Boosting magnetic reconnection by viscosity and thermal conduction

Abstract

Nonlinear evolution of magnetic reconnection is investigated by means of magnetohydrodynamic simulations including uniform resistivity, uniform viscosity, and anisotropic thermal conduction. When viscosity exceeds resistivity (the magnetic Prandtl number P r m > 1), the viscous dissipation dominates outflow dynamics and leads to the decrease in the plasma density inside a current sheet. The low-density current sheet supports the excitation of the vortex. The thickness of the vortex is broader than that of the current for P r m > 1. The broader vortex flow more efficiently carries the upstream magnetic flux toward the reconnection region, and consequently boosts the reconnection. The reconnection rate increases with viscosity provided that thermal conduction is fast enough to take away the thermal energy increased by the viscous dissipation (the fluid Prandtl number P r < 1). The result suggests the need to control the Prandtl numbers for the reconnection against the conventional resistive model.

Pickup Ion–Mediated Magnetic Reconnection in the Outer Heliosphere

“…These Prandtl numbers match a condition that corresponds to a boost in magnetic reconnection. Minoshima et al (2016) perform 2D single-fluid MHD simulations that include heat conduction and viscosity effects, and find that a small fluid Prandtl number and a large magnetic Prandtl number (Pr < 1 and Pr m > 1) preferentially boosts magnetic reconnection. Their interpretation is that having the viscosity stronger than the resistivity (Pr m > 1) results in a broader vortex layer than the current sheet layer, and the quadrupolar vortex excited in the vortex layer efficiently carries the upstream magnetic flux toward the reconnection region and the reconnection rate increases.…”

A High-order Weighted Finite Difference Scheme with a Multistate Approximate Riemann Solver for Divergence-free Magnetohydrodynamic Simulations

“…The nonlinearity of the numerical resistivity in the CUCT-AA method would be relatively weak, and it results in the elongated current sheet and the subsequent plasmoid formation that is a resemblance to the tearing instability for a uniform resistivity (Biskamp 1986). In order to test this hypothesis, we apply the schemes to the visco-resistive MHD equations (Minoshima et al 2016), and solve the same problem. Kinematic viscosity and resistivity coefficients, ν and η, are assumed constant and uniform, and the corresponding kinetic and magnetic Reynolds numbers are V A λ/ν = V A λ/η = 10 3 .…”