Variation in Path Lengths of Turbulent Magnetic Field Lines and Solar Energetic Particles

TL;DR

This study investigates how variability in turbulent magnetic field line path lengths influences the modeling of solar energetic particle (SEP) observations, highlighting the importance of turbulence-induced path length fluctuations on SEP data interpretation.

Contribution

The paper introduces Monte Carlo simulations of magnetic turbulence to analyze path length variability and its impact on SEP time profiles and data fitting.

Findings

Path length variability exhibits systematic spatial patterns.

Turbulence amplitude and geometry significantly affect path length distributions.

Variations influence SEP time profile modeling and interpretation.

Abstract

Modeling of time profiles of solar energetic particle (SEP) observations often 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 adapted to 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 field line and particle path length statistics for $1^\circ\times 1^\circ$ pixels in heliolatitude and heliolongitude at 0.35 and 1 AU from the Sun, for different values of the turbulence amplitude $b/B_0$ and turbulence geometry as expressed by the slab fraction $f_s$. Maps of the most probable path lengths of field lines and particles at each pixel exhibit systematic patterns that reflect the fluctuation amplitudes experienced by the field lines, which in turn relate to the local topology of 2D turbulence. We describe the effects of such path length variations on 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.