MAGNETIC FIELD LINE RANDOM WALK IN ISOTROPIC TURBULENCE WITH ZERO MEAN FIELD

In astrophysical plasmas, magnetic field lines often guide the motions of thermal and non-thermal particles. The field line random walk (FLRW) is typically considered to depend on the Kubo number R = (b / B-0)(l(parallel to) /l(perpendicular to)) for rms magnetic fluctuation b, large-scale mean field B-0, and parallel and perpendicular coherence scales l(parallel to) and l(perpendicular to), respectively. Here we examine the FLRW when R -> infinity by taking B-0 -> 0 for finite bz (fluctuation component along B-0), which differs from the well-studied route with b(z) = 0 or b(z) << B-0 as the turbulence becomes quasi-two-dimensional (quasi-2D). Fluctuations with B-0 = 0 are typically isotropic, which serves as a reasonable model of interstellar turbulence. We use a non-perturbative analytic framework based on Corrsin's hypothesis to determine closed-form solutions for the asymptotic field line diffusion coefficient for three versions of the theory, which are directly related to the k(-1) or k(-2) moment of the power spectrum. We test these theories by performing computer simulations of the FLRW, obtaining the ratio of diffusion coefficients for two different parameterizations of a field line. Comparing this with theoretical ratios, the random ballistic decorrelation version of the theory agrees well with the simulations. All results exhibit an analog to Bohm diffusion. In the quasi-2D limit, previous works have shown that Corrsin-based theories deviate substantially from simulation results, but here we find that as B-0 -> 0, they remain in reasonable agreement. We conclude that their applicability is limited not by large R, but rather by quasi-two-dimensionality.