On magnetic field reconstruction in the nearest region of the nighttime magnetosphere at substorm preliminary phase

Development of pressure anisotropy in the night sector of the nearest magnetosphere was investigated in the framework of adiabatic approximation. It is shown that gradual decrease of Bz- component in near the equatorial region is considered to be the necessary condition for development of the stretched magnetic field with thin current layer typical for the end of substorm preliminary phase. In the begining of magnetic structure evolution the decrease is conditioned by outer reasons, i.e. the growth of magnetic flux in tail shares and then is supported due to the positive feedback between field configuration and plasma anisotropy. Other sources of plasma anisotropy development related with the azimuthal asymmetry of magnetosphere are discussed