Why Does the Plasma Depletion Layer Exist Near the Magnetopause?

In the late seventies of the last century, it has been found that a layer with increased modulus of the magnetic field and decreased density (called in different sources both the Plasma Depletion Layer or the Magnetic Barrier) does exist in the inner magnetosheath close to the Earth's magnetopause. Nevertheless, the paper of Zwan and Wolf (1976) where the physical explanation of this phenomenon was given seems to be controversial and was criticized, for example, by Southwood and Kivelson (1992,1995). While later MHD models have confirmed the existence of the PDL (plasma depletion layer), there has been given no clear answer on the question of what is the physical mechanism of the plasma depletion. In the present work with the help of our 3-D MHD model of the magnetosheath, we study in details which forces accelerate the plasma along the magnetopause. We clearly show the difference in the acceleration mechanism for hydrodynamic and MHD flows. The enhanced magnetic field in the PDL accelerates the plasma in the direction perpendicular to the magnetic field, while in the direction along the magnetic field the plasma flow is due to the plasma pressure gradient.