Transport of relativistic solar energetic particles during GLE73

We investigate the transport of solar energetic particles (SEPs) during the relativistic and longitudinally widespread event of 28 October 2021 - GLE73, with the aim of quantifying the roles of parallel and perpendicular diffusion and constraining the spatial extent of the injection region. Inverse modeling is performed using numerical simulations of focused particle transport that include cross-field diffusion, in order to reproduce multi-spacecraft observations from STEREO-A, Solar Orbiter, and near-Earth missions over a wide range of electron and proton energies. Simulated intensity and anisotropy time profiles are compared across multiple helio-longitudes to derive consistent transport parameters. The results yield parallel mean free paths compatible with predictions from dynamical turbulence models for pitch-angle scattering. The inferred perpendicular mean free paths constitute a significant fraction of the parallel values, amounting to approximately 1–3% for electrons and 5–10% for protons, with a tendency to increase with particle rigidity. The injection region is found to be relatively narrow (≤20°) and to decrease with increasing rigidity. These findings indicate that a localized injection combined with efficient perpendicular diffusion can account for the observed widespread SEP signatures.

Acknowledgement
This research was supported by the European Union’s Horizon Europe programme under grant agreement No. 101135044 (SPEARHEAD; https://spearhead-he.eu/).