern hemisphere at all latitudes. At solstice, the neutral winds, mainly the meridional wind, modulate the fountain differently at different altitudes and latitudes with a general interhemispheric flow from the summer to the winter hemisphere at altitudes above the F-region peaks. The interhemispheric flow causes stronger premidnight enhancements in IEC and Nmax and stronger daytime Nmax in the winter hemisphere, especially at latitudes equatorward of the anomaly crest. The altitude and latitude distributions of the daytime plasma flows combined with the longer daytime period can cause stronger daytime IEC in the summer hemisphere at all latitudes.

Modelling studies are carried out using the Sheffield University Plasmasphere-Ionosphere Model (SUPIM) to explain the observed north-south ionospheric differences at mid latitudes. The model results qualitatively reproduce and explain the observed differences in electron content and in the occurrence of nighttime increases.

Nighttime enhancements in ionospheric electron content (IEC) observed at conjugate stations in the equatorial anomaly and mid-latitude regions under moderate to high solar activity conditions are studied. The observations at equatorial anomaly latitudes show that when an enhancement occurs in one hemisphere then an enhancement usually occurs in the conjugate hemisphere. The enhancement characteristics (frequency of occurrence, time of occurrence, amplitude, and duration) and their seasonal and solar activity variations are in agreement with the fact that the primary source of an enhancement is the post-sunset increase in the equatorial fountain. It is suggested that the north-south differences in the enhancement characteristics, e.g. the enhancement being more frequent and stronger in the southern hemisphere than in the northern hemisphere, are due to the north-south differences in the neutral air wind velocity. At mid-latitudes, on the other hand, when an enhancement occurs in one hemisphere then either no enhancement or only a weak enhancement occurs in the conjugate hemisphere; on no observed night does a strong enhancement occur in both hemispheres. The occurrence and other characteristics of the enhancements demonstrate that the primary source for the nighttime enhancements in IEC at mid-latitudes (i.e. the downward flow of plasma from the protonosphere to the ionosphere) is asymmetric; a strong downward flow occurs in only one hemisphere on any one night.

Diurnal variations in the electron content (Nt) and peak density (Nm) of the ionosphere are calculated using a full time-varying model which includes the effects of electric fields, interhemispheric fluxes and neutral winds. The calculation is iterated, adjusting the assumed hourly values of neutral wind until a good match is obtained with mean experimental values of Nt and Nm. Using accurate ionospheric data for quiet conditions at 35°S and 43°S, winds are derived for summer, equinox and winter conditions near solar maximum and solar minimum. Solar maximum results are also obtained at 35°N. Changes in the neutral wind are found to be the major cause of seasonal changes in the ionosphere, and of differences between the two hemispheres. Calculated winds show little variation with latitude, but the winds increase by about 30% at solar minimum (in equinox and winter). The HWM90 wind model gives daytime winds which are nearly twice too large near solar maximum. The theoretical VSH model agrees better with observed daytime variations, and both models fit the observed winds reasonably well at night. Results indicate that modelling of the quiet, mid-latitude ionosphere should be adequate for many purposes when improved wind models are available. Model values for the peak height of the ionosphere are also provided; these show that wind calculations using servo theory are unreliable from sunrise to noon and for several hours after sunset.