Lagrangian Statistics for Dispersion in Magnetohydrodynamic Turbulence
Measurements in the heliosphere and high-resolution fluid simulations
give clear indications for the anisotropy of plasma turbulence in the
presence of magnetic fields. How this anisotropy affects transport
processes like diffusion and dispersion remains an open question. The
first efforts to characterize Lagrangian single-particle diffusion and
two-particle dispersion in incompressible magnetohydrodynamic (MHD)
turbulence were performed a decade ago. We revisit those pioneering
results through updated simulations performed at higher Reynolds number.
We present new investigations that use the dispersion of many Lagrangian
tracer particles to examine the extremes of dispersion and the
anisotropy in direct numerical simulations. We then point out directions
in which Lagrangian statistics need to be developed to address the
fundamental problem of anisotropic MHD turbulence and transport in solar
and stellar winds.