ИСТИНА

The main feature of processes in the magnetosphere of the Earth is its collisionless character which exclude the possibility to use the approximation of finite conductivity for the description of large scale magnetospheric dynamics. Using ideal MHD is limited only for the processes when plasma velocity is comparable with the Alfvén velocity. We obtain the 2D distribution of the ratio between the Alfvén and averaged plasma velocities using data of the THEMIS mission at geocentric distances <20 RE. We show that this relation is comparatively large in all magnetospheric regions which requires to use Hall MHD for the analysis of large scale magnetospheric processes. Many of such processes can be described using the condition of magnetostatic equilibrium. We summarize the results of observations demonstrating the validity of this condition at the Earth’s magnetopause and in the geomagnetic tail in spite of large level of turbulent fluctuations. Mechanism of large-scale solar wind magnetic field “penetration” into magnetosphere and “modulation” of its activity is discussed.