О структуре неитралянои верхнеи атмосферы

Abstract

M. N. Izakov: On the Structure of the Neutral Upper Atmosphere In the first section the available experimental data obtained mainly by means of rockets and satellites are summed up. Main attention is devoted to the thermosphere, as the most variable region where variations in structural parameters reach orders of magnitude. The conclusion is drawn that by the present time all the basic variations are detected, although their absolute values should be determined more precisely. This refers primarily to latitude-seasonal variations and especially variations in polar regions. For the main density and temperature variations at heights above 150–200 km connected with solar activity, time of the day and geomagnetic disturbances, empirical or semi-empirical dependencies on determining parameters have been found, which allows us to determine the atmospheric structure with sufficient accuracy, at least at low and middle latitudes. The structure variations, especially composition variations at a height interval of 100–200 km, is yet known to a lesser degree. This interval is hard for studies because of various complicated processes and experimental difficulties. Nevertheless, for this interval a complex picture of variations appears gradually, which shows that in the lower part of this interval latitude-seasonal variations prevail and in the upper part — diurnal and solar-activity variations. Atmospheric composition seems to be a very variable parameter depending on various factors. Some disagreements in data and their possible reasons are discussed in connection with the accuracies of the various methods. The second section considers models of the structure of the upper atmosphere, i.e. systems of interrelated structural parameters satisfying the main physical regularities and describing experimental results, and methods of their construction. It is important to evolve a theoretical model which will connect the structure of the atmosphere with factors determining it. The most adequate apparatus for constructing such a model is the apparatus of Bolzmann's modified equations and equations of physical hydrodynamics obtained from it, taking into account the interaction of the gas with radiation. The limits of the applicability of the hydrodynamic description are evaluated. The above apparatus in principle provides us with the possibility of constructing a comprehensive theoretical model of the structure and the dynamics of the upper atmosphere, taking into account the interaction of the neutral and ionized components, although on this path there are considerable difficulties, both physical ones connected with the clarification of cross-sections of some reactions, and mathematical ones connected with the necessity of solving the complex systems of equations. Due to the complexity of this problem, the models are at present constructed with a number of assumptions and simplifications, which lead to the fact that all available models are in essence semi-empirical. These models are compared with each other and with experimental data. Despite their limitation, they are of great significance as a form of generalizing experimental data and as the first steps in constructing a comprehensive theoretical model of the upper atmosphere's structure. The problem of constructing an exosphere model is developed considerably, and in recent papers some improvements have been included, viz. the computation of the critical layer instead of the critical level, and taking into account lateral flows. The models of exosphere give us the neutral-components distributions at heights of the order of several earth radii, which indicates that there a large layer of helium prevails when temperature is high and a small layer when temperature is low. Hydrogen concentration varies much with temperature at a height near 1000 km and remains near the value of 102 cM-3 at a height of 20000 km.