Abstract
Abstract. The concept of diffusion in collisionless space plasmas like those near the magnetopause and in the geomagnetic tail during reconnection is reexamined making use of the division of particle orbits into waiting orbits and break-outs into ballistic motion lying at the bottom, for instance, of Lévy flights. The rms average displacement in this case increases with time, describing superdiffusion, though faster than classical, is still a weak process, being however strong enough to support fast reconnection. Referring to two kinds of numerical particle-in-cell simulations we determine the anomalous diffusion coefficient, the anomalous collision frequency on which the diffusion process is based, and construct a relation between the diffusion coefficients and the resistive scale. The anomalous collision frequency from electron pseudo-viscosity in reconnection turns out to be of the order of the lower-hybrid frequency with the latter providing a lower limit, thus making similar assumptions physically meaningful. Tentative though not completely justified use of the κ distribution yields κ ≈ 6 in the reconnection diffusion region and, for the anomalous diffusion coefficient, the order of several times Bohm diffusivity.
Highlights
Anomalous diffusion is the summary heading of all processes where the ensemble averaged mean-square displacement x2 ∝ tγ deviates from linear time dependence γ = 1 with the classical (Einstein) diffusion coefficient Dcl = 2T νc/m, with T the temperature, and νc the classical binary collision frequency
On this note, based on available numerical simulations, we demonstrate by estimating the anomalous collision frequency νa that magnetic merging during reconnection can well be understood as a localised anomalous diffusion process
As we demonstrate, anomalous diffusion in reconnection results from processes leading to waiting statistics and causing gyro-viscosity
Summary
On this note, based on available numerical simulations, we demonstrate by estimating the anomalous collision frequency νa that magnetic merging during reconnection can well be understood as a localised anomalous diffusion process. This result satisfactorily unifies the two originally different views on plasma transport across an apparently impermeable boundary like the magnetopause. Most of the observed diffusive particle spectra (cf., e.g., Christon et al, 1989, 1991, for the most elaborate observations in near-Earth space) barely exhibit the shapes resulting from quasilinear diffusion They turn out to be power law distributions both in energy and momentum space, most frequently being described best by so-called κ distributions x2 −(κ+1+d/2). As we demonstrate, anomalous diffusion in reconnection results from processes leading to waiting statistics and causing gyro-viscosity
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