Cross-field transport and pitch-angle anisotropy of solar energetic particles in MHD turbulence

TL;DR

This paper investigates how solar energetic particles spread in the heliosphere, focusing on the effects of pitch-angle anisotropy and different MHD turbulence models, revealing a new scaling law for particle drift.

Contribution

It introduces a novel calculation of particle drift considering pitch-angle dependence in two turbulence models, challenging previous assumptions.

Findings

Drift scales as (1-μ^2)^2 with pitch-angle cosine μ.

Results differ from previous models of SEP transport.

Implications for improved heliospheric propagation models.

Abstract

Recent modelling of solar energetic particles (SEPs) propagation through the heliospheric turbulence, also discussed in this workshop, has investigated the role of the pitch-angle scattering and the perpendicular transport in spreading particles in heliolongitude, as shown by multi-spacecraft measurements (STEREO A/B, ACE, SOHO, etc.) at 1 AU in various energy ranges. In some events the first-order pitch-angle anisotropy of the particles distribution is not-negligible. We calculate the average perpendicular displacement due to the gradient/curvature drift in an inhomogeneous turbulence accounting for pitch-angle dependence for two MHD turbulence models: (a) 3-D isotropic, (b) anisotropic as conjectured by Goldreich-Sridhar. We find in both cases that the drift scales as $(1-\mu^2)^2$ with the cosine of pitch-angle $\mu$, in contrast with previous models for transport of SEPs. This result can impact the models of propagation of SEPs through the heliosphere.