Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009apj...690..619r&link_type=abstract
The Astrophysical Journal, Volume 690, Issue 1, pp. 619-643 (2009).
Astronomy and Astrophysics
Astronomy
8
Cosmic Rays, Magnetic Fields, Plasmas, Turbulence, Waves
Scientific paper
The recent result that pairs of nearby magnetic field lines diverge (converge) subexponentially rather than exponentially in a turbulent magnetized plasma is extended to a broad range of initial field-line separations. The substantial twist, Δθ, or rotation of the magnetic field lines relative to each other, simultaneous with the field-line divergence, is also demonstrated in a more general framework. Two main regimes of field-line dispersal emerge from the more general calculation of this paper. For the longer field-line separation distances ρ, for which the turbulence spectral index exceeds -1 near 3ρ-1, the field-line spreading lang(Δρ)2rang and mean-square cross-field displacement lang(Δr)2rang have very similar behaviors, and both ρ and Δr have Gaussian statistics. For shorter ρ, the statistics of ρ become log-normal. In that regime of more moderate ρ, magnetic field lines perform random walks relative to each other in the (θ, ln ρ) space that become diffusive on the field-aligned distance Δz of a few ζ B ≡ max{min(ζ II , k -1 II ), min [ζρ, (3ξ)-1ρ]}, kII being the wavenumber where the three-dimensional turbulence spectrum becomes steeper than (k 2 par + ξ2 k 2 bottom)-1, ξ the anisotropy parameter of the turbulence, and ζ II and ζρ the nonlinear scales associated with k -1 II and ρ, respectively. In the nonlinear regime of field-line dispersal where ζ II or ζρ determines ζ B , the decorrelation of the field-lines' relative random walk is driven by Δr. Application to the slow solar wind at 1 AU shows that the mixing of field-line bundles starts beyond Δz~ 0.01-0.1 AU and occurs from the smallest transversal scales up. Another major conclusion is that the closer the magnetic field lines, the faster they rotate relative to each other.
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