Abstract
Magnetic reconnection, topological changes in magnetic fields, is a fundamental process in magnetized plasmas. It is associated with energy release in regions of magnetic field annihilation, but this is only one facet of this process. Astrophysical fluid flows normally have very large Reynolds numbers and are expected to be turbulent, in agreement with observations. In strong turbulence, magnetic field lines constantly reconnect everywhere and on all scales, thus making magnetic reconnection an intrinsic part of the turbulent cascade. We note in particular that this is inconsistent with the usual practice of magnetic field lines as persistent dynamical elements. A number of theoretical, numerical, and observational studies starting with the paper done by Lazarian and Vishniac [Astrophys. J. 517, 700–718 (1999)] proposed that 3D turbulence makes magnetic reconnection fast and that magnetic reconnection and turbulence are intrinsically connected. In particular, we discuss the dramatic violation of the textbook concept of magnetic flux-freezing in the presence of turbulence. We demonstrate that in the presence of turbulence, the plasma effects are subdominant to turbulence as far as the magnetic reconnection is concerned. The latter fact justifies a magnetohydrodynamiclike treatment of magnetic reconnection on all scales much larger than the relevant plasma scales. We discuss the numerical and observational evidence supporting the turbulent reconnection model. In particular, we demonstrate that the tearing reconnection is suppressed in 3D, and unlike the 2D settings, 3D reconnection induces turbulence that makes magnetic reconnection independent of resistivity. We show that turbulent reconnection dramatically affects key astrophysical processes, e.g., star formation, turbulent dynamo, and acceleration of cosmic rays. We provide criticism of the concept of “reconnection-mediated turbulence” and explain why turbulent reconnection is very different from enhanced turbulent resistivity and hyper-resistivity and why the latter have fatal conceptual flaws.
Highlights
1976; Priest and Forbes, 2002)
Turbulence: Summary of basic facts For our discussion of turbulent reconnection, it is important that turbulence is ubiquitous in astrophysical environments and that Alfvenic modes usually represent the dominant component of the MHD turbulent cascade
It is difficult to understand the properties of turbulent fluids if magnetic reconnection does not happen on the dynamical time of the fluid motions
Summary
Magnetic fields are ubiquitous in astrophysical systems and critically affect the dynamics and properties of magnetized plasmas over an extended range of scales. It is often overlooked that the observations of solar activity indicate that the reconnection rates can significantly vary This presents serious problems for theories that are based on relating the rate of magnetic reconnection to the peculiar properties of plasmas. These properties do not change, for instance, as the flux gets accumulated prior to a solar flare and gets annihilated during the flare. 2D numerical work has already shown that magnetic reconnection can be accelerated significantly by the tearing instability (Loureiro et al, 2007; Lapenta, 2008; Daughton et al, 2009a, 2009b; Bhattacharjee et al, 2009; Cassak et al, 2009) In a sense, this model has some similarities to the turbulent reconnection.
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