Particle-in-cell simulation study of the scaling of asymmetric magnetic reconnection with in-plane flow shear

Abstract

We investigate magnetic reconnection in systems simultaneously containing asymmetric (anti-parallel) magnetic fields, asymmetric plasma densities and temperatures, and arbitrary in-plane bulk flow of plasma in the upstream regions. Such configurations are common in the high-latitudes of Earth's magnetopause and in tokamaks.We investigate the convection speed of the X-line, the scaling of the reconnection rate, and the condition for which the flow suppresses reconnection as a function of upstream flow speeds. We use two-dimensional particle-in-cell simulations to capture the mixing of plasma in the outflow regions better than is possible in fluid modeling.We perform simulations with asymmetric magnetic fields, simulations with asymmetric densities, and simulations with magnetopause-like parameters where both are asymmetric. For flow speeds below the predicted cutoff velocity, we find good scaling agreement with the theory presented in Doss et al., J. Geophys. Res., 120, 7748 (2015). Applications to planetary magnetospheres, tokamaks, and the solar wind are discussed.

An Investigation of Flow Shear and Diamagnetic Drift Effects on Magnetic Reconnection at Saturn's Dawnside Magnetopause

“…The predicted critical flow shear speed is proportional to the asymmetric Alfvén speed (Doss et al, , ) . The proportionality constant is plotted as a function of event number.…”

“…The conditions needed for magnetic reconnection onset are driven by plasma flowing into the reconnection site. If these flow structures are interrupted or never form, then the reconnection process is suppressed (Cassak & Otto, ; Desroche et al, ; Doss et al, , ). The mechanisms that suppress reconnection discussed in this study are suppression by diamagnetic drifts (Swisdak et al, , ) and suppression by a flow shear at the magnetopause (Cassak & Otto, ; Desroche et al, ; Doss et al, , ).…”

“…For symmetric reconnection (when the densities and magnetic field strengths are the same in the magnetosphere and magnetosheath), the critical speed is the local Alfvèn speed (Cassak & Otto, ; Desroche et al, ). Doss et al (, ) showed that for asymmetric reconnection the critical speed for flow shear suppression is …”

“…The c A,asym factor in front of the parentheses is defined as the asymmetric outflow speed attributed to the reconnecting magnetic fields and is given by the asymmetric Alfvèn speed. Doss et al (, ) identify the following assumptions: (1) 2‐D antiparallel reconnection with no guide field (magnetic shear ~180°); (2) the flow shears on either side of the magnetopause are assumed to be parallel or antiparallel to the reconnecting magnetic fields; (3) the flow shears on either side of the magnetopause may be parallel or antiparallel to each other; (4) the outflow speeds are considered to be equal in magnitude and opposite in direction; and (5) an isolated x line. Due to the antiparallel limitation of equation , diamagnetic effects are not included explicitly in the formulation.…”

“…Further discussion on the effects of treating both mechanisms separately is explored in section below. Under completely symmetric conditions, the factor in parentheses is unity, and c A,asym tends toward the symmetric Alfvèn speed (Doss et al, , ). Thus, the critical speed needed to suppress symmetric reconnection is approximately the Alfvèn speed.…”