On the boundary layer between a flowing plasma and a magnetic field. Part I : The conditions for equilibrium

A qualitative study is made of a plane charge-neutral magnetopause separating a field-free plasma from a plasma-free magnetic field, in the case where the plasma is flowing parallel to the applied field. It is shown that the existence of small-scale equilibrium dépends on boundary conditions which are governed by the large-scale structure of the System. On certain conditions, which are defined, equilibrium is possible at ail flow speeds. For a class of structures characterized by approximately axial flow, these conditions cannot be satisfied if the plasma encloses the magnetic field, but can if the magnetic field encloses the plasma. They are satisfied most naturally if, in addition, the magnetopause has the form of a torus. While intended originally as a contribution to magnetospheric physics, this study may be more relevant to the high-0 confinement of fusion plasmas