Magnetic Reconnection and Turbulent Mixing: From ISM to Clusters of Galaxies

Magnetic reconnection, or the ability of the magnetic field lines that are frozen in plasma to change their topology, is a fundamental problem of magnetohydrodynamics (MHD). Webriefly examine the problem starting with the well-known Sweet-Parker scheme, discuss effectsof tearing modes, anomalous resistivity and the concept of hyperresistivity. We show that the field stochasticity by itself provides a way toenable fast reconnection even if, at the scale of individual turbulent wiggles,the reconnection happens at the slow Sweet-Parker rate. We show that fast reconnectionallows efficient mixing of magnetic field in the direction perpendicular tothe local direction of magnetic field. While the idea of stochastic reconnection still requiresnumerical confirmation,our numerical simulations testify that mixing motions perpendicular to the local magnetic field are upto high degree hydrodynamical. This suggests that the turbulent heattransport should be similar to that in non-magnetized turbulent fluid, namely,should have a diffusion coefficient ～V _L L, whereV _L is the amplitude of the turbulent velocity and L is the scale of the turbulent motions. We present numericalsimulations which support this conclusion. The applicationof this idea to thermal conductivity in clusters of galaxies shows that thismechanism may dominate the diffusion of heat and may be efficient enoughto prevent cooling flow formation.