Solar energetic particle access to distant longitudes through turbulent field-line meandering

TL;DR

This paper introduces a novel model for solar energetic particle (SEP) propagation that accounts for turbulent magnetic field-line meandering, successfully explaining rapid and wide SEP access across longitudes, which traditional diffusion models cannot.

Contribution

The study presents a new non-diffusive SEP propagation model based on magnetic field-line meandering, improving the understanding of early wide SEP events beyond traditional diffusion approaches.

Findings

Reproduces observed SEP longitudinal extent of 30-50° with the new model.

Explains the early timing of SEP arrival at distant longitudes.

Shows traditional diffusion models underestimate SEP spread.

Abstract

Context. Current solar energetic particle (SEP) propagation models describe the effects of interplanetary plasma turbulence on SEPs as diffusion, using a Fokker-Planck (FP) equation. However, FP models cannot explain the observed fast access of SEPs across the average magnetic field to regions that are widely separated in longitude within the heliosphere without using unrealistically strong cross-field diffusion. Aims. We study whether the recently suggested early non-diffusive phase of SEP propagation can explain the wide SEP events with realistic particle transport parameters. Methods. We used a novel model that accounts for the SEP propagation along field lines that meander as a result of plasma turbulence. Such a non-diffusive propagation mode has been shown to dominate the SEP cross-field propagation early in the SEP event history. We compare the new model to the traditional approach, and to SEP observations. Results. Using the new model, we reproduce the observed longitudinal extent of SEP peak fluxes that are characterised by a Gaussian profile with $\sigma=30-50^\circ$, while current diffusion theory can only explain extents of 11$^\circ$ with realistic diffusion coefficients. Our model also reproduces the timing of SEP arrival at distant longitudes, which cannot be explained using the diffusion model. Conclusions. The early onset of SEPs over a wide range of longitudes can be understood as a result of the effects of magnetic field-line random walk in the interplanetary medium and requires an SEP transport model that properly describes the non-diffusive early phase of SEP cross-field propagation.