Flow-turbulence interaction in magnetic reconnection

Physics of Plasmas, 2013

Through the enhancement of transport, turbulence is expected to contribute to the fast reconnection. However the effects of turbulence are not so straightforward. In addition to the enhancement of transport, turbulence under some environment shows effects that suppress the transport. In the presence of turbulent cross helicity, such a dynamic balance between the transport enhancement and suppression occurs. As this result of dynamic balance, the region of effective enhanced magnetic diffusivity is confined to a narrow region, leading to the fast reconnection. In order to confirm this idea, a self-consistent turbulence model for the magnetic reconnection is proposed. With the aid of numerical simulations where turbulence effects are incorporated in a consistent manner through the turbulence model, the dynamic balance in the turbulence magnetic reconnection is confirmed.

Physics of Plasmas, 2013

Through the enhancement of transport, turbulence is expected to contribute to the fast reconnection. However the effects of turbulence are not so straightforward. In addition to the enhancement of transport, turbulence under some environment shows effects that suppress the transport. In the presence of turbulent cross helicity, such a dynamic balance between the transport enhancement and suppression occurs. As this result of dynamic balance, the region of effective enhanced magnetic diffusivity is confined to a narrow region, leading to the fast reconnection. In order to confirm this idea, a self-consistent turbulence model for the magnetic reconnection is proposed. With the aid of numerical simulations where turbulence effects are incorporated in a consistent manner through the turbulence model, the dynamic balance in the turbulence magnetic reconnection is confirmed.

Monthly Notices of the Royal Astronomical Society: Letters, 2009

Two-dimensional numerical simulations of the effect of background turbulence on 2D resistive magnetic reconnection are presented. For sufficiently small values of the resistivity (η) and moderate values of the turbulent power (), the reconnection rate is found to have a much weaker dependence on η than the Sweet-Parker scaling of η 1/2 and is even consistent with an η-independent value. For a given value of η, the dependence of the reconnection rate on the turbulent power exhibits a critical threshold in above which the reconnection rate is significantly enhanced.

Proceedings of the International Astronomical Union, 2008

Turbulent reconnection is studied by means of three dimensional (3D) compressible magnetohydrodynamical numerical calculations. The process of homogeneous turbulence is set up by adding three-dimensional solenoidal random forcing implemented in the spectral space at small wave numbers with no correlation between velocity and forcing. We apply the initial Harris current sheet configuration together with a density profile calculated from the numerical equilibrium of magnetic and gas pressures. We assume that there is no external driving of the reconnection. The reconnection develops as a result of the initial vector potential perturbation. We use open boundary conditions. Our main goal is to find the dependencies of reconnection rate on different properties of turbulence. The results of our simulations show that turbulence significantly affects the topology of magnetic field near the diffusion region. We present that the reconnection speed does not depend on the Reynolds numbers as well the magnetic diffusion. In addition, a fragmentation of current sheet decreases the disparity in inflow/outflow ratios. When we apply the large scale and more powerful turbulence the reconnection is faster.

Astrophysical fluids have very large Reynolds numbers and there- fore turbulence is their natural state. Magnetic reconnection is an important process in many astrophysical plasmas, which allows restructuring of magnetic fields and conversion of stored magnetic energy into heat and kinetic energy. Turbulence is known to dramatically change different transport processes and therefore it is not unexpected that turbulence can alter the dynamics of mag- netic field lines within the reconnection process. We shall review the interaction between turbulence and reconnection at different scales, showing how a state of turbulent reconnection is natural in astrophysical plasmas, with implica- tions for a range of phenomena across astrophysics. We consider the process of magnetic reconnection that is fast in magnetohydrodynamic (MHD) limit and discuss how turbulence - both externally driven and generated in the re- connecting system - can make reconnection independent on the microphysical properties of plasmas. We will also show how relaxation theory can be used to calculate the energy dissipated in turbulent reconnecting fields. As well as heating the plasma, the energy dissipated by turbulent reconnection may cause acceleration of non-thermal particles, which is briefly discussed here.

The Astrophysical Journal, 2015

Plasma flows with an MHD-like turbulent inertial range, such as the solar wind, vitiate many assumptions of standard theories of magnetic reconnection. In particular, the "roughness" of turbulent velocity and magnetic fields implies that magnetic field-lines are nowhere "frozen-in" in the usual sense. This situation demands an essential generalization of the so-called "General Magnetic Reconnection" (GMR) theory. Following ideas of Axford and Lazarian & Vishniac, we identify magnetic field-lines by "tagging" them with plasma fluid elements and then determine their slip-velocity relative to the plasma fluid by integrating in arc-length along the wandering field-lines. The main new concept introduced here is the slip-velocity source vector, which gives the rate of development of slip-velocity per unit arc-length of field line. The slip-source vector is the ratio of the curl of the non-ideal electric field R in the Generalized Ohm's Law and the magnetic field strength. It diverges at magnetic nulls, unifying GMR with theories of magnetic null-point reconnection. Only under restrictive assumptions is the slip-velocity related to the gradient of the line-voltage or "quasi-potential" obtained by integrating the parallel electric field R along field-lines. In an MHD turbulent inertial-range ∇×R becomes extremely large while R is tiny, so that line-slippage occurs freely even as a description by ideal MHD becomes accurate. This "paradox" is resolved by the understanding that ideal MHD is valid for a turbulent inertialrange not in the standard sense but in a "weak" sense which does not imply magnetic linefreezing. The mathematical notion of "weak solution" is here explained in physical terms of spatial coarse-graining and renormalization-group (RG) theory. We give a new first-principles argument for the "weak" validity of the ideal Ohm's law in the inertial range, via rigorous estimates of the terms in the Generalized Ohm's Law for an electron-ion plasma. Particular attention is paid to the conditions in the solar wind, a collisionless, magnetized plasma. Coarsegrained to inertial-range lengths, all of the non-ideal terms (from collisional resistivity, Hall field, electron pressure anisotropy, and electron inertia) are shown to be irrelevant in the RG sense and large-scale reconnection is thus governed solely by ideal dynamics. We briefly discuss some implications for heliospheric reconnection, in particular for deviations from the Parker spiral model of interplanetary magnetic field. Solar wind observations show that reconnection in a turbulence-broadened heliospheric current sheet, consistent with the Lazarian & Vishniac (1999) theory, leads to slip velocities that cause field-lines to lag relative to the spiral model.

Physics of Plasmas, 2010

The nonlinear dynamics of magnetic reconnection in turbulence is investigated through direct numerical simulations of decaying, incompressible, two-dimensional magnetohydrodynamics. Recently, it was shown by Servidio et al. ͓Phys. Rev. Lett. 102, 115003 ͑2009͔͒ that in fully developed turbulence complex processes of reconnection occur locally. Here, the main statistical features of these multiscale reconnection events are further described, providing details on the methodology. It is found that is possible to describe the reconnection process in turbulence as a generalized local Sweet-Parker process in which the parameters are locally controlled by the turbulence cascade, thus providing a step toward reconciling classical turbulence analysis with reconnection theory. This general description of reconnection may be useful for laboratory and space plasmas, where the presence of turbulence plays a crucial role.

Physical Review Letters, 2008

Within a MHD approach we find magnetic reconnection to progress in two entirely different ways. The first is well-known: the laminar Sweet-Parker process. But a second, completely different and chaotic reconnection process is possible. This regime has properties of immediate practical relevance: i) it is much faster, developing on scales of the order of the Alfvén time, and ii) the areas of reconnection become distributed chaotically over a macroscopic region. The onset of the faster process is the formation of closed circulation patterns where the jets going out of the reconnection regions turn around and forces their way back in, carrying along copious amounts of magnetic flux.

2000

La reconección es el proceso mediante el cual los campos magnéticos cambian de topología en un medio conductor y es fundamental para entender diferentes procesos, incluyendo la turbulencia interestelar y los dinamos estelares y galácticos. Para explicar el campo galáctico y las ráfagas y el ciclo solar, la reconección debe ser rápida y propagarse a la velocidad de Alfvén. Trabajos anteriores consideraban fluidos laminares y obtenían tasas de reconección pequeñas. Mostramos que la presencia de una componente aleatoria del campo magntico permite una reconnección rápida ya que, a diferencia del caso laminar donde el proceso avanza línea por línea, en el caso turbulento participan muchas líneas simultáneamente. Una fracción importante de la energía magnética se va a la turbulencia MHD, lo cual aumenta la tasa de reconección al aumentar la parte aleatoria del campo. Como consecuencia, los dinamos solares y galácticos también se vuelven rápidos.

Physical Review Letters, 2013

We report simulation results for turbulent magnetic reconnection obtained using a newly developed Reynolds-averaged magnetohydrodynamics model. We find that the initial Harris current sheet develops in three ways, depending on the strength of turbulence: laminar reconnection, turbulent reconnection, and turbulent diffusion. The turbulent reconnection explosively converts the magnetic field energy into both kinetic and thermal energy of plasmas, and generates open fast reconnection jets. This fast turbulent reconnection is achieved by the localization of turbulent diffusion. Additionally, localized structure forms through the interaction of the mean field and turbulence.