Аннотация:Plasma and magnetic field parameters of LLBL crossings are studied using data of THEMIS mission. More than 70 cases of LLBL crossings are selected. Method of LLBL thickness determination using results of plasma velocity
measurements is developed. The dependences of LLBL thickness on Bz IMF and VBz (V-solar wind velocity)are studied. The results are explained using the hypothesis of regular and quasi-diffusional transport in Y-direction
inside the magnetosphere. The problem of magnetosheath plasma penetration inside the magnetosphere and LLBL formation is discussed.