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

TL;DR

This paper investigates how plasma flow gradients in space and astrophysical plasmas can naturally induce thin current sheets, using analytic solutions of stationary magnetohydrodynamics equations for incompressible, 2D equilibria.

Contribution

It provides the first analytic solutions demonstrating the formation of thin current sheets caused by plasma flow gradients in stationary MHD equilibria.

Findings

Strong plasma flow gradients induce thin current sheets.

Current sheet strength depends on Alfvén Mach number and magnetic field.

Analytic solutions for stationary MHD equilibria with flow gradients are derived.

Abstract

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 a plasma flow, which can lead to strong plasma flow gradients at the separatrices between field lines with different magnetic topologies, e.g., planetary magnetospheres, helmet streamers in the solar corona, or at the boundary between the heliosphere and interstellar medium. Within this work we make a first step to understand the influence of these flows towards the occurrence of current sheets in a stationary state situation. We concentrate here on incompressible plasma flows and 2D equilibria, which allow us to find analytic solutions of the stationary magnetohydrodynamics equations (SMHD). First we solve the magnetohydrostatic (MHS) equations with the help of a Grad-Shafranov equation and then we transform these static equilibria into a stationary state with plasma flow. We are in particular interested to study SMHD-equilibria with strong plasma flow gradients perpendicular to separatrices. We find that induced thin current sheets occur naturally in such situations. The strength of the induced currents depend on the Alfv\'en Mach number and its gradient, and on the magnetic field.