Magnetic field line path length variations and effects on solar energetic particle transport

Abstract

Modeling of time profiles of solar energetic particle (SEP) observations typically considers transport along a large-scale magnetic field with a fixed path length from the source to the observer. Here we point out that variability in the turbulent field line path length can affect the fits to SEP data and the inferred mean free path and injection profile. To explore such variability, we perform Monte Carlo simulations in representations of homogeneous 2D MHD + slab turbulence in spherical geometry and trace trajectories of field lines and full particle orbits, considering proton injection from a narrow or wide angular region near the Sun, corresponding to an impulsive or gradual solar event, respectively. We analyze our simulation results in terms of path length statistics within and among 1◦ × 1◦ pixels in heliolatitude and heliolongitude at 0.35 and 1 AU from the Sun. Field line path lengths relate to the fluctuation amplitudes experienced by the field lines, which in turn relate to the local topology of 2D turbulence. There are also systematic patterns in the peak path lengths of energetic particles arriving at different locations, because of variations in the underlying magnetic field line path lengths and variations in the pitch angle scattering experienced by the particles. We describe the effects of such path length variations on observed SEP time profiles, both in terms of path length variability at specific locations and motion of the observer with respect to turbulence topology during the course of the observations.