Charged energetic particles diffusion in solar wind with flux-tube-structures

Recent studies suggest that there may be flux-tube-like structures exist in the solar wind, where upon crossing the boundaries of such structures, the magnetic field directions change significantly. In such a scenario, the turbulence in the solar wind is confined in individual flux tubes and is described locally by the underlying solar wind MHD turbulence. The drastic changes of magnetic field direction introduce another source of MHD turbulence intermittency and can affect the transport of energetic particles. However, to theoretically study transport of energetic particles in solar wind, magnetic field is usually assumed to be composed by a uniform large scale background magnetic field with a superposed turbulent field. In this paper we construct a model turbulence of the solar wind by including the flux-tube-like structures. We start by cutting the solar wind randomly into smaller cells. In each of these cells, the magnetic field consists of a uniform mean magnetic field B ₀ with random direction and a model of transverse magnetic fluctuations (e.g. a slab turbulence model). In order to study the effect of flux tubes on the particles' transport, we numerically calculate test particles' trajectories in the new model of the turbulence. Our results show that the effects of flux tubes in the solar wind can affect the transport of energetic particles significantly by causing much stronger scatering in both directions parallel and perpendicular to the large scale background magnetic field. As a consequence, large scale perpendicular true diffusion is recovered from subdiffusion with local intrinsic pure slab turbulence.