Diffusion Confusion: Field Line Random Walk in Magnetic Turbulence

Details Diffusion Confusion: Field Line Random Walk in Magnetic Turbulence Diffusion Confusion: Field Line Random Walk in Magnetic Turbulence

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsh42a..05b&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #SH42A-05

Physics

2104 Cosmic Rays, 2149 Mhd Waves And Turbulence (2752, 6050, 7836), 2164 Solar Wind Plasma, 7859 Transport Processes, 7863 Turbulence (4490)

Scientific paper

Magnetic field lines in a plasma with both a mean field and a turbulent component will generally spread away from the mean field direction in a process called field line random walk (FLRW). Theoretical treatments generally agree that the character of this spreading is governed by the behavior of the turbulence spectrum at very large scales. In slab turbulence, subdiffusion, superdiffusion, and ordinary classical diffusion are all mathematically possible, while in two-dimensional (2D) turbulence diffusive and superdiffusive regimes occur. This work aims to delimit the possible behaviors of FLRW in terms of the turbulence correlation function and wave spectrum. In the case of slab turbulence, a magnetic correlation function that has bounded support (i.e., correlations vanish beyond some sufficiently large distance) rules out the superdiffusive regime of FLRW. Subdiffusion remains technically possible, but only for correlation functions with special properties. In the case of 2D turbulence, the homogeneity condition suffices to place FLRW in the diffusive regime. Further, even for inhomogeneous systems that display asymptotic superdiffusive behavior, this behavior is obtained only for the idealized case of an infinitely large system with turbulence wave modes populating arbitrarily large scales. Imposing a finite system size (or an upper wavelength limit) restores asymptotic diffusive behavior to FLRW. Supported by NASA Heliophysics Guest Investigator grant NNX07AH73G, by NASA Heliophysics Theory grant NNX08AI47G, and by the Thailand Research Fund.

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