Simulations of a gradual solar energetic particle event observed by Helios 1, Helios 2, and IMP 8

TL;DR

This study simulates a gradual solar energetic particle event observed by multiple spacecraft using a numerical model of particle transport, revealing key parameters for shock acceleration and diffusion processes in interplanetary space.

Contribution

It introduces a detailed numerical simulation of SEP events incorporating shock acceleration efficiency and perpendicular diffusion, fitting observed data from Helios and IMP 8 spacecraft.

Findings

Perpendicular diffusion coefficient is about 1-3% of parallel diffusion at 1 AU.

Simulation reproduces the reservoir phenomenon and flux decay gradients.

Shock acceleration efficiency significantly influences longitudinal flux variations.

Abstract

In this work, a gradual solar energetic particle (SEP) event observed by multispacecraft has been simulated. The time profiles of SEP fluxes accelerated by an interplanetary shock in the three-dimensional interplanetary space are obtained by solving numerically the Fokker-Planck focused transport equation. The interplanetary shock is modeled as a moving source of energetic particles. By fitting the 1979/03/01 SEP fluxes observed by Helios 1, Helios 2, and IMP 8 with our simulations, we obtain the best parameters for the shock acceleration efficiency model. And we also find that the particle perpendicular diffusion coefficient with the level of $ \sim 1\% - 3\% $ of parallel diffusion coefficient at 1 AU should be included. The reservoir phenomenon is reproduced in the simulations, and the longitudinal gradient of SEP fluxes in the decay phase, which is observed by three spacecraft at different locations, is more sensitive to the shock acceleration efficiency parameters than that is to the perpendicular diffusion coefficient.