1301.7424 (Andrey Beresnyak)

On the Rate of Spontaneous Magnetic Reconnection    [PDF]

Andrey Beresnyak

Magnetic reconnection is a topological rearrangement of the magnetic field lines, leading to the release of magnetic energy, which is thought to be associated with solar flares, coronal mass ejections and magnetospheric storms. Despite magnetic field lines are supposed to be frozen into the well-conducting plasma, the reconnection observed in nature is, typically, fast, so that the rate of convergence of the magnetic field lines is the fraction of the Alfven speed, v_A. The Sweet-Parker solution predicts reconnection rates which are negligible for the solar or astrophysical conditions, this have prompted research into collisionless reconnection. The stochasticity of magnetic field lines due to ambient turbulence leads to fast reconnection and the rate was predicted to be proportional to kinetic energy density of ambient turbulence. Also, tearing instability of the thin current sheet was proposed as a driver of resistivity-independent reconnection, which was shown to be consistent with two-dimensional simulations. In this Letter I report three-dimensional high resolution simulations of the nonlinear evolution of the thin current sheet which spontaneously evolves into a turbulent current layer and shows a constant reconnection rate of ~ 0.015 v_A and the dissipation rate per unit area ~ 0.006 \rho v_A^3, independent of the Lundquist number. Unlike the two-dimensional case, where reconnection is dominated by the ejection of large plasmoids and is very unsteady, I observed surprisingly steady rate. The turbulence is being fueled by the free energy of the oppositely directed magnetic fields. Around 40% of this energy is dissipated and 60% is converted into the turbulent fields. I conclude that the reconnection rate and the heating rate per unit area in the nearly-ideal current layers has a robust lower limit due to its inherent stochasticity.

View original: http://arxiv.org/abs/1301.7424