Thin current sheets caused by plasma flow gradients in space and astrophysical plasma

摘要

Strong gradients in plasma flows play a major role in space and astrophysical plasmas.A typical situation is that a static plasma equilibrium is surrounded by aplasma flow, which can lead to strong plasma flow gradients at the separatricesbetween field lines with different magnetic topologies, e.g.,planetary magnetospheres, helmet streamers in the solar corona,or at the boundary between the heliosphereand interstellar medium. Within this work we make a first step to understandthe influence of these flows towards the occurrence of current sheets in astationary state situation. We concentrate here on incompressibleplasma flows and 2-D equilibria, which allow us to find analytic solutionsof the stationary magnetohydrodynamics equations (SMHD).First we solvethe magnetohydrostatic (MHS) equations with the help of aGrad-Shafranov equation and then we transform these static equilibria intoa stationary state with plasma flow. We are in particular interestedto study SMHD-equilibria with strong plasma flow gradientsperpendicular to separatrices.We find that induced thin current sheets occur naturally insuch situations. The strength of the induced currentsdepend on the Alfvén Mach number and its gradient, and on the magneticfield.