F-region Altitudes Research Articles

Comparison of the measured and modelled electron densities and temperatures in the ionosphere and plasmasphere during 20-30 January, 1993

Abstract. We present a comparison of the electron density and temperature behaviour in the ionosphere and plasmasphere measured by the Millstone Hill incoherent-scatter radar and the instruments on board of the EXOS-D satellite with numerical model calculations from a time-dependent mathematical model of the Earth's ionosphere and plasmasphere during the geomagnetically quiet and storm period on 20–30 January, 1993. We have evaluated the value of the additional heating rate that should be added to the normal photoelectron heating in the electron energy equation in the daytime plasmasphere region above 5000 km along the magnetic field line to explain the high electron temperature measured by the instruments on board of the EXOS-D satellite within the Millstone Hill magnetic field flux tube in the Northern Hemisphere. The additional heating brings the measured and modelled electron temperatures into agreement in the plasmasphere and into very large disagreement in the ionosphere if the classical electron heat flux along magnetic field line is used in the model. A new approach, based on a new effective electron thermal conductivity coefficient along the magnetic field line, is presented to model the electron temperature in the ionosphere and plasmasphere. This new approach leads to a heat flux which is less than that given by the classical Spitzer-Harm theory. The evaluated additional heating of electrons in the plasmasphere and the decrease of the thermal conductivity in the topside ionosphere and the greater part of the plasmasphere found for the first time here allow the model to accurately reproduce the electron temperatures observed by the instruments on board the EXOS-D satellite in the plasmasphere and the Millstone Hill incoherent-scatter radar in the ionosphere. The effects of the daytime additional plasmaspheric heating of electrons on the electron temperature and density are small at the F-region altitudes if the modified electron heat flux is used. The deviations from the Boltzmann distribution for the first five vibrational levels of N2(v) and O2(v) were calculated. The present study suggests that these deviations are not significant at the first vibrational levels of N2 and O2 and the second level of O2, and the calculated distributions of N2(v) and O2(v) are highly non-Boltzmann at vibrational levels v > 2. The resulting effect of N2(v > 0) and O2(v > 0) on NmF2 is the decrease of the calculated daytime NmF2 up to a factor of 1.5. The modelled electron temperature is very sensitive to the electron density, and this decrease in electron density results in the increase of the calculated daytime electron temperature up to about 580 K at the F2 peak altitude giving closer agreement between the measured and modelled electron temperatures. Both the daytime and night-time densities are not reproduced by the model without N2(v > 0) and O2(v > 0), and inclusion of vibrationally excited N2 and O2 brings the model and data into better agreement.Key words: Ionosphere (ionospheric disturbances; ionosphere-magnetosphere interactions; plasma temperature and density)

Helicobacter felis infection in mouse alters the amount of peripheral blood phagocytic leukocytes

The electron density and temperature in the ionosphere and plasmasphere measured by the Millstone Hill incoherent-scatter radar and the instruments on board of the EXOS-D satellite are compared with calculations from a time-dependent mathematical model of the Earth's ionosphere and plasmasphere during 14–16 May 1991. Use of [O]/[N2] correction factors with the NRLMSISE-00 model of the neutral atmosphere was found to bring the modeled and measured F-region main peak electron densities into agreement. It was found that the nighttime additional heating rate should be added to the normal photoelectron heating in the electron energy equation, in the nighttime plasmasphere region, in order for the model to reproduce the observed high plasmaspheric electron temperature within the Millstone Hill magnetic field flux tube in the Northern Hemisphere. The additional heating brings the measured and modeled electron temperatures into agreement in the plasmasphere and into a very large disagreement in the ionosphere, if the classical electron heat flux along magnetic field lines is used. An approach of Pavlov et al. (2000, 2001) based on a new effective electron thermal conductivity coefficient along the magnetic field line and the evaluated additional heating of electrons in the plasmasphere is used to explain the observed electron temperature in the ionosphere and plasmasphere. This approach leads to a heat flux which is less than that given by the classical theory. The effects of the additional plasmaspheric heating of electrons on the electron temperature and density are small at the F-region altitudes if the modified electron heat flux is used. We found that the resulting effect of vibrationally excited N2 and O2 on NmF2 is the decrease of the calculated NmF2 by up to a factor of about 2.7 by day and up to a factor of about 2.5 by night. The modeled electron temperature is very sensitive to the electron density, and this decrease in electron density results in an increase of the calculated daytime electron temperature up to about 960K at the F-region main peak altitude giving closer agreement between the measured and modeled electron temperatures. Both the daytime and nighttime electron densities and temperatures are not reproduced by the model without vibrationally excited N2 and O2, and inclusion of vibrationally excited N2 and O2 brings the model and data into agreement. The model with including vibrationally excited N2 and O2 in the loss rate of O+(4S) ions produces ion temperatures close to those given by the middle latitude model without including vibrationally excited N2 and O2. The detailed investigation of the ionospheric electron energy balance was carried out. It is shown for the first time that the revised electron cooling rates derived by Pavlov (1998a, c), and Pavlov and Berrington (1999), and Lobzin et al. (1999) produce a better fit of the calculated electron temperature and density to the radar data than the outdated cooling rates of Schunk and Nagy (1978). Revised (decreased) electron cooling rates increase the electron temperature and decrease the electron density. The difference between the revised and outdated cooling rates of thermal electrons leads to the maximum difference of 230 K between the calculated electron temperatures at the F2-peak altitude and to the increase of the calculated F-region main peak electron density by up to a factor of 1.13. Contrary to previous studies given by Richards (1986), Richards and Khazanov (1997), and Aponte et al. (1999), we found that the resulting effect of N(2D) electron quenching included in thermal electron heating on the electron temperature at the F2 peak altitude is the very weak increase of the calculated electron temperature up to about 35K. It is found that the effect of including the N(2D) diffusion results in a decrease in the calculated daytime N(2D) number density above about 290km and in a decrease of the daytime integral intensity at 520nm up to a factor of 1.11.

A study of the Joule and Lorentz inputs in the production of atmospheric gravity waves in the upper thermosphere

Abstract. First results of a modelling study of atmospheric gravity waves (AGWs) are presented. A fully-coupled global thermosphere-ionosphere-plasmasphere model is used to examine the relative importance of Lorentz forcing and Joule heating in the generation of AGWs. It is found that Joule heating is the dominant component above 110km. The effects of the direction of the Lorentz forcing component on the subsequent propagation of the AGW are also addressed. It is found that enhancement of zonal E×B forcing results in AGWs at F-region altitudes of similar magnitudes travelling from the region of forcing in both poleward and equatorward directions, whilst enhancement of equatorward meridional E×B forcing results in AGWs travelling both poleward and equatorward, but with the magnitude of the poleward wave severely attenuated compared with the equatorward wave

A study of the Joule and Lorentz inputs in the production of atmospheric gravity waves in the upper thermosphere

First results of a modelling study of atmospheric gravity waves (AGWs) are presented. A fully-coupled global thermosphere-ionosphere-plasmasphere model is used to examine the relative importance of Lorentz forcing and Joule heating in the generation of AGWs. It is found that Joule heating is the dominant component above 110km. The effects of the direction of the Lorentz forcing component on the subsequent propagation of the AGW are also addressed. It is found that enhancement of zonal E×B forcing results in AGWs at F-region altitudes of similar magnitudes travelling from the region of forcing in both poleward and equatorward directions, whilst enhancement of equatorward meridional E×B forcing results in AGWs travelling both poleward and equatorward, but with the magnitude of the poleward wave severely attenuated compared with the equatorward wave

On the origin of dusk radar scatter events

The possibility of the generation of decameter scale ionospheric plasma density irregularities, that must be responsible for dusk scatter, by the plasma gradient drift instability (GDI) at F-region altitudes is considered. It is shown that the dusk scatter could be produced by the ion density perturbations which appear as a result of the development of the GDI produced by the maximum westward plasma drift in the region poleward of the trough minimum. Possible reasons for the appearance of growth of the GDI waves as a result of the development of the trough plasma GDI during or just after sunset in the F-region are discussed. It is shown that, if the GDI begins after sunset, then the influence of the drift velocity shear results in the action of the GDI during 1–2 hours after sunset, which is close to the duration of dusk scatter.

Electron density profile changes in a pre-earthquake period

Recent investigations show that there are a number of specific effects at F-region altitudes which could be considered as earthquakes ionospheric precursors. One of such effects is a night-time increase of F2 critical frequency 1 to 2 days before the seismic event. Distortions of the electron density profiles which occur at the same time are investigated in this report. N(h)-profiles obtained from vertical soundings near the epicenter differ noticeably from IRI predictions.

Vibrational nitrogen concentration in the ionosphere and its dependence on season and solar cycle

Abstract. A fully time-dependent mathematical model, SUPIM, of the Earth's plasmasphere is used in this investigation. The model solves coupled time-dependent equations of continuity, momentum and energy balance for the O+, H+, He+, N+2, O+2, NO+ ions and electrons; in the present study, the geomagnetic field is represented by an axial-centred dipole. Calculation of vibrationally excited nitrogen molecules, which has been incorporated into the model, is presented here. The enhanced model is then used to investigate the behaviour of vibrationally excited nitrogen molecules with F10.7 and solar EUV flux, during summer, winter and equinox conditions. The presence of vibrational nitrogen causes a reduction in the electron content. The diurnal peak in electron content increases linearly up to a certain value of F10.7, and above this value increases at a lesser rate, in agreement with previous observations and modelling work. The value of F10.7 at which this change in gradient occurs is reduced by the presence of vibrational nitrogen. Vibrational nitrogen is most effective at F-region altitudes during summer daytime conditions when a reduction in the electron density is seen. A lesser effect is seen at equinox, and in winter the effect is negligible. The summer reduction in electron density due to vibrational nitrogen therefore reinforces the seasonal anomaly.

Investigation of OI 557.7 nm and OI 630.0 nm nightglow intensity ratios during the occurrence of equatorial F-region plasma bubbles

It is well-known that, at equatorial and low-latitudes, the OI 557.7 nm emission emanates both from the mesosphere and F-region. Earlier investigations using, in general, ground-based simultaneous observations indicate that the contribution to the OI 557.7 nm emission from the F-region is about 20% of the F-region OI 630.0 nm emission. Both of these emissions are excited by the dissociative recombination process (O 2 + + e → O ∗( 1S, 1D) + O) in the F-region. The nocturnal intensity variations of these emissions sometimes show large short-lived depletions, associated with the passage of equatorial spread-F plasma bubbles through the field of view of a photometer. Regular observations of these emissions have been carried out at Cachoeira Paulista (22.7°S, 45.0°W), Brazil, since 1973. In this paper, we present and discuss simultaneous observations of these emissions, during periods of strong equatorial spread-F conditions, when both emissions show large intensity drop-outs. Twenty-five nights of measurements from Cachoeira Paulista have been analysed for this study, which indicate that the average ratio of the intensity depletions ( ΔI557.7 nm ΔI630 nm ) is about 0.26±0.07 for F-region altitudes, and this component has a dependence on the height of the electron density profile.

Dayside ion frictional heating: EISCAT observations and comparison with model results

An extended period of intense dayside ion frictional heating was observed on 3 April 1992 with the UHF radar of the European Incoherent Scatter (EISCAT) facility during a run of the common program CP-1-J. The elevated F-region ion temperature resulted from an enhancement in the ion drift velocity, principally in the field-orthogonal zonal direction, which steadily increased, in a westward direction, to a value in excess of 2 km s −1 during a 4 h interval commencing at approximately 10 UT (12 MLT). The maximum enhancement in the field-parallel ion temperature measured at 300 km altitude exceeded 700 K. The electron concentration in the F-region was substantially depleted during the interval of ion frictional heating, which further resulted in an increase in the F-region electron temperature as, at the local time of the event, the plasma was solar illuminated. A zonal E ∧ B velocity signature modelling that observed over EISCAT during the aforementioned interval was imposed on the Sheffield University plasmasphere and ionosphere model (SUPIM), previously used to study the effects of sub-auroral ion drifts (SAID). The model yields densities, temperatures and field-aligned velocities for the six major ion species and for the electron, and has recently been modified to account for O + temperature anisotropy. The plasma parameters modelled at F-region altitudes were compared with those measured by the EISCAT radar and, although they tended to exhibit the same general trends, the model results predicted substantially larger field-parallel ion temperature enhancements than were observed. However, the inclusion of a time-dependent calculated neutral wind, caused by ion drag, reduced the modelled field-parallel ion temperature to the values measured by EISCAT without severely affecting the other parameters. During extended periods of high ion flow, particularly on the dayside, an enhanced neutral wind becomes highly significant in determining the extent of frictional heating of the ion population.

An optimised method for calculating the O<sup>+</sup>-O collision parameter from aeronomical measurements

Abstract. A study has been made of the interaction between the thermosphere and the ionosphere at high latitudes, with particular regard to the value of the O+-O collision parameter. The European incoherent scatter radar (EISCAT) was used to make tristatic measurements of plasma parameters at F-region altitudes while simultaneous measurements of the neutral wind were made by a Fabry-Perot interferometer (FPI). The radar data were used to derive the meridional neutral winds in a way similar to that used by previous authors. The accuracy of this technique at high latitudes is reduced by the dynamic nature of the auroral ionosphere and the presence of significant vertical winds. The derived winds were compared with the meridional winds measured by the FPI. For each night, the value of the O+-O collision parameter which produced the best agreement between the two data sets was found. The precision of the collision frequency found in this way depends on the accuracy of the data. The statistical method was critically examined in an attempt to account for the variability in the data sets. This study revealed that systematic errors in the data, if unaccounted for by the analysis, have a tendency to increase the value of the derived collision frequency. Previous analyses did not weight each data set in order to account for the quality of the data; an improved method of analysis is suggested.

F-region ionospheric irregularities over King George Island and Argentine Islands – a comparative study

Spread-F is caused by the presence of ionospheric electron concentration irregularities of scale-size of order 5 km at F-region altitudes. Estimates of spread-F in the vicinity of the maximum plasma frequency of the Flayer (foF2) have been determined at 15 min intervals from ionograms recorded over a ten day period (1–10 May 1986) both at Marsh (62.2°S, 58.9°W), King George Island, and Faraday (65.2°S, 64.3°W), Argentine Islands. The interval, at low solar activity, includes periods of quiet and disturbed geomagnetic activity. Spread-F is observed on every night at both stations. It is more frequent, slightly more intense and starts earlier at Argentine Islands than at King George Island. On most nights, spread-F ceases about local sunrise at 120 km altitude at both stations. On the days of highest geomagnetic activity, the onset of spread-F is delayed compared with days of lower activity. Spread-F is usually most intense on the night(s) following largest geomagnetic activity level, as measured by the geomagnetic index, Kp. The growth rate of the plasma instability processes causing the ionospheric irregularities is inversely related to electron concentration (foF22), amongst other parameters. Thus the lower foF2 values over Argentine Islands are consistent with more spread-F being observed by the higher latitude observatory. However, no firm relationship between the absolute value of foF2, the horizontal gradient of foF2 between the two observatories, and the onset of spread-F, is found. Thus it has not been possible to determine uniquely the instability process responsible for the formation of the plasma irregularities.

Observations of neutral winds in the polar cap during northward IMF

Long term remote observations of neutral winds at F-region altitudes have been performed at Thule Air Base (lat. 76.5°N, long. 69.0°W), Greenland, and Søndre Strømfjord (lat. 67.0°N, long. 50.9°W), Greenland. The former site is very close to the geomagnetic pole, while the latter site is within the polar cap for several hours each night on either side of geomagnetic midnight. Wind data corresponding to clear sky conditions and Kp ⩽ 4 were sorted according to the sign of the IMF B z component. The averaged maximum poleward flow near midnight LST was reduced by approximately one third during B z northward conditions. If the magnitude of B y was less than the magnitude of the northward B z component, then the averaged poleward flow was further reduced by one half. In addition, if B z > 5 nT, then sunward directed horizontal neutral winds were observed at the very highest latitudes near noon LST.

The temporal evolution of the ionospheric signatures of subauroral ion drifts

A model of the ionosphere and plasmasphere is used to investigate the effects of an imposed westward plasma drift on O + and molecular ion behaviour in the nightside ionosphere. A closed subauroral tube of plasma is considered and the velocity input persists for 30 min. The rapid increase in the F-region ion temperature resulting from ion-neutral frictional heating causes an immediate surge in the O + fieldaligned velocity, upwards in the topside ionosphere and downwards below the F2-peak, but after about 10 min into the event the surge in the topside disappears. After the event there is a return flow of O + from the plasmasphere. The relative abundance of O + decreases during the event due to the increased rate of conversion of O + into NO + and O + 2; the decrease is more marked for greater values of the imposed westward ion drift. At a given F-region altitude, a low relative abundance of O + is more likely under sunspot maximum atmospheric conditions. There are also significant percentages of NO + in the topside ionosphere during the event. The bearing of these results on satellite observations of SAID (subauroral ion drift events) and on EISCAT incoherent scatter radar observations of ion heating events is discussed.

Numerical modelling of the thermosphere-ionosphere-protonosphere system

The results of a numerical calculation of the thermospheric circulation, the ion and electron concentrations and temperatures as well as electric fields of magnetospheric and dynamo origin are presented for solar minimum, low geomagnetic activity and spring (equinox) conditions. The results are obtained from a numerical hydrodynamical model of the Earth's ionosphere and protonosphere for the height range from 80 km to a geocentric distance of 15 R E . The concentrations and temperature of the neutral atmosphere are computed from the MSIS-83 empirical model of the thermosphere. The effects of small electric fields on the global distributions of the thermospheric circulation, concentrations and temperatures of the charged particles in the ionosphere and protonosphere are discussed. It is shown that electric fields smaller than even 20 mV m −1 form all the main structures of the global plasma distribution at F-region altitudes and in the protonosphere, such as the main ionospheric trough, the light ion trough, the plasmapause and ‘hot spots’ in the ion and electron temperature distributions.

F-region seasonal behavior as measured by the MU radar

We present MU radar ionospheric incoherent scatter results from the period December 1986 to August 1988. We extend our previous 3-month model of the ion vector velocity to a full year and determine seasonal variations. We also present solstitial and equinoctial patterns of electron-density and plasma-temperature behavior. Compared with our previous 3-month model, the full-year velocity model is largely diurnal in nature, the higher harmonic being very variable and tending to average out as noise over a long period. The seasonal behavior of the electron density may be explained largely in terms of the behaviors of the neutral composition and of the timing of the diurnal change of direction of the neutral wind with respect to the time of sunset. According to the neutral composition a high-altitude O + layer and a low-altitude molecular-ion layer compete for density predominance. The molecular-ion layer often becomes stronger during summer, and also during times of strong magnetic disturbance. The mean equinox daytime F-region electron and ion temperatures ( T e and T i ), and particularly their ratio ( T r = T e T i ), is controlled largely by the electron density, with other specific time-of-day effects. At night T r = 1, and at sunrise T r > 1 at all F-region altitudes measured. During midday we have two cases. For a peak F-layer density of about 10 6 cm −3 or greater, T r > 1 below 300 km altitude and T r = 1 above 300 km altitude; for lower electron densities T r > 1 at all F-region altitudes. Near sunset T r generally increases as the electron density decreases while photoionization is continuing.

Asymmetric incoherent scatter spectra from a relaxation collision model

A relaxation collision model for ion flow through a stationary neutral gas has been used to obtain ion velocity distributions and line-of-sight incoherent scatter spectra for a range of values of collision frequency and electric field. The mean velocity of the line-of-sight ion velocity distribution has been compared with the Doppler shift of the corresponding spectra. The latter is not always a good estimate of the former, because the ion velocity distribution in the plane perpendicular to the magnetic field direction is highly distorted. For ion-neutral collision frequency to ion gyrofrequency ratios 0.1 ≤V i,/Ω i≤0.5, the greatest inaccuracies in mean velocity estimation take place along the electric field direction, while for 0.5 ≤ V i/Ω i ≤ 1.0 the greatest inaccuracies occur across the electric field direction. These inaccuracies would be reduced but not eliminated in a more realistic model. At F-region altitudes, other processes must be invoked to explain observed asymmetrical spectra, but the comparison of mean line-of-sight ion velocity and spectrum Doppler shift may still have relevance.

The response of the thermosphere and ionosphere to magnetospheric forcing

During the past six years, rapid advances in three observational techniques (groundbased radars, optical interferometers and satellite-borne instruments) have provided a means of observing a wide range of spectacular interactions between the coupled magnetosphere, ionosphere and thermosphere system. Perhaps the most fundamental gain has come from the combined data-sets from the NASA Dynamics Explorer (DE) Satellites. These have unambiguously described the global nature of thermospheric flows, and their response to magnetospheric forcing. TheDEspacecraft have also described, at the same time, the magnetospheric particle precipitation and convective electric fields which force the polar thermosphere and ionosphere. The response of the thermosphere to magnetospheric forcing is far more complex than merely the rare excitation of 1 km s-1wind speeds and strong heating; the heating causes large-scale convection and advection within the thermosphere. These large winds grossly change the compositional structure of the upper thermosphere at high and middle latitudes during major geomagnetic disturbances. Some of the major seasonal and geomagnetic storm-related anomalies of the ionosphere are directly attributable to the gross windinduced changes of thermospheric composition; the mid-latitude ionospheric storm ‘negative phase’, however, is yet to be fully understood. The combination of very strong polar wind velocities and rapid plasma convection forced by magnetospheric electric fields strongly and rapidly modify F-region plasma distributions generated by the combination of local solar and auroral ionization sources. Until recently, however, it has been difficult to interpret the observed complex spatial and timedependent structures and motions of the thermosphere and ionosphere because of their strong and nonlinear coupling. It has recently been possible to complete a numerical and computational merging of the University College London (UCL) global thermospheric model and the Sheffield University ionospheric model. This has produced a self-consistent coupled thermospheric-ionospheric model, which has become a valuable diagnostic tool for examining thermospheric-ionospheric interactions in the polar regions. In particular, it is possible to examine the effects of induced winds, ion transport, and the seasonal and diurnal U.T. variations of solar heating and photoionization within the polar regions. Polar and high-latitude plasma density structure at F-region altitudes can be seen to be strongly controlled by U.T., and by season, even for constant solar and geomagnetic activity. In the winter, the F-region polar plasma density is generally dominated by the effects of transport of plasma from the dayside (sunlit cusp). In the summer polar region, however, an increase in the proportion of molecular to atomic species, created by the global seasonal circulation and augmented by the geomagnetic forcing, controls the plasma composition and generally depresses plasma densities at all U.Ts. A number of these complex effects can be seen in data obtained from ground-based radars, Fabry-Perot interferometers and in the combinedDEdata-sets. Several of these observations will be used, in combination with simulations using the UCL-Sheffield coupled model, to illustrate the major features of large-scale thermosphere-ionosphere interactions in response to geomagnetic forcing.

Global thermospheric studies of neutral dynamics using incoherent-scatter radars

Among the parameters we can deduce from incoherent-scatter radar measurements in the thermosphere is the meridional wind - the horizontal neutral wind in the magnetic north-south direction. Since this was first realized in 1967, radar observations have contributed greatly to the study of thermospheric dynamics. At mid and low latitudes, they have shown the important impact that tides propagating upward, from the lower atmosphere, have on the dynamics at F-region altitudes. At high latitudes, radar measurements have demonstrated what effects momentum and energy transfer from the magnetosphere have on the dynamics at F-region altitudes; including local ion drag, equatorward surge near midnight, IMF B y, local Joule heating, high-latitude heating, and midnight abatement and reversal. More generally the radar data have been used to confirm basic ideas about the global meridional circulation. Initially this was done with individual radars; now coordinated observations by a network of radars take data for periods of from one to five days. These global observations provide measurements of the full diurnal pattern at multiple locations as a function of season, magnetic activity, solar cycle, and of many other geophysical parameters. These multiday campaigns have contributed significantly to major ongoing programs such as CEDAR and WITS, and are planned to support future ones, such as STEP. The breadth of these programs is especially significant today. We are entering a new era when radar observations, combined with those from other instruments, can be used in conjunction with major theoretical models to test our quantitative understanding of the myriad physical processes and interactions that give rise to the winds and other geophysical parameters in the upper atmosphere.

Simulations of the seasonal variations of the thermosphere and ionosphere using a coupled, three-dimensional, global model, including variations of the interplanetary magnetic field

The University College London Thermospheric Model and the Sheffield University Ionospheric Convection Model have been integrated and improved to produce a self-consistent coupled global thermospheric/high latitude ionospheric model. The neutral thermospheric equations for wind velocity, composition, density and energy are solved, including their full interactions with the evolution of high latitude ion drift and plasma density, as these respond to convection, precipitation, solar photoionisation and changes of the thermosphere, particularly composition and wind velocity. Four 24 h Universal Time (UT) simulations have been performed. These correspond to positive and negative values of the IMF BY component at high solar activity, for a level of moderate geomagnetic activity, for each of the June and December solstices. In this paper we will describe the seasonal and IMF reponses of the coupled ionosphere/thermosphere system, as depicted by these simulations. In the winter polar region the diurnal migration of the polar convection pattern into and out of sunlight, together with ion transport, plays a major role in the plasma density structure at F-region altitudes. In the summer polar region an increase in the proportion of molecular to atomic species, created by the global seasonal thermospheric circulation and augmented by the geomagnetic forcing, controls the plasma densities at all Universal Times. The increased destruction of F-region ions in the summer polar region reduces the mean level of ionization to similar mean levels seen in winter, despite the increased level of solar insolation. In the upper thermosphere in winter for BY negative, a tongue of plasma is transported anti-sunward over the dusk side of the polar cap. To effect this transport, co-rotation and plasma convection work in the same sense. For IMF BY positive, plasma convection and co-rotation tend to oppose so that, despite similar cross-polar cap electric fields, a smaller polar cap plasma tongue is produced, distributed more centrally across the polar cap. In the summer polar cap, the enhanced plasma destruction due to enhancement of neutral molecular species and thus a changed ionospheric composition, causes F-region plasma minima at the same locations where the polar cap plasma maxima are produced in winter.

A three-dimensional model of ionospheric F-region

The three-dimensional stationary in solar-terrestrial system numerical model of the global ionosphere at F-region altitudes is developed. The input parameters are: the structural parameters of empirical thermospheric models (temperature, composition), electric fields, solar UV-radiation spectrum, corpuscular flows at high latitudes. The model includes the calculations of thermospheric circulation, electron density, electron and ion temperatures. The model reproduces the main morphological peculiarities of the distribution of thermospheric circulation and ionospheric plasma parameters. A comparison analysis of the results of the model calculation corresponding to different thermospheric models (DTM, MSIS, Jacchia-77, MSIS-83) is carried out. It is shown, that thermospheric circulation systems are the effective indicator of faithfulness of thermospheric models.