Speaker
Madalina Vlad
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/O3.J404.pdf
Average flows and stochastic islands in the
magnetic field line random walk
M. Vlad, F. Spineanu
National Institute of Laser, Plasma and Radiation Physics, Magurele-Bucharest, Romania
The nonlinear effects that appear in the field line random walk (FLRW) in magnetic
turbulence are discussed. The magnetic lines show both random and quasi-coherent aspects.
The random motion leads to diffusive transport while the coherent motion is associated with
trapping or eddying in the structure of the stochastic field. The strength of each of these
aspects depends on the parameters of the turbulence reflected in the magnetic Kubo number
𝑅 = 𝐿𝐼𝐼 𝛽/𝐿⊥ 𝐵0 , where 𝛽 is the amplitude of the stochastic magnetic field b, B0 is the
average magnetic field, 𝐿𝐼𝐼 , 𝐿⊥ are the correlation lengths of b along and perpendicular to
B0, respectively. Trapping is negligible for R < 1, and it is statistically relevant for R > 1, with
effects that become stronger as R increases. At large R the field line statistics is a mixture of
random and quasi-coherent behaviour, with diffusive spreading combined with the
generation of stochastic islands. In the limit R →∞, the transport is subdiffusive.
In special conditions, orientated steps of the field lines can appear even if the average of b is
zero. They lead to quasi-coherent flows of the magnetic lines. We identify and analyse two
sources of flows in FLRW, the large scale variation of the amplitude 𝛽 and of B0.
We have shown that the gradient of 𝛽 determines a quasi-coherent flow along its direction.
The average velocity Vf is an increasing function of R at R<1, it reaches a maximum around
R=1 and eventually decreases. The decrease is determined by the stochastic magnetic islands,
which yield an average velocity in the opposite direction. This process determines the
tendency of field line accumulation on the maxima of the amplitude 𝛽.
The gradient of B0 also determines a flow, but with different properties and physical reasons.
It is similar to a process found in confined plasmas [3].
The transport coefficients, the average size of the stochastic magnetic islands and the average
flows of the magnetic lines are determined. Our instrument, the decorrelation trajectory
method [4] is essentially analytical.
[1] Vlad M and Spineanu F 2014 Astrophys. J. 791 56
[2] Vlad M, Spineanu F and Croitoru A 2015 Astrophys. J. 815 11
[3] Vlad M, Spineanu F and Benkadda S 2006 Phys. Rev. Lett. 96 085001
[4] Vlad M, Spineanu F, Misguich J H and Balescu R 1998 Phys. Rev. E 58 7359
