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.
Highlights
Solar energetic particles (SEPs) are accelerated up to relativistic energies during solar eruptions
We introduce a new model for heliospheric solar energetic particle (SEP) propagation that is capable of reproducing the fast transport in longitude that is observed at the beginning of SEP events
It should be noted that particle propagation as a beam along field lines that random-walk according to the field-line diffusion coefficient in Eq (9) causes transport across the mean field much faster than diffusion with the coefficient given by Eq (3)
Summary
Solar energetic particles (SEPs) are accelerated up to relativistic energies during solar eruptions Their propagation in interplanetary space is controlled by the large-scale Parker spiral magnetic field. The field-aligned propagation is affected by small-scale inhomogeneities (Jokipii 1966), whereas the cross-field propagation is described as diffusion caused by random-walking field lines (Jokipii 1966; Matthaeus et al 2003; Shalchi 2010) These theories give the ratio of the cross-field and field-aligned diffusion coefficients κ⊥/κ ∼ 0.01 near Earth, and these values are supported by cosmic ray observations (Burger et al 2000; Potgieter et al 2014) and particle simulations (Giacalone & Jokipii 1999). We compare the new model to the traditional diffusion approach (the FP model)
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