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Lookup NU author(s): Dr Andrew Snodin
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In astrophysical plasmas, the magnetic field line random walk (FLRW) plays an important role in guiding particle transport. The FLRW behavior is scaled by the Kubo number R = (b / B0) (l|| / l⟂) for rms magnetic fluctuation b, large-scale mean field B0, and coherence scales parallel (l||) and perpendicular (l⟂) to B0. Here we use a nonperturbative analytic framework based on Corrsin's hypothesis, together with direct computer simulations, to examine the R-scaling of the FLRW for varying B0 with finite b and isotropic fluctuations with l|| / l⟂ = 1, instead of the well-studied route of varying l|| / l⟂for b ≪ B0. The FLRW for isotropic magnetic fluctuations is also of astrophysical interest regarding transport processes in the interstellar medium. With a mean field, fluctuations may have variance anisotropy, so we consider limiting cases of isotropic variance and transverse variance (with bz = 0). We obtain analytic theories, and closed-form solutions for extreme cases. Padé approximants are provided to interpolate all versions of theory and simulations to any B0. We demonstrate that, for isotropic turbulence, Corrsin-based theories generally work well, and with increasing R there is a transition from quasilinear to Bohm diffusion. This holds even with bz = 0, when different routes to R→∞ are mathematically equivalent; in contrast with previous studies, we find that a Corrsin-based theory with random ballistic decorrelation works well even up to R = 400, where the effects of trapping are barely perceptible in simulation results.
Author(s): Sonsrettee W, Subedi P, Ruffolo D, Matthaeus WH, Snodin AP, Wongpan P, Chuychai P, Rowlands G, Vyas S
Publication type: Article
Publication status: Published
Journal: Astrophysical Journal Supplement Series
Online publication date: 05/08/2016
Acceptance date: 31/05/2016
ISSN (print): 0067-0049
ISSN (electronic): 1538-4365
Publisher: IOP Publishing Ltd.
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