Trapping and diffusive escape of field lines in two-component magnetic turbulence

Abstract

Recent studies have shown that transport along magnetic field lines in turbulent plasmas admits a surprising degree of persistent trapping due to small-scale topological structures. This underlies the partial filamentation of magnetic connection from small regions of the solar corona to Earth orbit, as indicated by the observed dropouts (i.e., inhomogeneity and sharp gradients) of solar energetic particles. We explain the persistence of such topological trapping using a two-component model of magnetic turbulence with slab and two-dimensional (2D) fluctuations, which has provided a useful description of transport phenomena in the solar wind. In the presence of slab turbulence, the diffusive escape of field lines from 2D orbits is suppressed by either a strong or an irregular 2D field. For slab turbulence superposed on a 2D field with a single, circular island, we present an analytic theory, confirmed by numerical simulations, for the trapping length and its dependence on various parameters. For a turbulent 2D+slab field, we find that the filamentation of magnetic connectivity to the source is sharply delineated by local trapping boundaries, defined by a local maximum in the mean squared field along the 2D orbit, because of a similar suppression effect. We provide a quasi-linear theory for field-line diffusion in a turbulent 2D+slab field, which indicates that irregularity of the 2D orbit enhances the suppression of slab diffusion. The theory is confirmed by computer simulations. These concepts provide a physical explanation of the persistence and sharpness of dropouts of solar energetic particles at Earth orbit. © 2007. The American Astronomical Society. All rights reserved.