Small scale topology of solar wind turbulence and energetic particle transport

Abstract

The observed dropouts of solar energetic particles from impulsive solar events (i.e., the inhomogeneity and sharp gradients in particle density) indicate the partial filamentation of magnetic connection from small regions of the solar corona to Earth orbit. This can be understood in terms of trapping of field lines due to small-scale topological structures in the solar wind. We further explore how this turbulence structure should be manifest in particle observations. Using a set of numerical experiments using the two-component (2D+slab) description of magnetic turbulence, we evaluate particle trajectories using the fundamental Newton-Lorentz equations. We adapt the two-component model to a 2D Gaussian field in Cartesian geometry, and a 2D turbulent field in spherical geometry, which includes the focusing of particles. The 2D turbulent field is generated by a 2D fast Fourier transform, a valid approximation over a small angular region. The charged particles can be temporarily trapped in flux tubes and then escape due to random turbulent perturbations in the magnetic field. The overall effect is a delay in the onset of time-asymptotic transport. For the 2D turbulent field, at 1 AU our simulations of particle density integrated over time for various energies match quite closely with the magnetic field line density. That means the particles of energy < 1 GeV follow the field lines very closely, so "dropout" patterns can be modeled by field line tracing (in spherical geometry) as well as particle tracing.