Asymmetric Diffusion of Magnetic Field Lines

Charged particles in magnetized plasmas move preferentially along magnetic field lines. The perpendicular transport and mixing are suppressed in quiet, laminar plasmas. In turbulent plasmas, however, magnetic field lines are stochastic and this accounts for a big part of perpendicular transport. Magnetic field lines separate faster than diffusively in turbulent plasma, which is called superdiffusion. Furthermore, we discovered that this superdiffusion is, in general, asymmetric, so that the separation of field lines along the magnetic field direction is different from the separation in the opposite direction, if the symmetry of the flow is broken by the so-called imbalance or cross-helicity. The difference between forward and backward diffusion, however, is not directly due to imbalance, but a non-trivial consequence of both imbalance and non-reversibility of turbulence. The asymmetric diffusion perpendicular to the mean magnetic field entails a variety of new physical phenomena, such as the production of parallel particle streaming in the presence of perpendicular gradients. Such streaming and associated instabilities are important for particle transport in laboratory, space, and astrophysical plasmas.