7. Conclusions

[25]  The representative data of the Intercosmos 19 satellite and the technique of calculations developed make it possible to find the main factors and to estimate the contribution of each into variations in h_mF2 at middle, subauroral, and auroral latitudes. The approach is tested using the summer near-midnight conditions, but one can solve similar problem for any other conditions. Now we formulate the main results of the studies carried out.

[26]  The distributions of the height of the F2 -layer maximum within the invariant latitude belt from 40^o to 65^o in the both hemispheres for the summer nighttime conditions were built. The data from the topside sounding onboard the Intercosmos 19 satellite were carefully selected for very quiet conditions to minimize the influence of the electric fields and AGW. This made it possible to derive stable longitudinal variations in h_mF2 related to the stable system of neutral winds. The character of LE in h_mF2 varies slightly at the transition from middle to auroral latitudes and its amplitude is about 45-50 km in the Northern Hemisphere and 65-70 km in the Southern Hemisphere. The IRI model does not adequately reproduce the h_mF2 variations at high latitudes and needs further correction.

[27]  On the basis of the approach developed by Karpachev and Gasilov [2000] the inverse problem was solved: using the longitudinal variations in the plasma vertical drift velocity W calculated from h_mF2 by means of the servo model of the ionosphere, the meridional and zonal components of the neutral wind for the fixed invariant latitudes 40^o, 50^o, 60^o, and 65^o were calculated with the accuracy up to the first harmonic. The approach is based on the expansion into the finite Fourier series of the longitudinal variations in W, geomagnetic field parameters, and wind velocity and on solution of the obtained system of algebraic equations by the Tikhonov regularization method. The calculations performed by this method show that there is a stable solution for the meridional wind, whereas the accuracy of derivation of the longitudinal variations in h_mF2 is not enough for an adequate determination of the zonal wind.

[28]  The direct problem was also solved: using the HWM model of the neutral wind, servo model of the ionosphere, and MSIS thermospheric model, variations in h_mF2 for the considered conditions were calculated. It was shown that the HWM model inadequately reproduces the h_mF2 variations, especially in the Southern Hemisphere where the model is based on a limited set of data. A correction of the HWM model for the considered conditions was performed: the zonal component of the wind was smoothed by one first harmonic and the meridional component was calculated by the regularization method. The wind system obtained in such a way reproduces fairly accurately the longitudinal and latitudinal variations in h_mF2 in the entire belt of the latitudes considered. Therefore the global data of the topside sounding can be used for correction the neutral wind model, such correction being an actual task because of a serious lack in the wind velocity measurements.

[29]  The analysis performed makes it possible to find the main factors which determine the longitudinal and latitudinal variations in h_mF2 in the summer nighttime conditions and to estimate the contribution of each factor into these variations. At middle latitudes, the temperature of the neutral atmosphere T_n and plasma vertical drift W induced by the neutral wind are the main factors. The contributions of these factors are from 25% to 30% and from 70% to 75% correspondingly. The meridional wind component provides a larger contribution into the wind effect than the zonal component (  80% and 20%, correspondingly). With an increase in latitude, the relative contribution of the wind into the longitudinal variations in h_mF2 decreases slightly, although both the mean values of the neutral wind components and the amplitudes of their longitudinal variations increase. This is mainly due to a decrease of the contribution of the wind meridional component into the drift longitudinal variations in the Northern Hemisphere, whereas in the Southern Hemisphere this is related to the reverse of the zonal wind direction. However, the decrease of the wind contribution is compensated by the increase in the contribution of h_m0; so finally the LE amplitude in h_mF2 almost does not change with latitude.

[30]  The results obtained promote a deeper understanding of the causes of the asymmetry between the ionospheric parameters in the Northern and Southern hemispheres. In the geomagnetic coordinate system the different geometry of the magnetic field in different hemispheres manifests mainly in the effect of the neutral wind via the variations in the magnetic field inclination. The difference between the geomagnetic and geographic coordinates manifests mainly in variations in h_m0 via the longitudinal variations in the composition and temperature of the thermosphere. As for the longitudinal variations in the wind velocity, they are practically unexplored and so it is too early to consider a complete understanding of a problem of the asymmetry.

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