Electric and magnetic field pressures in double layers

Summary form only given. We have recently shown that the net momentum delivered by the large electric field inside a one-dimensional double layer is zero. This is demonstrated through an analysis of the momentum balance in the double layer at the boundary between the ionosphere and the aurora cavity. It is argued that even in a double layer of a varying cross-section along the flow, that momentum is usually negligible. In particular, for the recently observed double layer in laboratory current-free plasma expanding along a divergent magnetic field, the increase of plasma thrust is shown to result from the magnetic field force balancing the plasma pressure in the direction of acceleration. The plasma acceleration follows an efficient conversion of plasma pressure to momentum, rather than the result of electrostatic acceleration. We identify the source of localized double-layer acceleration as the simultaneous and abrupt cross-section expansion and ionization variation. Abrupt divergence of magnetic field and localized ionization could result in fast particle beams in space. Since the scale length of the acceleration is determined by variations in the magnetic field topology and by the ionization mean-free-path, solar wind acceleration, for example, could occur, according to a recent intriguing suggestion, in the sun chromosphere, rather than in the corona. Implications to space plasmas and to laboratory plasmas will be discussed. For a thruster a steady-state finite-length configuration will be described