Heat Transfer and Reconnection Diffusion in Turbulent Magnetized Plasmas

TL;DR

This paper reviews how turbulence and magnetic reconnection influence heat transfer in astrophysical plasmas, emphasizing reconnection diffusion and comparing it with electron streaming along stochastic magnetic fields.

Contribution

It introduces the concept of reconnection diffusion as a key mechanism for heat transfer in turbulent magnetized plasmas, integrating modern MHD turbulence understanding.

Findings

Reconnection diffusion enables plasma to diffuse between magnetized eddies.

Eddy advection of heat often dominates over electron streaming in astrophysical conditions.

Superdiffusion of heat can occur on scales smaller than turbulence injection scale.

Abstract

It is well known that magnetic fields constrain motions of charged particles, impeding the diffusion of charged particles perpendicular to magnetic field direction. This modification of transport processes is of vital importance for a wide variety of astrophysical processes including cosmic ray transport, transfer of heavy elements in the interstellar medium, star formation etc. Dealing with these processes one should keep in mind that in realistic astrophysical conditions magnetized fluids are turbulent. In this review we single out a single transport process, namely, heat transfer and consider how it occurs in the presence of the magnetized turbulence. We show that the ability of magnetic field lines to constantly change topology and connectivity is at the heart of the correct description of the 3D magnetic field stochasticity in turbulent fluids. This ability is ensured by fast magnetic reconnection in turbulent fluids and puts forward the concept of reconnection diffusion at the core of the physical picture of heat transfer in astrophysical plasmas. Appealing to reconnection diffusion we describe the ability of plasma to diffuse between different magnetized eddies explaining the advection of the heat by turbulence. Adopting the structure of magnetic field that follows from the modern understanding of MHD turbulence, we also discuss thermal conductivity that arises as electrons stream along stochastic magnetic field lines. We compare the effective heat transport that arise from the two processes and conclude that in many astrophysically-motivated cased eddy advection of heat dominates. Finally, we discuss the concepts of sub and superdiffusion and show that the subdiffusion requires rather restrictive settings. At the same time, accelerated diffusion or superdiffusion of heat is possible on the scales less than the injection scale of the turbulence.