Formation of discontinuities and expansion waves in the outflow region of magnetic reconnection in an asymmetric current sheet

Abstract

The current sheets observed in the solar wind, magnetopause, and nightside plasma sheet can be asymmetric, in which the plasma densities and/or magnetic field magnitudes on the two sides of the current sheet are not equal. A hybrid code is used to simulate the 1D Riemann problem for the generation and evolution of magnetohydrodynamic discontinuities and expansion waves in the outflow region of magnetic reconnection in an asymmetric current sheet. In a symmetric current sheet, four types of compound structures are found: (a) rotational discontinuity (RD)-show shock (SS) compound structure: SS is attached to the downstream of RD, (b) SS-RD: SS is followed by an adjacent RD, (c) SS-RD-SS: RD is trapped inside SS, and (d) switch-off slow shock. In the asymmetric current sheet, the rotational angle of magnetic field across an RD on the side with a higher plasma density is usually larger than that with a lower plasma density. In the asymmetric cases, a single SS may appear. For an asymmetric current sheet, a compound structure of diffusive contact discontinuity/slow expansion wave is formed behind an SS-RD compound structure or a pure RD on the side with a very high plasma density.