Solar Energetic Particle anisotropy in the field-line meandering model of interplanetary transport

Abstract:

Recent multi-spacecraft Solar Energetic Particle (SEP) observations have challenged the traditional view of SEP production and interplanetary transport, with the observations suggesting fast SEP access to a wide range of heliographic longitudes. For many events the anisotropy of the SEPs has been found to depend on the observer's longitude, being stronger at locations that are well magnetically connected to the assumed SEP source region, as compared to wider longitudinal reaches. This suggests that interplanetary transport is an important factor for the SEP cross-field extent. For some events, on the other hand, strong anisotropies are observed at a wider longitudinal range. In this report, we study the temporal and spatial evolution of SEP anisotropy using a new SEP transport model which incorporates field-line meandering into the Fokker-Planck (FP) transport modelling framework. The new model, FP+FLRW, was introduced by Laitinen et al (2013), who found using full-orbit simulations that the cross-field propagation of particles early in an SEP event is not diffusive, but dominated by deterministic propagation along turbulently meandering field-lines. We have recently incorporated the FP+FLRW model into the Parker spiral geometry, and shown that it is able to reproduce the observed fast access of SEPs to a wide range of longitudes, and the observed longitudinal peak intensity range with σ=30-50°, already for a narrow source region, while using realistic interplanetary transport conditions. Here, we compare the anisotropy evolution of an SEP event given by the FP+FLRW model to that given by the traditional FP approach, and discuss the implications of our findings for the SEP event origins, source width and the role of interplanetary turbulence in the interpretation of the SEP observations.