Title : Asymmetric diffusion of magnetic field lines Speaker Name: Andrey Beresenyak Speaker Institution : Naval Research Laboratory
Abstract : Particles interact via fields in a collisionless plasma. This interaction is a key to understand the transport, for example, in the context of solar wind - how particles from an active region on the Sun spread around its initial magnetic flux tube. Perhaps even more importantly, we need to understand how plasmas end up in non-equilibrium states and how free energy from large scales drives this process. I looked at field line diffusion, which is the main contributor to perpendicular transport in collisionless, turbulent magnetized plasmas. Amazingly, this diffusion, which is a purely geometric property of magnetic field lines, does not have to be symmetric, e.g. particles following the field line along the direction of the magnetic field can have a different perpendicular diffusion coefficient than particles propagating in the opposite direction. This can create free energy in particles entirely due to this peculiar geometry of field lines, for example, perpendicular particle gradients in such a system will result in particle streaming. Although it seems like magic, this does not break the second law of thermodynamics, because it relies on time-irreversibility of turbulence, which, in turn, follows from the fact that turbulence produces entropy. In an artificial system having time-symmetric random fields, such an effect is absent. MHD and PIC simulations of imbalanced turbulence show asymmetric diffusion. It should be important in the solar wind, which is turbulent, imbalanced and has high perpendicular gradients in particles.