Plasma Physics seminar

Wed, Jan 15, 2020 10:00 am - 11:00 am
Energy Research Facility, Room 1207


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.

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