# Early propagation of energetic particles across the mean field in turbulent plasmas

## TL;DR

This study uses full-orbit simulations to analyze early non-diffusive propagation of energetic particles along meandering magnetic field lines in turbulent plasmas, highlighting the importance of non-diffusive models for solar energetic particle events.

## Contribution

It demonstrates that early particle propagation is dominated by meandering field lines, challenging traditional diffusive models and emphasizing the need for non-diffusive transport descriptions.

## Key findings

- Early propagation is consistent with meandering field lines.
- Particles exhibit a wide cross-field extent at initial arrival.
- Cross-field distribution remains constant for hours before diffusing.

## Abstract

Propagation of energetic particles across the mean field direction in turbulent magnetic fields is often described as spatial diffusion. Recently, it has been suggested that initially the particles propagate systematically along meandering field lines, and only later reach the time-asymptotic diffusive cross-field propagation. In this paper, we analyse cross-field propagation of 1--100 MeV protons in composite 2D-slab turbulence superposed on a constant background magnetic field, using full-orbit particle simulations, to study the non-diffusive phase of particle propagation with a wide range of turbulence parameters. We show that the early-time non-diffusive propagation of the particles is consistent with particle propagation along turbulently meandering field lines. This results in a wide cross-field extent of the particles already at the initial arrival of particles to a given distance along the mean field direction, unlike when using spatial diffusion particle transport models. The cross-field extent of the particle distribution remains constant for up to tens of hours in turbulence environment consistent with the inner heliosphere during solar energetic particle events. Subsequently, the particles escape from their initial meandering field lines, and the particle propagation across the mean field reaches time-asymptotic diffusion. Our analysis shows that in order to understand solar energetic particle event origins, particle transport modelling must include non-diffusive particle propagation along meandering field lines.