The effect of turbulence strength on meandering field lines and Solar Energetic Particle event extents

TL;DR

This paper investigates how turbulence strength in the solar wind influences the propagation and spatial extent of Solar Energetic Particles, highlighting the importance of early-time non-diffusive transport along meandering magnetic field lines.

Contribution

It demonstrates the impact of turbulence strength on SEP event extents and provides a scaling law linking turbulence parameters to particle distribution widths.

Findings

Wider SEP events occur under slow parallel transport conditions.

The maximum intensity distribution scales with turbulence strength as (1/λ∥*)^{1/4}.

High-intensity regions shift westward with stronger scattering.

Abstract

Insights into the processes of Solar Energetic Particle (SEP) propagation are essential for understanding how solar eruptions affect the radiation environment of near-Earth space. SEP propagation is influenced by turbulent magnetic fields in the solar wind, resulting in stochastic transport of SEPs to Earth. Multi-spacecraft observations suggest that the cross-field propagation shapes the SEP fluxes at Earth strongly. However, modelling SEP cross-field transport as spatial diffusion has been shown to be insufficient without use of unrealistically large cross-field diffusion coefficients. Recent work has shown that the early-time propagation of energetic particles across the mean field direction in turbulent fields is not diffusive, as the particles propagating along meandering field lines. This early-time transport mode results in fast access of the particles across the mean field direction, in agreement with the SEP observations. In this work, we demonstrate the significance of turbulence strength on evolution of the SEP radiation environment near Earth. We calculate the transport parameters with a turbulence transport model, parametrised by the SEP parallel scattering mean free path at 1~AU, $\lambda_{\parallel}^{*}$, and show that the parallel and cross-field transport are connected, with conditions resulting in slow parallel transport corresponding to wider events. We find a scaling $\sigma_{\phi,\, \mathrm{max}}\propto (1/\lambda_{\parallel}^{*})^{1/4}$ for the Gaussian fitting of the longitudinal distribution of maximum intensities. The longitudes with highest intensities are shifted towards the west for strong scattering conditions. Our results emphasise the importance of understanding both the SEP transport and the interplanetary turbulence conditions for modelling and predicting the SEP radiation environment at Earth.