Trapping and Diffusive Escape of Field Lines in Two‐Component Magnetic Turbulence

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.,inhomogeneityandsharpgradients)ofsolarenergeticparticles.Weexplainthepersistenceof suchtopological 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 turbu- lence, the diffusive escape offield 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 sup- pressioneffect.Weprovideaquasi-lineartheoryforfield-linediffusioninaturbulent2D+slabfield,whichindicates 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.